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<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/no_exceptions/" rel="canonical"><link href=../namespace/ rel=prev><link href=../object_order/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>Working Without Exceptions - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.ab4e12ef.min.css><link rel="stylesheet" href="../../assets/external/fonts.googleapis.com/css.61a430c9.css"><style>:root{--md-text-font:"Roboto";--md-code-font:"JetBrains Mono"}</style><link rel=stylesheet href=../../css/custom.css><script>__md_scope=new 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<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;iostream&gt;</span>
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;vector&gt;</span>
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;nlohmann/json.hpp&gt;</span>
<span class=k>using</span><span class=w> </span><span class=n>json</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>nlohmann</span><span class=o>::</span><span class=n>json</span><span class=p>;</span>
<span class=c1>// None of the calls below throws, so this example also works with -fno-exceptions.</span>
<span class=kt>int</span><span class=w> </span><span class=nf>main</span><span class=p>()</span>
<span class=p>{</span>
<span class=w> </span><span class=k>const</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>string</span><span class=w> </span><span class=n>invalid</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=sa>R</span><span class=s>"</span><span class=dl>(</span><span class=s>{"name": "Mary", "age": </span><span class=dl>)</span><span class=s>"</span><span class=p>;</span>
<span class=w> </span><span class=c1>// parse() with allow_exceptions = false returns a discarded value</span>
<span class=w> </span><span class=n>json</span><span class=w> </span><span class=n>j</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>parse</span><span class=p>(</span><span class=n>invalid</span><span class=p>,</span><span class=w> </span><span class=nb>nullptr</span><span class=p>,</span><span class=w> </span><span class=nb>false</span><span class=p>);</span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>boolalpha</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=s>"parse: discarded = "</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>j</span><span class=p>.</span><span class=n>is_discarded</span><span class=p>()</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=sc>'\n'</span><span class=p>;</span>
<span class=w> </span><span class=c1>// accept() only tells whether the input is valid JSON</span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=s>"accept: "</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>accept</span><span class=p>(</span><span class=n>invalid</span><span class=p>)</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=sc>'\n'</span><span class=p>;</span>
<span class=w> </span><span class=c1>// the binary formats have the same parameter</span>
<span class=w> </span><span class=k>const</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>vector</span><span class=o>&lt;</span><span class=n>std</span><span class=o>::</span><span class=kt>uint8_t</span><span class=o>&gt;</span><span class=w> </span><span class=n>truncated_cbor</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=p>{</span><span class=mh>0x83</span><span class=p>,</span><span class=w> </span><span class=mh>0x01</span><span class=p>};</span>
<span class=w> </span><span class=n>json</span><span class=w> </span><span class=n>c</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>from_cbor</span><span class=p>(</span><span class=n>truncated_cbor</span><span class=p>,</span><span class=w> </span><span class=nb>true</span><span class=p>,</span><span class=w> </span><span class=nb>false</span><span class=p>);</span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=s>"from_cbor: discarded = "</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>c</span><span class=p>.</span><span class=n>is_discarded</span><span class=p>()</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=sc>'\n'</span><span class=p>;</span>
<span class=w> </span><span class=c1>// valid input is parsed as usual</span>
