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std::bad_variant_access

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< cpp‎ | utility‎ | variant
 
C++
 
Utilities library
General utilities
Relational operators (deprecated in C++20)
 
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

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

[edit] Notes

Feature-test macro Value Std Feature
__cpp_lib_constexpr_exceptions 202502L (C++26) constexpr std::bad_variant_access

[edit] 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

[edit] 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) [edit]
(C++17)
 calls the provided functor with the arguments held by one or more variants
(function template) [edit]
(C++26)
 calls the provided functor with the argument held by the variant
(public member function) [edit]
(C++17)
 exception indicating checked access to an optional that doesn't contain a value
(class) [edit]
(C++23)
 exception indicating checked access to an expected that contains an unexpected value
(class template) [edit]
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