stl_construct.h
Go to the documentation of this file.
// nonstandard construct and destroy functions -*- C++ -*-
// Copyright (C) 2001-2026 Free Software Foundation, Inc.
//
// This file is part of the GNU ISO C++ Library. This library is free
// software; you can redistribute it and/or modify it under the
// terms of the GNU General Public License as published by the
// Free Software Foundation; either version 3, or (at your option)
// any later version.
// This library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
// GNU General Public License for more details.
// Under Section 7 of GPL version 3, you are granted additional
// permissions described in the GCC Runtime Library Exception, version
// 3.1, as published by the Free Software Foundation.
// You should have received a copy of the GNU General Public License and
// a copy of the GCC Runtime Library Exception along with this program;
// see the files COPYING3 and COPYING.RUNTIME respectively. If not, see
// <http://www.gnu.org/licenses/>.
/*
*
* Copyright (c) 1994
* Hewlett-Packard Company
*
* Permission to use, copy, modify, distribute and sell this software
* and its documentation for any purpose is hereby granted without fee,
* provided that the above copyright notice appear in all copies and
* that both that copyright notice and this permission notice appear
* in supporting documentation. Hewlett-Packard Company makes no
* representations about the suitability of this software for any
* purpose. It is provided "as is" without express or implied warranty.
*
*
* Copyright (c) 1996,1997
* Silicon Graphics Computer Systems, Inc.
*
* Permission to use, copy, modify, distribute and sell this software
* and its documentation for any purpose is hereby granted without fee,
* provided that the above copyright notice appear in all copies and
* that both that copyright notice and this permission notice appear
* in supporting documentation. Silicon Graphics makes no
* representations about the suitability of this software for any
* purpose. It is provided "as is" without express or implied warranty.
*/
/** @file bits/stl_construct.h
* This is an internal header file, included by other library headers.
* Do not attempt to use it directly. @headername{memory}
*/
#ifndef _STL_CONSTRUCT_H
#define _STL_CONSTRUCT_H 1
#include <new>
#include <bits/move.h>
#include <bits/stl_iterator_base_types.h> // for iterator_traits
#include <bits/stl_iterator_base_funcs.h> // for advance
/* This file provides the C++17 functions std::destroy_at, std::destroy, and
* std::destroy_n, and the C++20 function std::construct_at.
* It also provides std::_Construct, std::_Destroy,and std::_Destroy_n functions
* which are defined in all standard modes and so can be used in C++98-14 code.
* The _Destroy functions will dispatch to destroy_at during constant
* evaluation, because calls to that function are intercepted by the compiler
* to allow use in constant expressions.
*/
namespace std _GLIBCXX_VISIBILITY(default)
{
_GLIBCXX_BEGIN_NAMESPACE_VERSION
#if __glibcxx_raw_memory_algorithms // >= C++17
template <typename _Tp>
_GLIBCXX20_CONSTEXPR inline void
destroy_at(_Tp* __location)
{
if constexpr (__cplusplus > 201703L && is_array_v<_Tp>)
{
for (auto& __x : *__location)
std::destroy_at(std::addressof(__x));
}
else
__location->~_Tp();
}
#if __cpp_constexpr_dynamic_alloc // >= C++20
template<typename _Tp, typename... _Args>
requires (!is_unbounded_array_v<_Tp>)
&& requires { ::new((void*)0) _Tp(std::declval<_Args>()...); }
constexpr _Tp*
construct_at(_Tp* __location, _Args&&... __args)
noexcept(noexcept(::new((void*)0) _Tp(std::declval<_Args>()...)))
{
void* __loc = __location;
// _GLIBCXX_RESOLVE_LIB_DEFECTS
// 3436. std::construct_at should support arrays
if constexpr (is_array_v<_Tp>)
{
static_assert(sizeof...(_Args) == 0, "std::construct_at for array "
"types must not use any arguments to initialize the "
"array");
return ::new(__loc) _Tp[1]();
}
else
return ::new(__loc) _Tp(std::forward<_Args>(__args)...);
}
#endif // C++20
#endif// C++17
/**
* Constructs an object in existing memory by invoking an allocated
* object's constructor with an initializer.
