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# Working Without Exceptions
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By default, the library reports errors by throwing exceptions (see [Exceptions](../home/exceptions.md)). This page
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collects what to use when exceptions are unwanted or unavailable, for example in embedded systems, game engines, or
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code compiled with `-fno-exceptions`.
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## Two different questions
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1. **Can my code avoid *catching* exceptions?** Yes. For parsing and for accessing values there are functions that
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report failure through a return value. They work in a normal build and need no `#!cpp try`/`#!cpp catch`.
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2. **Can the library be *compiled* without exceptions?** Yes, with `-fno-exceptions` or by defining
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[`JSON_NOEXCEPTION`](../api/macros/json_noexception.md). Both have the same effect: every `#!cpp throw` in the
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library becomes a call to `std::abort()`. The functions listed below never reach a `#!cpp throw` for the failures
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they report, so they keep working in this configuration. Anything else that fails aborts the program.
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## Parsing
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| function | on invalid input |
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|---------------------------------------------------------------------------------------------|-----------------------------------------------------|
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| `#!cpp json::parse(input, nullptr, false)` | returns a *discarded* value; test with `is_discarded()` |
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| [`accept`](../api/basic_json/accept.md) | returns `#!cpp false` |
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| `#!cpp json::from_cbor(input, true, false)` (and the other `from_*` functions) | returns a *discarded* value |
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| [`sax_parse`](../api/basic_json/sax_parse.md) with your own handler | returns `#!cpp false` after calling `parse_error` |
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??? example "Example: parse and validate without exceptions"
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```cpp
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#include <cstdint>
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#include <iostream>
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#include <vector>
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#include <nlohmann/json.hpp>
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using json = nlohmann::json;
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// None of the calls below throws, so this example also works with -fno-exceptions.
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int main()
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{
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const std::string invalid = R"({"name": "Mary", "age": )";
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// parse() with allow_exceptions = false returns a discarded value
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json j = json::parse(invalid, nullptr, false);
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std::cout << std::boolalpha << "parse: discarded = " << j.is_discarded() << '\n';
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// accept() only tells whether the input is valid JSON
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std::cout << "accept: " << json::accept(invalid) << '\n';
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// the binary formats have the same parameter
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const std::vector<std::uint8_t> truncated_cbor = {0x83, 0x01};
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json c = json::from_cbor(truncated_cbor, true, false);
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std::cout << "from_cbor: discarded = " << c.is_discarded() << '\n';
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// valid input is parsed as usual
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json valid = json::parse(R"({"name": "Mary"})", nullptr, false);
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std::cout << "valid input: " << valid << '\n';
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}
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```
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Output:
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```
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parse: discarded = true
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accept: false
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from_cbor: discarded = true
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valid input: {"name":"Mary"}
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```
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None of the calls in this example throws, so it also compiles and runs with `-fno-exceptions`.
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The discarded value carries no message. If you need the position or the reason, use the SAX interface; see
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[Parsing and exceptions](parsing/parse_exceptions.md).
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## Accessing values
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The access functions are not equally safe. Only some of them never throw:
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| expression | if the key is missing or the type is wrong |
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|-------------------------------------|-------------------------------------------------------------------------------|
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| `#!cpp j.contains("k")` | returns `#!cpp false` for a missing key; never throws |
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| `#!cpp j.find("k")` | returns `end()` for a missing key; never throws |
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| `#!cpp j.is_string()`, ... | the `is_*` functions test the type; never throw |
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| `#!cpp j.get_ptr<const std::string*>()` | returns `#!cpp nullptr` for a different type; never throws |
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| `#!cpp j.at("k")` | throws `out_of_range.403` for a missing key |
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| `#!cpp j.get<int>()` | throws `type_error.302` for a value that is not a number |
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| `#!cpp j.value("k", 0)` | returns the default for a missing key, but **throws `type_error.302` if `k` exists with another type** |
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| `#!cpp j["k"]` (non-const) | inserts `null` for a missing key |
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| `#!cpp j["k"]` (const) | undefined behavior for a missing key |
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So reading a value that may be absent or of the wrong type without exceptions takes two steps: look up, then test the
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type.
