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std::visit

From cppreference.com
< cpp‎ | utility‎ | variant
 
 
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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)
template< class... Ts >
auto&& as-variant( std::variant<Ts...>& var );
(3) (exposition only*)
template< class... Ts >
auto&& as-variant( const std::variant<Ts...>& var );
(4) (exposition only*)
template< class... Ts >
auto&& as-variant( std::variant<Ts...>&& var );
(5) (exposition only*)
template< class... Ts >
auto&& as-variant( const std::variant<Ts...>&& var );
(6) (exposition only*)

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

Given VariantBases as decltype(as-variant(std::forward<Variants>(vars))... (a pack of sizeof...(Variants) types):

1) Invokes vis as if by

INVOKE(std::forward<Visitor>(vis),
       std::get<indices>(std::forward<VariantBases>(vars))...)
,

where indices is as-variant(vars).index()....
2) Invokes vis as if by

INVOKE<R>(std::forward<Visitor>(vis),
          std::get<indices>(std::forward<VariantBases>(vars))...)
,

where indices is as-variant(vars).index()....

These overloads participate in overload resolution only if every type in VariantBases is a valid type. If the expression denoted by INVOKE or INVOKE<R>(since C++20) is invalid, or the results of INVOKE or INVOKE<R>(since C++20) have different types or value categories for different indices, the program is ill-formed.

3-6) The exposition-only as-variant function templates accept a value whose type can be deduced for std::variant<Ts...> (i.e. either std::variant<Ts...> or a type derived from std::variant<Ts...>), and return the std::variant value with the same const-qualification and value category.
3,4) Returns var.
5,6) Returns std::move(var).

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 result of the INVOKE operation. The return type is the type obtained from applying decltype to the result.
2) Nothing if R is (possibly cv-qualified) void; otherwise the result of the INVOKE<R> operation.
3-6) A std::variant value converted from var.

[edit] Exceptions

Throws std::bad_variant_access if as-variant(vars_i).valueless_by_exception() is true for any variant vars_i in vars.

[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 the number of types can be stored in the variant.

If the number of variants is larger than one, 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.

Feature-test macro Value Std Feature
__cpp_lib_variant 202102L (C++17)
(DR)
std::visit for classes derived from std::variant

[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
        // Note: The `(auto arg)` template operator() will bind to `int` and `long`
        //       in this case, but in its absence the `(double arg)` operator()
        //       *will also* bind to `int` and `long` because both are implicitly
        //       convertible to double. When using this form, care has to be taken
        //       that implicit conversions are handled correctly.
        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] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
LWG 2970 C++17 the return type of overload (1) did not preserve the
value category of the result of the INVOKE operation
preserves
LWG 3052
(P2162R2)
C++17 the effects were unspecified if any type
in Variants is not a std::variant
specified

[edit] See also

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