JSON_ASSERT(x)

#define JSON_ASSERT(x) /* value */

This macro controls which code is executed for runtime assertions of the libraries. It is set by the library, but can be overridden by defining it before including the library.

Parameters

x (in): expression of scalar type

Default definition

The default value is #!cpp 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 "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[]
```

Version history

Added in version 3.9.0.

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