<span class=w> </span><span class=n>json</span><span class=w> </span><span class=n>valid</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>parse</span><span class=p>(</span><span class=sa>R</span><span class=s>"</span><span class=dl>(</span><span class=s>{"name": "Mary"}</span><span class=dl>)</span><span class=s>"</span><span class=p>,</span><span class=w> </span><span class=nb>nullptr</span><span class=p>,</span><span class=w> </span><span class=nb>false</span><span class=p>);</span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=s>"valid input: "</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>valid</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=sc>'\n'</span><span class=p>;</span>
<span class=p>}</span>
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code>parse: discarded = true
accept: false
from_cbor: discarded = true
valid input: {"name":"Mary"}
</code></pre></div> </details> <p>None of the calls in this example throws, so it also compiles and runs with <code>-fno-exceptions</code>.</p> <p>The discarded value carries no message. If you need the position or the reason, use the <abbr title="Simple API for XML">SAX</abbr> interface; see <a href=../parsing/parse_exceptions/ >Parsing and exceptions</a>.</p> <h2 id=accessing-values>Accessing values<a class=headerlink href=#accessing-values title="Permanent link">&para;</a></h2> <p>The access functions are not equally safe. Only some of them never throw:</p> <table> <thead> <tr> <th>expression</th> <th>if the key is missing or the type is wrong</th> </tr> </thead> <tbody> <tr> <td><code class=highlight><span class=n>j</span><span class=p>.</span><span class=n>contains</span><span class=p>(</span><span class=s>"k"</span><span class=p>)</span></code></td> <td>returns <code class=highlight><span class=nb>false</span></code> for a missing key; never throws</td> </tr> <tr> <td><code class=highlight><span class=n>j</span><span class=p>.</span><span class=n>find</span><span class=p>(</span><span class=s>"k"</span><span class=p>)</span></code></td> <td>returns <code>end()</code> for a missing key; never throws</td> </tr> <tr> <td><code class=highlight><span class=n>j</span><span class=p>.</span><span class=n>is_string</span><span class=p>()</span></code>, ...</td> <td>the <code>is_*</code> functions test the type; never throw</td> </tr> <tr> <td><code class=highlight><span class=n>j</span><span class=p>.</span><span class=n>get_ptr</span><span class=o>&lt;</span><span class=k>const</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>string</span><span class=o>*&gt;</span><span class=p>()</span></code></td> <td>returns <code class=highlight><span class=nb>nullptr</span></code> for a different type; never throws</td> </tr> <tr> <td><code class=highlight><span class=n>j</span><span class=p>.</span><span class=n>at</span><span class=p>(</span><span class=s>"k"</span><span class=p>)</span></code></td> <td>throws <code>out_of_range.403</code> for a missing key</td> </tr> <tr> <td><code class=highlight><span class=n>j</span><span class=p>.</span><span class=n>get</span><span class=o>&lt;</span><span class=kt>int</span><span class=o>&gt;</span><span class=p>()</span></code></td> <td>throws <code>type_error.302</code> for a value that is not a number</td> </tr> <tr> <td><code class=highlight><span class=n>j</span><span class=p>.</span><span class=n>value</span><span class=p>(</span><span class=s>"k"</span><span class=p>,</span><span class=w> </span><span class=mi>0</span><span class=p>)</span></code></td> <td>returns the default for a missing key, but <strong>throws <code>type_error.302</code> if <code>k</code> exists with another type</strong></td> </tr> <tr> <td><code class=highlight><span class=n>j</span><span class=p>[</span><span class=s>"k"</span><span class=p>]</span></code> (non-const)</td> <td>inserts <code>null</code> for a missing key</td> </tr> <tr> <td><code class=highlight><span class=n>j</span><span class=p>[</span><span class=s>"k"</span><span class=p>]</span></code> (const)</td> <td>undefined behavior for a missing key</td> </tr> </tbody> </table> <p>So reading a value that may be absent or of the wrong type without exceptions takes two steps: look up, then test the type.</p> <details class=example> <summary>Example: look up a key and test its type</summary> <div class=highlight><pre><span></span><code><span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;iostream&gt;</span>
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;nlohmann/json.hpp&gt;</span>
<span class=k>using</span><span class=w> </span><span class=n>json</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>nlohmann</span><span class=o>::</span><span class=n>json</span><span class=p>;</span>
