# JSON_ASSERT

```cpp
#define JSON_ASSERT(x) /* value */
```

This macro controls which code is executed for [runtime assertions](../../features/assertions.md) of the library.

## Parameters

`x` (in)
:   expression of a scalar type

## Default definition

The default value is [`#!cpp assert(x)`](https://en.cppreference.com/w/cpp/error/assert).

```cpp
#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[]](../basic_json/operator%5B%5D.md) with a missing object key on a `const` object). See
  page [runtime assertions](../../features/assertions.md) 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 "Example 1: default behavior"

    The following code will trigger an assertion at runtime:

    ```cpp
    #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 "Example 2: user-defined behavior"

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

    ```cpp
    #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[]
    ```

## See also

- [Runtime Assertions](../../features/assertions.md) - overview documentation

## Version history

- Added in version 3.9.0.