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std:: format_error

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Text processing library
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Standard format specification
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Format strings
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Formatter
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Format error
format_error
(C++20)
Defined in header <format>
class format_error : public runtime_error
(since C++20)

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

All member functions of std::format_error are constexpr : it is possible to create and use std::format_error objects in the evaluation of a constant expression.

However, std::format_error objects generally cannot be constexpr , because any dynamically allocated storage must be released in the same evaluation of constant expression.

(since C++26)
cpp/error/exception cpp/error/runtime error std-format error-inheritance.svg

Inheritance diagram

Contents

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) (constexpr since C++26)
format_error ( const char * what_arg ) ;
(2) (constexpr since C++26)
format_error ( const format_error & other ) noexcept ;
(3) (constexpr since C++26)
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 . No exception can be thrown from the copy constructor.

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 ;
(constexpr since C++26)

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. No exception can be thrown from the copy assignment operator.

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

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

Notes

Feature-test macro Value Std Feature
__cpp_lib_constexpr_exceptions 202502L (C++26) constexpr exception types

Example

Run this code 
#include <format>
#include <print>
#include <string_view>
#include <utility>
int main()
{
    try
    {
        auto x13{37};
        auto args{std::make_format_args(x13)};
        std::ignore = std::vformat("{:()}", args); // throws
    }
    catch(const std::format_error& ex)
    {
        std::println("{}", ex.what());
    }
}

Possible output: 

format error: failed to parse format-spec

See also