<span class=kt>int</span><span class=w> </span><span class=nf>main</span><span class=p>()</span>
<span class=p>{</span>
<span class=w> </span><span class=n>json</span><span class=w> </span><span class=n>j</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>parse</span><span class=p>(</span><span class=sa>R</span><span class=s>"</span><span class=dl>(</span><span class=s>{"name": "Mary", "age": 42}</span><span class=dl>)</span><span class=s>"</span><span class=p>);</span>
<span class=w> </span><span class=c1>// look up the key, then test the type: neither step throws</span>
<span class=w> </span><span class=k>const</span><span class=w> </span><span class=k>auto</span><span class=w> </span><span class=n>keys</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=p>{</span><span class=s>"name"</span><span class=p>,</span><span class=w> </span><span class=s>"age"</span><span class=p>,</span><span class=w> </span><span class=s>"city"</span><span class=p>};</span>
<span class=w> </span><span class=k>for</span><span class=w> </span><span class=p>(</span><span class=k>const</span><span class=w> </span><span class=kt>char</span><span class=o>*</span><span class=w> </span><span class=n>key</span><span class=w> </span><span class=o>:</span><span class=w> </span><span class=n>keys</span><span class=p>)</span>
<span class=w> </span><span class=p>{</span>
<span class=w> </span><span class=k>const</span><span class=w> </span><span class=k>auto</span><span class=w> </span><span class=n>it</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>j</span><span class=p>.</span><span class=n>find</span><span class=p>(</span><span class=n>key</span><span class=p>);</span>
<span class=w> </span><span class=k>if</span><span class=w> </span><span class=p>(</span><span class=n>it</span><span class=w> </span><span class=o>==</span><span class=w> </span><span class=n>j</span><span class=p>.</span><span class=n>end</span><span class=p>())</span>
<span class=w> </span><span class=p>{</span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>key</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=s>": missing</span><span class=se>\n</span><span class=s>"</span><span class=p>;</span>
<span class=w> </span><span class=p>}</span>
<span class=w> </span><span class=k>else</span><span class=w> </span><span class=k>if</span><span class=w> </span><span class=p>(</span><span class=n>it</span><span class=o>-&gt;</span><span class=n>is_string</span><span class=p>())</span>
<span class=w> </span><span class=p>{</span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>key</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=s>": string </span><span class=se>\"</span><span class=s>"</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>it</span><span class=o>-&gt;</span><span class=n>get_ref</span><span class=o>&lt;</span><span class=k>const</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>string</span><span class=o>&amp;&gt;</span><span class=p>()</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=s>"</span><span class=se>\"\n</span><span class=s>"</span><span class=p>;</span>
<span class=w> </span><span class=p>}</span>
<span class=w> </span><span class=k>else</span>
<span class=w> </span><span class=p>{</span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>key</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=s>": not a string, but "</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>it</span><span class=o>-&gt;</span><span class=n>type_name</span><span class=p>()</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=sc>'\n'</span><span class=p>;</span>
<span class=w> </span><span class=p>}</span>
<span class=w> </span><span class=p>}</span>
<span class=w> </span><span class=c1>// get_ptr returns a null pointer if the value has another type</span>
<span class=w> </span><span class=k>const</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>string</span><span class=o>*</span><span class=w> </span><span class=n>name</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>j</span><span class=p>[</span><span class=s>"name"</span><span class=p>].</span><span class=n>get_ptr</span><span class=o>&lt;</span><span class=k>const</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>string</span><span class=o>*&gt;</span><span class=p>();</span>
