NAME¶

std::visit - std::visit

Synopsis¶

Defined in header <variant>
template< class Visitor, class... Variants >
constexpr /* see below */ visit( Visitor&& vis, Variants&&... (1) (since C++17)
vars );
template< class R, class Visitor, class... Variants > (2) (since C++20)
constexpr R visit( Visitor&& vis, Variants&&... vars );
template< class... Ts > (3) (exposition only*)
auto&& as-variant( std::variant<Ts...>& var );
template< class... Ts > (4) (exposition only*)
auto&& as-variant( const std::variant<Ts...>& var );
template< class... Ts > (5) (exposition only*)
auto&& as-variant( std::variant<Ts...>&& var );
template< class... Ts > (6) (exposition only*)
auto&& as-variant( const std::variant<Ts...>&& var );

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).

Parameters¶

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

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.

Exceptions¶

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

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.

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) std::visit for classes derived from std::variant
(DR)

Example¶

// Run this code

#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 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(false, "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"

Defect reports

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

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

See also¶

swap swaps with another variant
(public member function)