# std::format_error

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< cpp‎ | utility‎ | format

Defined in header `<format>` |  |
class format_error; |  | (since C++20)

Defines the type of exception object thrown to report errors in the formatting library.

Inheritance diagram

### Member functions

(constructor) | constructs a new `format_error` object with the given message (public member function)
operator= | replaces the `format_error` object (public member function)

## std::format_error::format_error

format_error( const std::string& what_arg ); | (1) |
format_error( const char* what_arg ); | (2) |
format_error( const format_error& other ) noexcept; | (3) |

1) Constructs the exception object with what_arg as explanatory string. After construction, std::strcmp(what(), what_arg.c_str()) == 0.
2) Constructs the exception object with what_arg as explanatory string. After construction, std::strcmp(what(), what_arg) == 0.
3) Copy constructor. If *this and other both have dynamic type `std::format_error` then std::strcmp(what(), other.what()) == 0.

### Parameters

what_arg | - | explanatory string
other | - | another exception object to copy

### Exceptions

1-2) May throw std::bad_alloc

### Notes

Because copying `std::format_error` is not permitted to throw exceptions, this message is typically stored internally as a separately-allocated reference-counted string. This is also why there is no constructor taking `std::string&&`: it would have to copy the content anyway.

A derived standard exception class must have a publicly accessible copy constructor. It can be implicitly defined as long as the explanatory strings obtained by `what()` are the same for the original object and the copied object.

## std::format_error::operator=

format_error& operator=( const format_error& other ) noexcept; |  |

Assigns the contents with those of other. If *this and other both have dynamic type `std::format_error` then std::strcmp(what(), other.what()) == 0 after assignment.

### Parameters

other | - | another exception object to assign with

### Return value

*this

### Notes

A derived standard exception class must have a publicly accessible copy assignment operator. It can be implicitly defined as long as the explanatory strings obtained by `what()` are the same for the original object and the copied object.

## Inherited from std::runtime_error

## Inherited from std::exception

### Member functions

(destructor) [virtual] | destroys the exception object (virtual public member function of `std::exception`)
what [virtual] | returns an explanatory string (virtual public member function of `std::exception`)

### Example

| This section is incomplete Reason: no example

### See also

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