<span class=w> </span><span class=k>const</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>string</span><span class=o>*</span><span class=w> </span><span class=n>age</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>j</span><span class=p>[</span><span class=s>"age"</span><span class=p>].</span><span class=n>get_ptr</span><span class=o>&lt;</span><span class=k>const</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>string</span><span class=o>*&gt;</span><span class=p>();</span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=s>"get_ptr&lt;string*&gt;: name = "</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=p>(</span><span class=n>name</span><span class=w> </span><span class=o>!=</span><span class=w> </span><span class=nb>nullptr</span><span class=p>)</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=s>", age = "</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=p>(</span><span class=n>age</span><span class=w> </span><span class=o>!=</span><span class=w> </span><span class=nb>nullptr</span><span class=p>)</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=sc>'\n'</span><span class=p>;</span>
<span class=w> </span><span class=c1>// value() is NOT exception-free: it throws if the key exists with a different type</span>
<span class=w> </span><span class=k>try</span>
<span class=w> </span><span class=p>{</span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>j</span><span class=p>.</span><span class=n>value</span><span class=p>(</span><span class=s>"name"</span><span class=p>,</span><span class=w> </span><span class=mi>0</span><span class=p>)</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=sc>'\n'</span><span class=p>;</span>
<span class=w> </span><span class=p>}</span>
<span class=w> </span><span class=k>catch</span><span class=w> </span><span class=p>(</span><span class=k>const</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>type_error</span><span class=o>&amp;</span><span class=w> </span><span class=n>e</span><span class=p>)</span>
<span class=w> </span><span class=p>{</span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>e</span><span class=p>.</span><span class=n>what</span><span class=p>()</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=sc>'\n'</span><span class=p>;</span>
<span class=w> </span><span class=p>}</span>
<span class=p>}</span>
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code>name: string "Mary"
age: not a string, but number
city: missing
get_ptr&lt;string*&gt;: name = 1, age = 0
[json.exception.type_error.302] type must be number, but is string
</code></pre></div> </details> <p>The last lines of the example show why <code>value</code> is not on the safe list: it returns the default only for a <em>missing</em> key.</p> <p>For numbers, test with <code>is_number()</code> (or <code>is_number_integer()</code>, <code>is_number_unsigned()</code>, <code>is_number_float()</code>) before calling <code>get&lt;int&gt;()</code>, and see <a href=../types/number_handling/ >number handling</a> for range and precision. Whether a value fits into the target type is not checked by <code>get</code>.</p> <h2 id=converting-to-your-own-types>Converting to your own types<a class=headerlink href=#converting-to-your-own-types title="Permanent link">&para;</a></h2> <p><code>from_json</code> functions (including the ones generated by the <a href=../arbitrary_types/#simplify-your-life-with-macros><code>NLOHMANN_DEFINE_TYPE_*</code> macros</a>) report problems by throwing. There is no variant that returns an error code. Without exceptions you have two options:</p> <ul> <li>Validate the <abbr title="JavaScript Object Notation">JSON</abbr> value yourself, as shown above, and fill your struct field by field.</li> <li>Convert in a function that is compiled <strong>with</strong> exceptions enabled and catches them at the boundary, returning <code>std::optional</code> or an error code to the rest of your code:</li> </ul> <details class=example> <summary>Example: convert to a struct and report failure as std::optional</summary> <div class=highlight><pre><span></span><code><span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;iostream&gt;</span>
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;optional&gt;</span>
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;string&gt;</span>
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;nlohmann/json.hpp&gt;</span>
<span class=k>using</span><span class=w> </span><span class=n>json</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>nlohmann</span><span class=o>::</span><span class=n>json</span><span class=p>;</span>
