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std:: bad_variant_access

From cppreference.net
C++
Utilities library
std::variant
Defined in header <variant>
class bad_variant_access : public std:: exception
(since C++17)

std::bad_variant_access is the type of the exception thrown in the following situations:

(since C++26)

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

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

(since C++26)

Contents

Member functions

(constructor)
constructs a new bad_variant_access object
(public member function)
operator=
replaces the bad_variant_access object
(public member function)
what
returns the explanatory string
(public member function)

std::bad_variant_access:: bad_variant_access

bad_variant_access ( ) noexcept ;
(1) (since C++17)
(constexpr since C++26)
bad_variant_access ( const bad_variant_access & other ) noexcept ;
(2) (since C++17)
(constexpr since C++26)

Constructs a new bad_variant_access object with an implementation-defined null-terminated byte string which is accessible through what() .

1) Default constructor.
2) Copy constructor. If * this and other both have dynamic type std::bad_variant_access then std:: strcmp ( what ( ) , other. what ( ) ) == 0 .

Parameters

other - another exception object to copy

std::bad_variant_access:: operator=

bad_variant_access & operator = ( const bad_variant_access & other ) noexcept ;
(since C++17)
(constexpr since C++26)

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

Parameters

other - another exception object to assign with

Return value

* this

std::bad_variant_access:: what

virtual const char * what ( ) const noexcept ;
(since C++17)
(constexpr since C++26)

Returns the explanatory string.

Return value

Pointer to an implementation-defined null-terminated string with explanatory information. The string is suitable for conversion and display as a std::wstring . The pointer is guaranteed to be valid at least until the exception object from which it is obtained is destroyed, or until a non-const member function (e.g. copy assignment operator) on the exception object is called.

The returned string is encoded with the ordinary literal encoding during constant evaluation.

(since C++26)

Notes

Implementations are allowed but not required to override what() .

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 std::bad_variant_access

Example

Run this code 
#include <iostream>
#include <variant>
int main()
{
    std::variant<int, float> v;
    v = 12;
    try
    {
        std::get<float>(v);
    }
    catch (const std::bad_variant_access& e)
    {
        std::cout << e.what() << '\n';
    }
}

Possible output: 

bad_variant_access

See also

(C++17)
reads the value of the variant given the index or the type (if the type is unique), throws on error
(function template)
(C++17)
calls the provided functor with the arguments held by one or more variant s
(function template)
(C++26)
calls the provided functor with the argument held by the variant
(public member function)
(C++17)
exception indicating checked access to an optional that doesn't contain a value
(class)
(C++23)
exception indicating checked access to an expected that contains an unexpected value
(class template)