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std::visit

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Defined in header <variant>
template <class Visitor, class... Variants>
constexpr /*see below*/ visit( Visitor&& vis, Variants&&... vars );
(1) (since C++17)
template <class R, class Visitor, class... Variants>
constexpr R visit( Visitor&& vis, Variants&&... vars );
(2) (since C++20)

Applies the visitor vis (Callable that can be called with any combination of types from variants) to the variants vars.

Every type in std::remove_reference_t<Variants>... may be a (possibly const-qualified) specialization of std::variant. It is unspecified whether other argument types, e.g. a class derived from a std::variant, are supported.

Effectively returns

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

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

(until C++23)

These overloads participate in overload resolution only if every type in std::remove_reference_t<Variants>... is a (possibly const-qualified) specialization of std::variant, or a (possibly const-qualified) class C such that there is exactly one std::variant specialization that is a base class of C and it is a public and unambiguous base class.

Effectively returns

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

, where every type in VariantBases... is the unique std::variant specialization determined above, except that const, &, or && is added to it if the corresponding argument is of a const-qualified type, is an lvalue, or is an rvalue, respectively, and is... is std::forward<VariantBases>(vars).index()....

(since C++23)
1) The return type is deduced from the returned expression as if by decltype. 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.
2) The return type is R. If R is (possibly cv-qualified) void, the result of the invoke expression is discarded.

Contents

[edit] Parameters

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

[edit] Return value

1) The value returned by the selected invocation of the visitor.
2) Nothing if R is (possibly cv-qualified) void; otherwise the value returned by the selected invocation of the visitor, implicitly converted to R.

[edit] Exceptions

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

Whether any variant is valueless by exception is determined as if by (std::forward<VariantBases>(vars).valueless_by_exception() || ...).

(since C++23)

[edit] 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.

[edit] Notes

Let n be (1 * ... * std::variant_size_v<std::remove_reference_t<VariantBases>>), implementations usually generate a table equivalent to an (possibly multidimensional) array of n function pointers for every specialization of std::visit, which is similar to the implementation of virtual functions.

Implementations may also generate a switch statement with n branches for std::visit (e.g. the MSVC STL implementation uses a switch statement when n is not greater than 256).

On typical implementations, the time complexity of the invocation of vis can be considered equal to that of access to an element in an (possibly multidimensional) array or execution of a switch statement.

[edit] Example

#include <iomanip>
#include <iostream>
#include <string>
#include <type_traits>
#include <variant>
#include <vector>
 
// the variant to visit
using var_t = std::variant<int, long, double, std::string>;
 
// helper constant for the visitor #3
template<class> inline constexpr bool always_false_v = false;
 
// helper type for the visitor #4
template<class... Ts> struct overloaded : Ts... { using Ts::operator()...; };
// explicit deduction guide (not needed as of C++20)
template<class... Ts> overloaded(Ts...) -> overloaded<Ts...>;
 
int main() {
    std::vector<var_t> vec = {10, 15l, 1.5, "hello"};
    for(auto& v: vec) {
 
        // 1. void visitor, only called for side-effects (here, for I/O)
        std::visit([](auto&& arg){std::cout << arg;}, v);
 
        // 2. value-returning visitor, demonstrates the idiom of returning another variant
        var_t w = std::visit([](auto&& arg) -> var_t {return arg + arg;}, v);
 
        // 3. type-matching visitor: a lambda that handles each type differently
        std::cout << ". After doubling, variant holds ";
        std::visit([](auto&& arg) {
            using T = std::decay_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_v<T>, "non-exhaustive visitor!");
        }, w);
    }
 
    for (auto& v: vec) {
        // 4. another type-matching visitor: a class with 3 overloaded operator()'s
        std::visit(overloaded {
            [](auto arg) { std::cout << arg << ' '; },
            [](double arg) { std::cout << std::fixed << arg << ' '; },
            [](const std::string& arg) { std::cout << std::quoted(arg) << ' '; },
        }, v);
    }
}

Output:

10. After doubling, variant holds int with value 20
15. After doubling, variant holds long with value 30
1.5. After doubling, variant holds double with value 3
hello. After doubling, variant holds std::string with value "hellohello"
10 15 1.500000 "hello"

[edit] See also

swaps with another variant
(public member function) [edit]