<span class=k>struct</span><span class=w> </span><span class=nc>person</span>
<span class=p>{</span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>string</span><span class=w> </span><span class=n>name</span><span class=p>;</span>
<span class=w> </span><span class=kt>int</span><span class=w> </span><span class=n>age</span><span class=p>;</span>
<span class=p>};</span>
<span class=n>NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE</span><span class=p>(</span><span class=n>person</span><span class=p>,</span><span class=w> </span><span class=n>name</span><span class=p>,</span><span class=w> </span><span class=n>age</span><span class=p>)</span>
<span class=c1>// Compile this function with exceptions enabled and call it from code that does not use them.</span>
<span class=n>std</span><span class=o>::</span><span class=n>optional</span><span class=o>&lt;</span><span class=n>person</span><span class=o>&gt;</span><span class=w> </span><span class=n>to_person</span><span class=p>(</span><span class=k>const</span><span class=w> </span><span class=n>json</span><span class=o>&amp;</span><span class=w> </span><span class=n>j</span><span class=p>)</span>
<span class=p>{</span>
<span class=w> </span><span class=k>try</span>
<span class=w> </span><span class=p>{</span>
<span class=w> </span><span class=k>return</span><span class=w> </span><span class=n>j</span><span class=p>.</span><span class=n>get</span><span class=o>&lt;</span><span class=n>person</span><span class=o>&gt;</span><span class=p>();</span>
<span class=w> </span><span class=p>}</span>
<span class=w> </span><span class=k>catch</span><span class=w> </span><span class=p>(</span><span class=k>const</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>exception</span><span class=o>&amp;</span><span class=p>)</span>
<span class=w> </span><span class=p>{</span>
<span class=w> </span><span class=k>return</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>nullopt</span><span class=p>;</span>
<span class=w> </span><span class=p>}</span>
<span class=p>}</span>
<span class=kt>int</span><span class=w> </span><span class=n>main</span><span class=p>()</span>
<span class=p>{</span>
<span class=w> </span><span class=k>const</span><span class=w> </span><span class=n>json</span><span class=w> </span><span class=n>good</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=p>{{</span><span class=s>"name"</span><span class=p>,</span><span class=w> </span><span class=s>"Mary"</span><span class=p>},</span><span class=w> </span><span class=p>{</span><span class=s>"age"</span><span class=p>,</span><span class=w> </span><span class=mi>42</span><span class=p>}};</span>
<span class=w> </span><span class=k>const</span><span class=w> </span><span class=n>json</span><span class=w> </span><span class=n>bad</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=p>{{</span><span class=s>"name"</span><span class=p>,</span><span class=w> </span><span class=s>"Mary"</span><span class=p>},</span><span class=w> </span><span class=p>{</span><span class=s>"age"</span><span class=p>,</span><span class=w> </span><span class=s>"forty-two"</span><span class=p>}};</span>
<span class=w> </span><span class=k>const</span><span class=w> </span><span class=n>json</span><span class=w> </span><span class=n>incomplete</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=p>{{</span><span class=s>"name"</span><span class=p>,</span><span class=w> </span><span class=s>"Mary"</span><span class=p>}};</span>
<span class=w> </span><span class=k>const</span><span class=w> </span><span class=k>auto</span><span class=w> </span><span class=n>inputs</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=p>{</span><span class=n>good</span><span class=p>,</span><span class=w> </span><span class=n>bad</span><span class=p>,</span><span class=w> </span><span class=n>incomplete</span><span class=p>};</span>
<span class=w> </span><span class=k>for</span><span class=w> </span><span class=p>(</span><span class=k>const</span><span class=w> </span><span class=n>json</span><span class=o>&amp;</span><span class=w> </span><span class=n>j</span><span class=w> </span><span class=o>:</span><span class=w> </span><span class=n>inputs</span><span class=p>)</span>
<span class=w> </span><span class=p>{</span>
