std::variant<Types...>::visit
From cppreference.com
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 <print>
#include <string>
#include <string_view>
#include <variant>
struct Base {};
struct Derived : Base {};
// helper type for the visitor
template<class... Ts>
struct overloads : Ts... { using Ts::operator()...; };
// the variant to visit
using var_t = std::variant<int, std::string, Derived>;
int main()
{
const auto visitor = overloads
{
[](int i){ std::print("int = {}\n", i); },
[](std::string_view s){ std::println("string = “{}”", s); },
[](const Base&){ std::println("base"); }
};
const var_t var1 = 42, var2 = "abc", var3 = Derived();
#if (__cpp_lib_variant >= 202306L)
var1.visit(visitor);
var2.visit(visitor);
var3.visit(visitor);
#else
std::visit(visitor, var1);
std::visit(visitor, var2);
std::visit(visitor, var3);
#endif
}
Output:
int = 42
string = “abc”
base
See also
(C++17) |
calls the provided functor with the arguments held by one or more variants (function template) |