*/
#if __cplusplus >= 201103L
template<typename _Tp, typename... _Args>
_GLIBCXX20_CONSTEXPR
inline void
_Construct(_Tp* __p, _Args&&... __args)
{
#if __cpp_constexpr_dynamic_alloc // >= C++20
if (std::is_constant_evaluated())
{
// Allow std::_Construct to be used in constant expressions.
std::construct_at(__p, std::forward<_Args>(__args)...);
return;
}
#endif
::new(static_cast<void*>(__p)) _Tp(std::forward<_Args>(__args)...);
}
#else
template<typename _T1, typename _T2>
inline void
_Construct(_T1* __p, const _T2& __value)
{
// _GLIBCXX_RESOLVE_LIB_DEFECTS
// 402. wrong new expression in [some_]allocator::construct
::new(static_cast<void*>(__p)) _T1(__value);
}
#endif
template<typename _T1>
_GLIBCXX26_CONSTEXPR
inline void
_Construct_novalue(_T1* __p)
{ ::new(static_cast<void*>(__p)) _T1; }
template<typename _ForwardIterator>
_GLIBCXX20_CONSTEXPR void
_Destroy(_ForwardIterator __first, _ForwardIterator __last);
/**
* Destroy the object pointed to by a pointer type.
*/
template<typename _Tp>
_GLIBCXX14_CONSTEXPR inline void
_Destroy(_Tp* __pointer)
{
#if __cpp_constexpr_dynamic_alloc // >= C++20
std::destroy_at(__pointer);
#else
__pointer->~_Tp();
#endif
}
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wc++17-extensions" // for if-constexpr
#if __cplusplus < 201103L
template<bool>
struct _Destroy_aux
{
template<typename _ForwardIterator>
static _GLIBCXX20_CONSTEXPR void
__destroy(_ForwardIterator __first, _ForwardIterator __last)
{
for (; __first != __last; ++__first)
std::_Destroy(std::__addressof(*__first));
}
template<typename _ForwardIterator, typename _Size>
static _GLIBCXX20_CONSTEXPR _ForwardIterator
__destroy_n(_ForwardIterator __first, _Size __count)
{
for (; __count > 0; (void)++__first, --__count)
std::_Destroy(std::__addressof(*__first));
return __first;
}
};
template<>
struct _Destroy_aux<true>
{
template<typename _ForwardIterator>
static void
__destroy(_ForwardIterator, _ForwardIterator) { }
template<typename _ForwardIterator, typename _Size>
static _ForwardIterator
__destroy_n(_ForwardIterator __first, _Size __count)
{
std::advance(__first, __count);
return __first;
}
};
#endif
/**
* Destroy a range of objects. If the value_type of the object has
* a trivial destructor, the compiler should optimize all of this
* away, otherwise the objects' destructors must be invoked.
*/
template<typename _ForwardIterator>
_GLIBCXX20_CONSTEXPR inline void
_Destroy(_ForwardIterator __first, _ForwardIterator __last)
{
typedef typename iterator_traits<_ForwardIterator>::value_type
_Value_type;
#if __cplusplus >= 201103L
if constexpr (!is_trivially_destructible<_Value_type>::value)
for (; __first != __last; ++__first)
std::_Destroy(std::addressof(*__first));
#if __cpp_constexpr_dynamic_alloc // >= C++20
else if (std::is_constant_evaluated())
for (; __first != __last; ++__first)
std::destroy_at(std::addressof(*__first));
#endif
#else
std::_Destroy_aux<__has_trivial_destructor(_Value_type)>::
__destroy(__first, __last);
#endif
}
/**
* Destroy a range of objects. If the value_type of the object has
* a trivial destructor, the compiler should optimize all of this
* away, otherwise the objects' destructors must be invoked.