<span class=w> </span><span class=k>if</span><span class=w> </span><span class=p>(</span><span class=k>const</span><span class=w> </span><span class=k>auto</span><span class=w> </span><span class=n>p</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>to_person</span><span class=p>(</span><span class=n>j</span><span class=p>))</span>
<span class=w> </span><span class=p>{</span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=s>"person: "</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>p</span><span class=o>-&gt;</span><span class=n>name</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=s>", "</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>p</span><span class=o>-&gt;</span><span class=n>age</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=sc>'\n'</span><span class=p>;</span>
<span class=w> </span><span class=p>}</span>
<span class=w> </span><span class=k>else</span>
<span class=w> </span><span class=p>{</span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=s>"not a person: "</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>j</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=sc>'\n'</span><span class=p>;</span>
<span class=w> </span><span class=p>}</span>
<span class=w> </span><span class=p>}</span>
<span class=p>}</span>
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code>person: Mary, 42
not a person: {"age":"forty-two","name":"Mary"}
not a person: {"name":"Mary"}
</code></pre></div> </details> <h2 id=overriding-what-happens-instead-of-an-exception>Overriding what happens instead of an exception<a class=headerlink href=#overriding-what-happens-instead-of-an-exception title="Permanent link">&para;</a></h2> <p>With exceptions off, you can replace the <code>abort()</code> by your own code, for example to log the message first. See <a href=../../home/exceptions/#switch-off-exceptions>Switch off exceptions</a> and <a href=../../api/macros/json_throw_user/ ><code>JSON_THROW_USER</code></a>. The handler must not return normally.</p> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../../home/exceptions/ >Exceptions</a> - all exceptions and their ids</li> <li><a href=../parsing/parse_exceptions/ >Parsing and exceptions</a> - the parsing alternatives in detail</li> <li><a href=../parsing/untrusted_input/ >Parsing untrusted input</a></li> <li><a href=../../api/macros/json_noexception/ ><code>JSON_NOEXCEPTION</code></a></li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 25, 2026 11:07:13 UTC">October 25, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2025 Fonticons, Inc.--><path d="M173.9 397.4c0 2-2.3 3.6-5.2 3.6-3.3.3-5.6-1.3-5.6-3.6 0-2 2.3-3.6 5.2-3.6 3-.3 5.6 1.3 5.6 3.6m-31.1-4.5c-.7 2 1.3 4.3 4.3 4.9 2.6 1 5.6 0 6.2-2s-1.3-4.3-4.3-5.2c-2.6-.7-5.5.3-6.2 2.3m44.2-1.7c-2.9.7-4.9 2.6-4.6 4.9.3 2 2.9 3.3 5.9 2.6 2.9-.7 4.9-2.6 4.6-4.6-.3-1.9-3-3.2-5.9-2.9M252.8 8C114.1 8 8 113.3 8 252c0 110.9 69.8 205.8 169.5 239.2 12.8 2.3 17.3-5.6 17.3-12.1 0-6.2-.3-40.4-.3-61.4 0 0-70 15-84.7-29.8 0 0-11.4-29.1-27.8-36.6 0 0-22.9-15.7 1.6-15.4 0 0 24.9 2 38.6 25.8 21.9 38.6 58.6 27.5 72.9 20.9 2.3-16 8.8-27.1 16-33.7-55.9-6.2-112.3-14.3-112.3-110.5 0-27.5 7.6-41.3 23.6-58.9-2.6-6.5-11.1-33.3 2.6-67.9 20.9-6.5 69 27 69 27 20-5.6 41.5-8.5 62.8-8.5s42.8 2.9 62.8 8.5c0 0 48.1-33.6 69-27 13.7 34.7 5.2 61.4 2.6 67.9 16 17.7 25.8 31.5 25.8 58.9 0 96.5-58.9 104.2-114.8 110.5 9.2 7.9 17 22.9 17 46.4 0 33.7-.3 75.4-.3 83.6 0 6.5 4.6 14.4 17.3 12.1C436.2 457.8 504 362.9 504 252 504 113.3 391.5 8 252.8 8M105.2 352.9c-1.3 1-1 3.3.7 5.2 1.6 1.6 3.9 2.3 5.2 1 1.3-1 1-3.3-.7-5.2-1.6-1.6-3.9-2.3-5.2-1m-10.8-8.1c-.7 1.3.3 2.9 2.3 3.9 1.6 1 3.6.7 4.3-.7.7-1.3-.3-2.9-2.3-3.9-2-.6-3.6-.3-4.3.7m32.4 35.6c-1.6 1.3-1 4.3 1.3 6.2 2.3 2.3 5.2 2.6 6.5 1 1.3-1.3.7-4.3-1.3-6.2-2.2-2.3-5.2-2.6-6.5-1m-11.4-14.7c-1.6 1-1.6 3.6 0 5.9s4.3 3.3 5.6 2.3c1.6-1.3 1.6-3.9 0-6.2-1.4-2.3-4-3.3-5.6-2"/></svg> </a> <a href="https://www.linkedin.com/in/nielslohmann/" target="_blank" rel="noopener" title="www.linkedin.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 448 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2025 Fonticons, Inc.--><path d="M416 32H31.9C14.3 32 0 46.5 0 64.3v383.4C0 465.5 14.3 480 31.9 480H416cLine truncated
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# Working Without Exceptions
By default, the library reports errors by throwing exceptions (see [Exceptions](https://json.nlohmann.me/home/exceptions/index.md)). This page collects what to use when exceptions are unwanted or unavailable, for example in embedded systems, game engines, or code compiled with `-fno-exceptions`.