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??? example "Example: look up a key and test its type"
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```cpp
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#include <iostream>
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#include <nlohmann/json.hpp>
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using json = nlohmann::json;
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int main()
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{
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json j = json::parse(R"({"name": "Mary", "age": 42})");
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// look up the key, then test the type: neither step throws
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const auto keys = {"name", "age", "city"};
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for (const char* key : keys)
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{
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const auto it = j.find(key);
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if (it == j.end())
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{
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std::cout << key << ": missing\n";
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}
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else if (it->is_string())
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{
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std::cout << key << ": string \"" << it->get_ref<const std::string&>() << "\"\n";
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}
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else
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{
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std::cout << key << ": not a string, but " << it->type_name() << '\n';
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}
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}
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// get_ptr returns a null pointer if the value has another type
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const std::string* name = j["name"].get_ptr<const std::string*>();
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const std::string* age = j["age"].get_ptr<const std::string*>();
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std::cout << "get_ptr<string*>: name = " << (name != nullptr) << ", age = " << (age != nullptr) << '\n';
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// value() is NOT exception-free: it throws if the key exists with a different type
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try
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{
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std::cout << j.value("name", 0) << '\n';
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}
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catch (const json::type_error& e)
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{
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std::cout << e.what() << '\n';
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}
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}
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```
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Output:
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```
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name: string "Mary"
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age: not a string, but number
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city: missing
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get_ptr<string*>: name = 1, age = 0
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[json.exception.type_error.302] type must be number, but is string
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```
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The last lines of the example show why `value` is not on the safe list: it returns the default only for a *missing*
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key.
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For numbers, test with `is_number()` (or `is_number_integer()`, `is_number_unsigned()`, `is_number_float()`) before
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calling `get<int>()`, and see [number handling](types/number_handling.md) for range and precision. Whether a value
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fits into the target type is not checked by `get`.
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## Converting to your own types
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`from_json` functions (including the ones generated by the
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[`NLOHMANN_DEFINE_TYPE_*` macros](arbitrary_types.md#simplify-your-life-with-macros)) report problems by throwing.
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There is no variant that returns an error code. Without exceptions you have two options:
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- Validate the JSON value yourself, as shown above, and fill your struct field by field.
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- Convert in a function that is compiled **with** exceptions enabled and catches them at the boundary, returning
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`std::optional` or an error code to the rest of your code:
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??? example "Example: convert to a struct and report failure as std::optional"
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```cpp
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#include <iostream>
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#include <optional>
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#include <string>
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#include <nlohmann/json.hpp>
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using json = nlohmann::json;
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struct person
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{
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std::string name;
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int age;
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};
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NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE(person, name, age)
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// Compile this function with exceptions enabled and call it from code that does not use them.
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std::optional<person> to_person(const json& j)
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{
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try
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{
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return j.get<person>();
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}
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catch (const json::exception&)
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{
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return std::nullopt;
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}
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}
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int main()
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{
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const json good = {{"name", "Mary"}, {"age", 42}};
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const json bad = {{"name", "Mary"}, {"age", "forty-two"}};
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const json incomplete = {{"name", "Mary"}};
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const auto inputs = {good, bad, incomplete};
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for (const json& j : inputs)
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{
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if (const auto p = to_person(j))
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{
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std::cout << "person: " << p->name << ", " << p->age << '\n';
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}
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else
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{
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std::cout << "not a person: " << j << '\n';
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}
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}
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}
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```
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Output:
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```
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person: Mary, 42
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not a person: {"age":"forty-two","name":"Mary"}
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not a person: {"name":"Mary"}
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```
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## Overriding what happens instead of an exception
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With exceptions off, you can replace the `abort()` by your own code, for example to log the message first. See
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[Switch off exceptions](../home/exceptions.md#switch-off-exceptions) and
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[`JSON_THROW_USER`](../api/macros/json_throw_user.md). The handler must not return normally.
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## See also
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- [Exceptions](../home/exceptions.md) - all exceptions and their ids
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- [Parsing and exceptions](parsing/parse_exceptions.md) - the parsing alternatives in detail
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- [Parsing untrusted input](parsing/untrusted_input.md)
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- [`JSON_NOEXCEPTION`](../api/macros/json_noexception.md)
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