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std::optional<T>::emplace

From cppreference.com
< cpp‎ | utility‎ | optional
 
 
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template< class... Args >
T& emplace( Args&&... args );
(1) (since C++17)
(constexpr since C++20)
template< class U, class... Args >
T& emplace( std::initializer_list<U> ilist, Args&&... args );
(2) (since C++17)
(constexpr since C++20)

Constructs the contained value in-place. If *this already contains a value before the call, the contained value is destroyed by calling its destructor.

1) Initializes the contained value by direct-initializing (but not direct-list-initializing) with std::forward<Args>(args)... as parameters.
2) Initializes the contained value by calling its constructor with ilist, std::forward<Args>(args)... as parameters. This overload participates in overload resolution only if std::is_constructible<T, std::initializer_list<U>&, Args&&...>::value is true.

Contents

[edit] Parameters

args... - the arguments to pass to the constructor
ilist - the initializer list to pass to the constructor
Type requirements
-
T must be constructible from Args... for overload (1)
-
T must be constructible from std::initializer_list and Args... for overload (2)

[edit] Return value

A reference to the new contained value.

[edit] Exceptions

Any exception thrown by the selected constructor of T. If an exception is thrown, *this does not contain a value after this call (the previously contained value, if any, had been destroyed).

Feature-test macro Value Std Feature
__cpp_lib_optional 202106L (C++20)
(DR20)
Fully constexpr (1,2)

[edit] Example

#include <iostream>
#include <optional>
 
struct A
{
    std::string s;
 
    A(std::string str) : s(std::move(str)), id{n++} { note("+ constructed"); }
    ~A() { note("~ destructed"); }
    A(const A& o) : s(o.s), id{n++} { note("+ copy constructed"); }
    A(A&& o) : s(std::move(o.s)), id{n++} { note("+ move constructed"); }
 
    A& operator=(const A& other)
    {
        s = other.s;
        note("= copy assigned");
        return *this;
    }
 
    A& operator=(A&& other)
    {
        s = std::move(other.s);
        note("= move assigned");
        return *this;
    }
 
    inline static int n{};
    int id{};
    void note(auto s) { std::cout << "  " << s << " #" << id << '\n'; }
};
 
int main()
{
    std::optional<A> opt;
 
    std::cout << "Assign:\n";
    opt = A("Lorem ipsum dolor sit amet, consectetur adipiscing elit nec.");
 
    std::cout << "Emplace:\n";
    // As opt contains a value it will also destroy that value
    opt.emplace("Lorem ipsum dolor sit amet, consectetur efficitur.");
 
    std::cout << "End example\n";
}

Output:

Assign:
  + constructed #0
  + move constructed #1
  ~ destructed #0
Emplace:
  ~ destructed #1
  + constructed #2
End example
  ~ destructed #2

[edit] Defect reports

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

DR Applied to Behavior as published Correct behavior
P2231R1 C++20 emplace was not constexpr while the required operations can be constexpr in C++20 made constexpr

[edit] See also

assigns contents
(public member function) [edit]