## Two different questions
1. **Can my code avoid *catching* exceptions?** Yes. For parsing and for accessing values there are functions that report failure through a return value. They work in a normal build and need no `try`/`catch`.
1. **Can the library be *compiled* without exceptions?** Yes, with `-fno-exceptions` or by defining [`JSON_NOEXCEPTION`](https://json.nlohmann.me/api/macros/json_noexception/index.md). Both have the same effect: every `throw` in the library becomes a call to `std::abort()`. The functions listed below never reach a `throw` for the failures they report, so they keep working in this configuration. Anything else that fails aborts the program.
## Parsing
| function | on invalid input |
| ----------------------------------------------------------------------------------------------- | ------------------------------------------------------- |
| `json::parse(input, nullptr, false)` | returns a *discarded* value; test with `is_discarded()` |
| [`accept`](https://json.nlohmann.me/api/basic_json/accept/index.md) | returns `false` |
| `json::from_cbor(input, true, false)` (and the other `from_*` functions) | returns a *discarded* value |
| [`sax_parse`](https://json.nlohmann.me/api/basic_json/sax_parse/index.md) with your own handler | returns `false` after calling `parse_error` |
Example: parse and validate without exceptions
```
#include <cstdint>
#include <iostream>
#include <vector>
#include <nlohmann/json.hpp>
using json = nlohmann::json;
// None of the calls below throws, so this example also works with -fno-exceptions.
int main()
{
const std::string invalid = R"({"name": "Mary", "age": )";
// parse() with allow_exceptions = false returns a discarded value
json j = json::parse(invalid, nullptr, false);
std::cout << std::boolalpha << "parse: discarded = " << j.is_discarded() << '\n';
// accept() only tells whether the input is valid JSON
std::cout << "accept: " << json::accept(invalid) << '\n';
// the binary formats have the same parameter
const std::vector<std::uint8_t> truncated_cbor = {0x83, 0x01};
json c = json::from_cbor(truncated_cbor, true, false);
std::cout << "from_cbor: discarded = " << c.is_discarded() << '\n';
// valid input is parsed as usual
json valid = json::parse(R"({"name": "Mary"})", nullptr, false);
std::cout << "valid input: " << valid << '\n';
}
```
Output:
```
parse: discarded = true
accept: false
from_cbor: discarded = true
valid input: {"name":"Mary"}
```
None of the calls in this example throws, so it also compiles and runs with `-fno-exceptions`.
The discarded value carries no message. If you need the position or the reason, use the SAX interface; see [Parsing and exceptions](https://json.nlohmann.me/features/parsing/parse_exceptions/index.md).
## Accessing values
The access functions are not equally safe. Only some of them never throw:
| expression | if the key is missing or the type is wrong |
| --------------------------------- | ------------------------------------------------------------------------------------------------------ |
| `j.contains("k")` | returns `false` for a missing key; never throws |
| `j.find("k")` | returns `end()` for a missing key; never throws |
| `j.is_string()`, ... | the `is_*` functions test the type; never throw |
| `j.get_ptr<const std::string*>()` | returns `nullptr` for a different type; never throws |
| `j.at("k")` | throws `out_of_range.403` for a missing key |
| `j.get<int>()` | throws `type_error.302` for a value that is not a number |
| `j.value("k", 0)` | returns the default for a missing key, but **throws `type_error.302` if `k` exists with another type** |
| `j["k"]` (non-const) | inserts `null` for a missing key |
| `j["k"]` (const) | undefined behavior for a missing key |
So reading a value that may be absent or of the wrong type without exceptions takes two steps: look up, then test the type.
