W3cubDocs

/C++

std::visit

Defined in header `<variant>`
template <class Visitor, class... Variants> constexpr /see below/ visit(Visitor&& vis, Variants&&... vars);		(since C++17)

Applies the visitor vis to the variants vars.

Effectively returns.

std::invoke(std::forward<Visitor>(vis), std::get<is>(std::forward<Variants>(vars))...).

, where is... is vars.index()....

The call is ill-formed if the invocation above is not a valid expression of the same type and value category, for all combinations of alternative types of all variants.

Parameters

vis	-	a `Callable` that accepts every possible alternative from every variant
vars	-	list of variants to pass to the visitor

Return value

The value returned by the selected invocation of the visitor, converted to the common type of all possible std::invoke expressions.

Exceptions

Throws std::bad_variant_access if any variant in vars is valueless_by_exception.

Complexity

When the number of variants is zero or one, the invocation of the callable object is implemented in constant time, i.e. it does not depend on sizeof...(Types).

If the number of variants is larger than 1, the invocation of the callable object has no complexity requirements.

Example

#include <variant>
#include <iostream>
#include <type_traits>
#include <iomanip>
#include <vector>
 
 
template<class T> struct always_false : std::false_type {};
 
using var_t = std::variant<int, long, double, std::string>;
 
int main() {
    std::vector<var_t> vec = {10, 15l, 1.5, "hello"};
    for(auto v: vec) {
        // void visitor, only called for side-effects
        std::visit([](auto&& arg){std::cout << arg;}, v);
 
        // value-returning visitor. A common idiom is to return another variant
        var_t w = std::visit([](auto&& arg) -> var_t {return arg + arg;}, v);
 
        std::cout << ". After doubling, variant holds ";
        // type-matching visitor: can also be a class with 4 overloaded operator()'s
        std::visit([](auto&& arg) {
            using T = std::remove_cv_t<std::remove_reference_t<decltype(arg)>>;
            if constexpr (std::is_same_v<T, int>)
                std::cout << "int with value " << arg << '\n';
            else if constexpr (std::is_same_v<T, long>)
                std::cout << "long with value " << arg << '\n';
            else if constexpr (std::is_same_v<T, double>)
                std::cout << "double with value " << arg << '\n';
            else if constexpr (std::is_same_v<T, std::string>)
                std::cout << "std::string with value " << std::quoted(arg) << '\n';
            else 
                static_assert(always_false<T>::value, "non-exhaustive visitor!");
        }, w);
    }
}

Output: