Difference between revisions of "cpp/utility/variant/emplace"
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template <class T, class U, class... Args> | template <class T, class U, class... Args> | ||
| − | T& emplace(std::initializer_list<U> il, Args&&... args); | + | T& emplace( std::initializer_list<U> il, Args&&... args ); |
}} | }} | ||
{{dcl | num=3 | since=c++17|1= | {{dcl | num=3 | since=c++17|1= | ||
template <size_t I, class... Args> | template <size_t I, class... Args> | ||
| − | std::variant_alternative_t<I, variant>& emplace(Args&&... args); | + | std::variant_alternative_t<I, variant>& emplace( Args&&... args ); |
}} | }} | ||
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template <size_t I, class U, class... Args> | template <size_t I, class U, class... Args> | ||
| − | std::variant_alternative_t<I, variant>& emplace(std::initializer_list<U> il, Args&&... args); | + | std::variant_alternative_t<I, variant>& |
| + | emplace( std::initializer_list<U> il, Args&&... args ); | ||
}} | }} | ||
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Revision as of 15:01, 29 October 2019
| template <class T, class... Args> T& emplace(Args&&... args); |
(1) | (since C++17) |
| template <class T, class U, class... Args> T& emplace( std::initializer_list<U> il, Args&&... args ); |
(2) | (since C++17) |
| template <size_t I, class... Args> std::variant_alternative_t<I, variant>& emplace( Args&&... args ); |
(3) | (since C++17) |
| template <size_t I, class U, class... Args> std::variant_alternative_t<I, variant>& |
(4) | (since C++17) |
Creates a new value in-place, in an existing variant object
1) Equivalent to emplace<I>(std::forward<Args>(args)...), where
I is the zero-based index of T in Types.... This overload participates in overload resolution only if std::is_constructible_v<T, Args...> is true, and T occurs exactly once in Types...2) Equivalent to emplace<I>(il, std::forward<Args>(args)...), where
I is the zero-based index of T in Types.... This overload participates in overload resolution only if std::is_constructible_v<T, std::initializer_list<U>&, Args...> is true, and T occurs exactly once in Types...3) First, destroys the currently contained value (if any). Then direct-initializes the contained value as if constructing a value of type
T_I with the arguments std::forward<Args>(args).... If an exception is thrown, *this may become valueless_by_exception.
This overload participates in overload resolution only if std::is_constructible_v<T_I, Args...> is true. The behavior is undefined if I is not less than sizeof...(Types).4) First, destroys the currently contained value (if any). Then direct-initializes the contained value as if constructing a value of type
T_I with the arguments il, std::forward<Args>(args).... If an exception is thrown, *this may become valueless_by_exception.
This overload participates in overload resolution only if std::is_constructible_v<T_I, initializer_list<U>&, Args...> is true. The behavior is undefined if I is not less than sizeof...(Types).Contents |
Parameters
| args | - | constructor arguments to use when constructing the new value |
| il | - | initializer_list argument to use when constructing the new value |
Return value
A reference to the new contained value.
Exceptions
1-4) Any exception thrown during the initialization of the contained value.
Example
Run this code
#include <iostream> #include <string> #include <variant> int main() { std::variant<std::string> v1; v1.emplace<0>("abc"); // OK std::cout << std::get<0>(v1) << '\n'; v1.emplace<std::string>("def"); // OK std::cout << std::get<0>(v1) << '\n'; std::variant<std::string, std::string> v2; v2.emplace<1>("ghi"); // OK std::cout << std::get<1>(v2) << '\n'; // v2.emplace<std::string>("abc"); -> Error }
Output:
abc def ghi
See also
assigns a variant (public member function) |
