template < class U, class... Args > void construct( U* p, Args&&... args ); | (1) | (since C++17) |

template< class T1, class T2, class... Args1, class... Args2 > void construct( std::pair<T1, T2>* p, std::piecewise_construct_t, std::tuple<Args1...> x, std::tuple<Args2...> y ); | (2) | (since C++17) (until C++20) |

template< class T1, class T2 > void construct( std::pair<T1, T2>* p ); | (3) | (since C++17) (until C++20) |

template< class T1, class T2, class U, class V > void construct( std::pair<T1, T2>* p, U&& x, V&& y ); | (4) | (since C++17) (until C++20) |

template< class T1, class T2, class U, class V > void construct( std::pair<T1, T2>* p, const std::pair<U, V>& xy ); | (5) | (since C++17) (until C++20) |

template< class T1, class T2, class U, class V > void construct( std::pair<T1, T2>* p, std::pair<U, V>&& xy ); | (6) | (since C++17) (until C++20) |

Constructs an object in allocated, but not initialized storage pointed to by `p`

the provided constructor arguments. If the object is of type that itself uses allocators, or if it is std::pair, passes `this->resource()`

down to the constructed object.

1) Creates an object of the given type

`U`

by means of uses-allocator construction at the uninitialized memory location indicated by p, using `*this`

as the allocator. This overload only participates in overload resolution if `U`

is not a specialization of `std::pair`

. (until C++20)
2) First, if either `T1` or `T2` is allocator-aware, modifies the tuples `x` and `y` to include `this->resource()` , resulting in the two new tuples `xprime` and `yprime` , according to the following three rules: 2a) if `T1` is not allocator-aware (`std::uses_allocator<T1, polymorphic_allocator>::value==false` ) and `std::is_constructible<T1, Args1...>::value==true` , then `xprime` is `x` , unmodified. 2b) if `T1` is allocator-aware (`std::uses_allocator<T1, polymorphic_allocator>::value==true` ), and its constructor takes an allocator tag (`std::is_constructible<T1, std::allocator_arg_t, polymorphic_allocator, Args1...>::value==true` , then `xprime` is `std::tuple_cat(std::make_tuple(std::allocator_arg, *this), std::move(x))` 2c) if `T1` is allocator-aware (`std::uses_allocator<T1, polymorphic_allocator>::value==true` ), and its constructor takes the allocator as the last argument (`std::is_constructible<T1, Args1..., polymorphic_allocator>::value==true` ), then `xprime` is `std::tuple_cat(std::move(x), std::make_tuple(*this))` . 2d) Otherwise, the program is ill-formed. Same rules apply to `T2` and the replacement of `y` with `yprime` . Once `xprime` and `yprime` are constructed, constructs the pair `p` in allocated storage as if by `::new((void *) p) pair<T1, T2>(std::piecewise_construct, std::move(xprime), std::move(yprime));` 3) Equivalent to `construct(p, std::piecewise_construct, std::tuple<>(), std::tuple<>())` , that is, passes the memory resource on to the pair's member types if they accept them. 4) Equivalent to construct(p, std::piecewise_construct, std::forward_as_tuple(std::forward<U>(x)), std::forward_as_tuple(std::forward<V>(y))) construct(p, std::piecewise_construct, std::forward_as_tuple(xy.first), std::forward_as_tuple(xy.second)) construct(p, std::piecewise_construct, std::forward_as_tuple(std::forward<U>(xy.first)), std::forward_as_tuple(std::forward<V>(xy.second))) | (until C++20) |

p | - | pointer to allocated, but not initialized storage |

args... | - | the constructor arguments to pass to the constructor of `T` |

x | - | the constructor arguments to pass to the constructor of `T1` |

y | - | the constructor arguments to pass to the constructor of `T2` |

xy | - | the pair whose two members are the constructor arguments for `T1` and `T2` |

(none).

This function is called (through `std::allocator_traits`

) by any allocator-aware object, such as `std::pmr::vector`

(or another `std::vector`

that was given a `std::std::polymorphic_allocator`

as the allocator to use).

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

DR | Applied to | Behavior as published | Correct behavior |
---|---|---|---|

LWG 2969 | C++17 | uses-allocator construction passed `resource()` | passes `*this` |

LWG 2975 | C++17 | first overload is mistakenly used for pair construction in some cases | constrained to not accept pairs |

[static] | constructs an object in the allocated storage (function template) |

(deprecated in C++17)(removed in C++20) | constructs an object in allocated storage (public member function of `std::allocator<T>` ) |

© cppreference.com

Licensed under the Creative Commons Attribution-ShareAlike Unported License v3.0.

http://en.cppreference.com/w/cpp/memory/polymorphic_allocator/construct