*/
template<typename _ForwardIterator, typename _Size>
_GLIBCXX20_CONSTEXPR inline _ForwardIterator
_Destroy_n(_ForwardIterator __first, _Size __count)
{
typedef typename iterator_traits<_ForwardIterator>::value_type
_Value_type;
#if __cplusplus >= 201103L
if constexpr (!is_trivially_destructible<_Value_type>::value)
for (; __count > 0; (void)++__first, --__count)
std::_Destroy(std::addressof(*__first));
#if __cpp_constexpr_dynamic_alloc // >= C++20
else if (std::is_constant_evaluated())
for (; __count > 0; (void)++__first, --__count)
std::destroy_at(std::addressof(*__first));
#endif
else
std::advance(__first, __count);
return __first;
#else
return std::_Destroy_aux<__has_trivial_destructor(_Value_type)>::
__destroy_n(__first, __count);
#endif
}
#pragma GCC diagnostic pop
#if __glibcxx_raw_memory_algorithms // >= C++17
template <typename _ForwardIterator>
_GLIBCXX20_CONSTEXPR inline void
destroy(_ForwardIterator __first, _ForwardIterator __last)
{
std::_Destroy(__first, __last);
}
template <typename _ForwardIterator, typename _Size>
_GLIBCXX20_CONSTEXPR inline _ForwardIterator
destroy_n(_ForwardIterator __first, _Size __count)
{
return std::_Destroy_n(__first, __count);
}
#endif // C++17
#if __glibcxx_start_lifetime_as >= 202207L // C++ >= 23
template<typename _Tp>
[[__gnu__::__always_inline__]]
inline _Tp*
start_lifetime_as(void* __p) noexcept
{
#if __glibcxx_is_implicit_lifetime >= 202302L
static_assert(is_implicit_lifetime_v<_Tp>);
#endif
auto __q = reinterpret_cast<_Tp*>(__p);
__asm__ __volatile__("" : "=g" (__q), "=m" (*__q)
: "0" (__q), "m" (*__q));
return __q;
}
template<typename _Tp>
[[__gnu__::__always_inline__]]
inline const _Tp*
start_lifetime_as(const void* __p) noexcept
{
#if __glibcxx_is_implicit_lifetime >= 202302L
static_assert(is_implicit_lifetime_v<_Tp>);
#endif
auto __q = reinterpret_cast<const _Tp*>(__p);
auto __r = reinterpret_cast<_Tp*>(const_cast<void*>(__p));
__asm__ __volatile__("" : "=g" (__q), "=m" (*__r)
: "0" (__q), "m" (*__q));
return __q;
}
template<typename _Tp>
[[__gnu__::__always_inline__]]
inline volatile _Tp*
start_lifetime_as(volatile void* __p) noexcept
{
#if __glibcxx_is_implicit_lifetime >= 202302L
static_assert(is_implicit_lifetime_v<_Tp>);
#endif
auto __q = reinterpret_cast<volatile _Tp*>(__p);
auto __r = reinterpret_cast<_Tp*>(const_cast<void*>(__p));
__asm__ __volatile__("" : "=g" (__q), "=m" (*__r)
: "0" (__q), "m" (*__q));
return __q;
}
template<typename _Tp>
[[__gnu__::__always_inline__]]
inline const volatile _Tp*
start_lifetime_as(const volatile void* __p) noexcept
{
#if __glibcxx_is_implicit_lifetime >= 202302L
static_assert(is_implicit_lifetime_v<_Tp>);
#endif
auto __q = reinterpret_cast<const volatile _Tp*>(__p);
auto __r = reinterpret_cast<_Tp*>(const_cast<void*>(__p));
__asm__ __volatile__("" : "=g" (__q), "=m" (*__r)
: "0" (__q), "m" (*__q));
return __q;
}
template<typename _Tp>
[[__gnu__::__always_inline__]]
inline _Tp*
