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std::scoped...::construct(3) C++ Standard Libary std::scoped...::construct(3) NAME std::scoped_allocator_adaptor::construct - std::scoped_allocator_adap- tor::construct Synopsis Defined in header <scoped_allocator> template< class T, class... Args > (1) void construct( T* p, Args&&... args ); template< class T1, class T2, class... Args1, class... Args2 > void construct( std::pair<T1, T2>* p, std::piecewise_construct_t, (2) (until C++20) std::tuple<Args1...> x, std::tuple<Args2...> y ); template< class T1, class T2 > (3) (until C++20) void construct( std::pair<T1, T2>* p ); template< class T1, class T2, class U, class V > (4) (until C++20) void construct( std::pair<T1, T2>* p, U&& x, V&& y ); template< class T1, class T2, class U, class V > (5) (until C++20) void construct( std::pair<T1, T2>* p, const std::pair<U, V>& xy ); template< class T1, class T2, class U, class V > (6) (until C++20) void construct( std::pair<T1, T2>* p, std::pair<U, V>&& xy ); template< class T1, class T2, class NonPair > (7) (until C++20) void construct( std::pair<T1, T2>* p, NonPair&& non_pair ); Constructs an object in allocated, but not initialized storage pointed to by p using OuterAllocator and the provided constructor arguments. If the object is of type that itself uses allocators, or if it is std::pair, passes InnerAllocator down to the constructed object. First, retrieve the outermost allocator OUTERMOST by calling this->outer_allocator(), and then calling the outer_allocator() mem- ber function recursively on the result of this call until reaching an allocator that has no such member function. Define OUTERMOST_ALLOC_TRAITS(x) as std::allocator_traits<std::remove_reference_t<decltype(OUTER- MOST(x))>> 1) Creates an object of the given type T by means of uses-allocator construction at the uninitialized memory location indicated by p, using OUTERMOST as the allocator. After adjustment for uses-allocator convention expected by T's con- structor, calls OUTERMOST_ALLOC_TRAITS(*this)::construct. This overload participates in overload resolution only if U is not a (until C++20) specialization of std::pair. Equivalent to std::apply( [p, this](auto&&... newargs) { OUTERMOST_ALLOC_TRAITS(*this)::construct( OUTERMOST(*this), p, (since C++20) std::forward<decltype(newargs)>(newargs)...); }, std::uses_allocator_construction_args( inner_allocator(), std::forward<Args>(args)... ) ); 2) First, if either T1 or T2 is allocator-aware, modifies the tuples x and y to include the appropriate inner allocator, 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, inner_allocator_type>::value == false), then xprime is std::tuple<Args1&&...>(std::move(x)). (It is also required that std::is_constructible<T1, Args1...>::value == true). 2b) if T1 is allocator-aware (std::uses_allocator<T1, inner_allocator_type>::value == true), and its constructor takes an allocator tag std::is_constructible<T1, std::allocator_arg_t, inner_allocator_type&, Args1...>::value == true, then xprime is std::tuple_cat(std::tuple<std::allocator_arg_t, inner_alloca- tor_type&>( std::allocator_arg, inner_allocator() ), std::tuple<Args1&&...>(std::move(x))) 2c) if T1 is allocator-aware (std::uses_allocator<T1, inner_allocator_type>::value == true), and its constructor takes the allocator as the last argument std::is_constructible<T1, Args1..., inner_allocator_type&>::value == true, then xprime is std::tuple_cat(std::tuple<Args1&&...>(std::move(x)), std::tuple<inner_allocator_type&>(inner_alloca- tor())). Same rules apply to T2 and the replacement of y with yprime. Once xprime and yprime are constructed, constructs the pair p in al- located storage by calling (until std::allocator_traits<O>::construct(OUTERMOST, C++20) p, std::piecewise_construct, std::move(xprime), std::move(yprime)); 3) Equivalent to construct(p, std::piecewise_construct, std::tuple<>(), std::tu- ple<>()), that is, passes the inner allocator 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))) 5) Equivalent to construct(p, std::piecewise_construct, std::forward_as_tu- ple(xy.first), std::forward_as_tuple(xy.sec- ond)) 6) Equivalent to construct(p, std::piecewise_construct, std::forward_as_tuple(std::forward<U>(xy.first)), std::forward_as_tuple(std::forward<V>(xy.second))) 7) This overload participates in overload resolution only if given the exposition-only function template template<class A, class B> void /*deduce-as-pair*/(const std::pair<A, B>&);, /*deduce-as-pair*/(non_pair) is ill-formed when considered as an un- evaluated operand. Equivalent to construct<T1, T2, T1, T2>(p, std::forward<Non- Pair>(non_pair));. Parameters 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 non_pair - non-pair argument to convert to pair for further con- struction Return value (none) Notes This function is called (through std::allocator_traits) by any allo- cator-aware object, such as std::vector, that was given a std::scoped_alloca- tor_adaptor as the allocator to use. Since inner_allocator is itself an instance of std::scoped_allocator_adaptor, this function will also be called when the allocator-aware objects constructed through this function start con- structing their own members. Defect reports The following behavior-changing defect reports were applied retroac- tively to previously published C++ standards. DR Applied to Behavior as published Correct behavior first overload is mistakenly used constrained to not accept LWG 2975 C++11 for pair construction in some pairs cases P0475R1 C++11 pair piecewise construction may transformed to tuples of copy the arguments references to avoid copy LWG 3525 C++11 no overload could handle non-pair reconstructing overload added types convertible to pair See also construct constructs an object in the allocated storage [static] (function template) construct constructs an object in allocated storage (until C++20) (public member function of std::allocator<T>) http://cppreference.com 2024.06.10 std::scoped...::construct(3)
NAME | Synopsis | Parameters | Return value | Notes | See also
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