Example: look up a key and test its type
```
#include <iostream>
#include <nlohmann/json.hpp>
using json = nlohmann::json;
int main()
{
json j = json::parse(R"({"name": "Mary", "age": 42})");
// look up the key, then test the type: neither step throws
const auto keys = {"name", "age", "city"};
for (const char* key : keys)
{
const auto it = j.find(key);
if (it == j.end())
{
std::cout << key << ": missing\n";
}
else if (it->is_string())
{
std::cout << key << ": string \"" << it->get_ref<const std::string&>() << "\"\n";
}
else
{
std::cout << key << ": not a string, but " << it->type_name() << '\n';
}
}
// get_ptr returns a null pointer if the value has another type
const std::string* name = j["name"].get_ptr<const std::string*>();
const std::string* age = j["age"].get_ptr<const std::string*>();
std::cout << "get_ptr<string*>: name = " << (name != nullptr) << ", age = " << (age != nullptr) << '\n';
// value() is NOT exception-free: it throws if the key exists with a different type
try
{
std::cout << j.value("name", 0) << '\n';
}
catch (const json::type_error& e)
{
std::cout << e.what() << '\n';
}
}
```
Output:
```
name: string "Mary"
age: not a string, but number
city: missing
get_ptr<string*>: name = 1, age = 0
[json.exception.type_error.302] type must be number, but is string
```
The last lines of the example show why `value` is not on the safe list: it returns the default only for a *missing* key.
For numbers, test with `is_number()` (or `is_number_integer()`, `is_number_unsigned()`, `is_number_float()`) before calling `get<int>()`, and see [number handling](https://json.nlohmann.me/features/types/number_handling/index.md) for range and precision. Whether a value fits into the target type is not checked by `get`.
## Converting to your own types
`from_json` functions (including the ones generated by the [`NLOHMANN_DEFINE_TYPE_*` macros](https://json.nlohmann.me/features/arbitrary_types/#simplify-your-life-with-macros)) report problems by throwing. There is no variant that returns an error code. Without exceptions you have two options:
- Validate the JSON value yourself, as shown above, and fill your struct field by field.
- Convert in a function that is compiled **with** exceptions enabled and catches them at the boundary, returning `std::optional` or an error code to the rest of your code:
Example: convert to a struct and report failure as std::optional
```
#include <iostream>
#include <optional>
#include <string>
#include <nlohmann/json.hpp>
using json = nlohmann::json;
struct person
{
std::string name;
int age;
};
NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE(person, name, age)
// Compile this function with exceptions enabled and call it from code that does not use them.
std::optional<person> to_person(const json& j)
{
try
{
return j.get<person>();
}
catch (const json::exception&)
{
return std::nullopt;
}
}
int main()
{
const json good = {{"name", "Mary"}, {"age", 42}};
const json bad = {{"name", "Mary"}, {"age", "forty-two"}};
const json incomplete = {{"name", "Mary"}};
const auto inputs = {good, bad, incomplete};
for (const json& j : inputs)
{
if (const auto p = to_person(j))
{
std::cout << "person: " << p->name << ", " << p->age << '\n';
}
else
{
std::cout << "not a person: " << j << '\n';
}
}
}
```
Output:
```
person: Mary, 42
not a person: {"age":"forty-two","name":"Mary"}
not a person: {"name":"Mary"}
```
## Overriding what happens instead of an exception
With exceptions off, you can replace the `abort()` by your own code, for example to log the message first. See [Switch off exceptions](https://json.nlohmann.me/home/exceptions/#switch-off-exceptions) and [`JSON_THROW_USER`](https://json.nlohmann.me/api/macros/json_throw_user/index.md). The handler must not return normally.
## See also
- [Exceptions](https://json.nlohmann.me/home/exceptions/index.md) - all exceptions and their ids
- [Parsing and exceptions](https://json.nlohmann.me/features/parsing/parse_exceptions/index.md) - the parsing alternatives in detail
- [Parsing untrusted input](https://json.nlohmann.me/features/parsing/untrusted_input/index.md)
- [`JSON_NOEXCEPTION`](https://json.nlohmann.me/api/macros/json_noexception/index.md)