json/doc/mkdocs/docs/api/macros/json_throw_user.md

JSON_CATCH_USER, JSON_THROW_USER, JSON_TRY_USER

// (1)
#define JSON_CATCH_USER(exception) /* value */
// (2)
#define JSON_THROW_USER(exception) /* value */
// (3)
#define JSON_TRY_USER /* value */

Controls how exceptions are handled by the library.

1. This macro overrides #!cpp catch calls inside the library. The argument is the type of the exception to catch. As of version 3.8.0, the library only catches std::out_of_range exceptions internally to rethrow them as json::out_of_range exceptions. The macro is always followed by a scope.
2. This macro overrides #!cpp throw calls inside the library. The argument is the exception to be thrown. Note that JSON_THROW_USER should leave the current scope (e.g., by throwing or aborting), as continuing after it may yield undefined behavior.
3. This macro overrides #!cpp try calls inside the library. It has no arguments and is always followed by a scope.

Parameters

exception (in)

an exception type

Default definition

By default, the macros map to their respective C++ keywords:

#define JSON_CATCH_USER(exception) catch(exception)
#define JSON_THROW_USER(exception) throw exception
#define JSON_TRY_USER              try

When exceptions are switched off, the #!cpp try block is executed unconditionally, and throwing exceptions is replaced by calling std::abort to make reaching the #!cpp throw branch abort the process.

#define JSON_THROW_USER(exception) std::abort()
#define JSON_TRY_USER              if (true)
#define JSON_CATCH_USER(exception) if (false)

Examples

??? example

The code below switches off exceptions and creates a log entry with a detailed error message in case of errors.

```cpp
#include <iostream>

#define JSON_TRY_USER if(true)
#define JSON_CATCH_USER(exception) if(false)
#define JSON_THROW_USER(exception)                           \
    {std::clog << "Error in " << __FILE__ << ":" << __LINE__ \
               << " (function " << __FUNCTION__ << ") - "    \
               << (exception).what() << std::endl;           \
     std::abort();}

#include <nlohmann/json.hpp>
```

See also

• Switch off exceptions for more information how to switch off exceptions
• JSON_NOEXCEPTION - switch off exceptions

Version history

• Added in version 3.1.0.