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# JSON_ASSERT

    #define JSON_ASSERT(x) /* value */

This macro controls which code is executed for runtime assertions of the library.

## Parameters

`x` (in)
  expression of scalar type

## Default definition

The default value is `assert(x)`.

    #define JSON_ASSERT(x) assert(x)

Therefore, assertions can be switched off by defining `NDEBUG`.

## Notes

- The library uses numerous assertions to guarantee invariants and to abort in case of otherwise undefined behavior (e.g., when calling operator[] with a missing object key on a `const` object). See page runtime assertions for more information.
- Defining the macro to code that does not call `std::abort` may leave the library in an undefined state.
- The macro is undefined outside the library.

## Examples

Example 1: default behavior

The following code will trigger an assertion at runtime:

    #include <nlohmann/json.hpp>

    using json = nlohmann::json;

    int main()
    {
        const json j = {{"key", "value"}};
        auto v = j["missing"];
    }

Output:

    Assertion failed: (m_value.object->find(key) != m_value.object->end()), function operator[], file json.hpp, line 2144.

Example 2: user-defined behavior

The assertion reporting can be changed by defining `JSON_ASSERT(x)` differently.

    #include <cstdio>
    #include <cstdlib>
    #define JSON_ASSERT(x) if(!(x)){fprintf(stderr, "assertion error in %s\n", __FUNCTION__); std::abort();}

    #include <nlohmann/json.hpp>

    using json = nlohmann::json;

    int main()
    {
        const json j = {{"key", "value"}};
        auto v = j["missing"];
    }

Output:

    assertion error in operator[]

## Version history

- Added in version 3.9.0.

Last update: May 1, 2022
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