Home - News ( United States | United Kingdom | Italy | Germany ) - Football scores

Showing content from https://en.cppreference.com/w/cpp/language/../algorithm/../../cpp/utility/variant/visit2.html below:

std::visit - cppreference.com

template< class Visitor, class... Variants >
constexpr /* see below */ visit( Visitor&& v, Variants&&... values );

template< class R, class Visitor, class... Variants >
constexpr R visit( Visitor&& v, Variants&&... values );

Helper templates

Applies the visitor v (a Callable that can be called with any combination of types from Variants) to the Variants values.

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

Invokes

as if by

INVOKE(std::forward<Visitor>(v),
       std::get<indices>(std::forward<VariantBases>(values))...),

where

is

.

Invokes

as if by

INVOKE<R>(std::forward<Visitor>(v),
          std::get<indices>(std::forward<VariantBases>(values))...),

where

is

.

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.

The exposition-only

function templates accept a value whose type can be

for

(i.e., either

or a type derived from

), and return the

value with the same const-qualification and value category.

3,4) Returns value.

5,6) Returns std::move(value).

The result of the

operation. The return type is the type obtained from applying

to the result.

Nothing if

is (possibly cv-qualified)

; otherwise the result of the

operation.

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

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.

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 v can be considered equal to that of access to an element in an (possibly multidimensional) array or execution of a switch statement.

#include <iomanip>
#include <iostream>
#include <string>
#include <type_traits>
#include <variant>
#include <vector>
 
// the variant to visit
using value_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<value_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
        value_t w = std::visit([](auto&& arg) -> value_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"

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

RetroSearch is an open source project built by @garambo | Open a GitHub Issue

Search and Browse the WWW like it's 1997 | Search results from DuckDuckGo

HTML: 3.2 | Encoding: UTF-8 | Version: 0.7.4