Difference between revisions of "cpp/utility/variant/visit"
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hello. After doubling, variant holds std::string with value "hellohello" | hello. After doubling, variant holds std::string with value "hellohello" | ||
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Revision as of 11:17, 13 September 2018
| Defined in header <variant>
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| template <class Visitor, class... Variants> constexpr /*see below*/ visit(Visitor&& vis, Variants&&... vars); |
(since C++17) | |
Applies the visitor vis to the variants vars
Effectively returns
std::invoke(std::forward<Visitor>(vis), std::get<is>(std::forward<Variants>(vars))...)
, where is... is vars.index().... 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.
Contents |
Parameters
| vis | - | a Callable that accepts every possible alternative from every variant |
| vars | - | list of variants to pass to the visitor |
Return value
The value returned by the selected invocation of the visitor.
Exceptions
Throws std::bad_variant_access if any variant in vars is valueless_by_exception.
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.
Example
#include <iomanip> #include <iostream> #include <string> #include <type_traits> #include <variant> #include <vector> template<class T> struct always_false : std::false_type {}; using var_t = std::variant<int, long, double, std::string>; template<class... Ts> struct overloaded : Ts... { using Ts::operator()...; }; template<class... Ts> overloaded(Ts...) -> overloaded<Ts...>; int main() { std::vector<var_t> vec = {10, 15l, 1.5, "hello"}; for(auto& v: vec) { // void visitor, only called for side-effects std::visit([](auto&& arg){std::cout << arg;}, v); // value-returning visitor. A common idiom is to return another variant var_t w = std::visit([](auto&& arg) -> var_t {return arg + arg;}, v); std::cout << ". After doubling, variant holds "; // type-matching visitor: can also be a class with 4 overloaded operator()'s 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<T>::value, "non-exhaustive visitor!"); }, w); } for (auto& v: vec) { 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"
See also
swaps with another variant (public member function) |
