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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)