std::variant<Types...>:: visit

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C++
Utilities library
General utilities
Relational operators (deprecated in C++20)
std::variant
template < class Self, class Visitor >
constexpr decltype ( auto ) visit ( this Self && self, Visitor && vis ) ;
(1) (since C++26)
template < class R, class Self, class Visitor >
constexpr R visit ( this Self && self, Visitor && vis ) ;
(2) (since C++26)

Applies the visitor vis (a Callable that can be called with any combination of types from the variant) to the variant held by self .

Given type V as decltype ( std:: forward_like < Self > ( std:: declval < variant > ( ) ) ) , the equivalent call is:

1) return std:: visit ( std:: forward < Visitor > ( vis ) , ( V ) self ) ; .
2) return std:: visit < R > ( std:: forward < Visitor > ( vis ) , ( V ) self ) ; .

Parameters

vis - a Callable that accepts every possible alternative from the variant
self - variant to pass to the visitor

Return value

1) The result of the std:: visit invocation.
2) Nothing if R is (possibly cv-qualified) void ; otherwise the result of the std:: visit < R > invocation.

Exceptions

Only throws if the call to std:: visit throws.

Notes

Feature-test macro Value Std Feature
__cpp_lib_variant 202306L (C++26) member visit

Example

Run this code 
#include <iostream>
#include <string>
#include <variant>
 
// helper type for the visitor
template<class... Ts>
struct overloads : Ts... { using Ts::operator()...; };
 
int main()
{
    std::variant<int, std::string> var1{42}, var2{"abc"};
 
    auto use_int = [](int i){ std::cout << "int = " << i << '\n'; };
    auto use_str = [](std::string s){ std::cout << "string = " << s << '\n'; };
 
#if (__cpp_lib_variant >= 202306L)
    var1.visit(overloads{use_int, use_str});
    var2.visit(overloads{use_int, use_str});
#else
    std::visit(overloads{use_int, use_str}, var1);
    std::visit(overloads{use_int, use_str}, var2);
#endif
}

Output: 

int = 42
string = abc
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