start_lifetime_as_array(void* __p, size_t __n) noexcept
{
auto __q = reinterpret_cast<_Tp*>(__p);
if (!__n)
return __q;
auto __r = (__extension__ reinterpret_cast<_Tp(*)[__n]>(__p));
__asm__ __volatile__("" : "=g" (__q), "=m" (*__r)
: "0" (__q), "m" (*__r));
return __q;
}
template<typename _Tp>
[[__gnu__::__always_inline__]]
inline const _Tp*
start_lifetime_as_array(const void* __p, size_t __n) noexcept
{
auto __q = reinterpret_cast<const _Tp*>(__p);
if (!__n)
return __q;
auto __r = (__extension__ reinterpret_cast<const _Tp(*)[__n]>(__p));
auto __s = (__extension__
reinterpret_cast<_Tp(*)[__n]>(const_cast<void*>(__p)));
__asm__ __volatile__("" : "=g" (__q), "=m" (*__s)
: "0" (__q), "m" (*__r));
return __q;
}
template<typename _Tp>
[[__gnu__::__always_inline__]]
inline volatile _Tp*
start_lifetime_as_array(volatile void* __p, size_t __n) noexcept
{
auto __q = reinterpret_cast<volatile _Tp*>(__p);
if (!__n)
return __q;
auto __r = (__extension__ reinterpret_cast<volatile _Tp(*)[__n]>(__p));
auto __s = (__extension__
reinterpret_cast<_Tp(*)[__n]>(const_cast<void*>(__p)));
__asm__ __volatile__("" : "=g" (__q), "=m" (*__s)
: "0" (__q), "m" (*__r));
return __q;
}
template<typename _Tp>
[[__gnu__::__always_inline__]]
inline const volatile _Tp*
start_lifetime_as_array(const volatile void* __p, size_t __n) noexcept
{
auto __q = reinterpret_cast<const volatile _Tp*>(__p);
if (!__n)
return __q;
auto __r = (__extension__ reinterpret_cast<const volatile _Tp(*)[__n]>(__p));
auto __s = (__extension__
reinterpret_cast<_Tp(*)[__n]>(const_cast<void*>(__p)));
__asm__ __volatile__("" : "=g" (__q), "=m" (*__s)
: "0" (__q), "m" (*__r));
return __q;
}
#endif // C++23
_GLIBCXX_END_NAMESPACE_VERSION
} // namespace std
#endif /* _STL_CONSTRUCT_H */
new
move.h
stl_iterator_base_types.h
stl_iterator_base_funcs.h
std::declval
auto declval() noexcept -> decltype(__declval< _Tp >(0))
Definition type_traits:2714
std::addressof
constexpr _Tp * addressof(_Tp &__r) noexcept
Returns the actual address of the object or function referenced by r, even in the presence of an over...
Definition move.h:176
std::__addressof
constexpr _Tp * __addressof(_Tp &__r) noexcept
Same as C++11 std::addressof.
Definition move.h:52
std::forward
constexpr _Tp && forward(typename std::remove_reference< _Tp >::type &__t) noexcept
Forward an lvalue.
Definition move.h:72
std
ISO C++ entities toplevel namespace is std.
std::_Destroy_n
constexpr _ForwardIterator _Destroy_n(_ForwardIterator __first, _Size __count)
Definition stl_construct.h:245
std::_Construct
constexpr void _Construct(_Tp *__p, _Args &&... __args)
Definition stl_construct.h:123
std::advance
constexpr void advance(_InputIterator &__i, _Distance __n)
A generalization of pointer arithmetic.
Definition stl_iterator_base_funcs.h:262
std::_Destroy
constexpr void _Destroy(_ForwardIterator __first, _ForwardIterator __last)
Definition stl_construct.h:219
std::is_trivially_destructible
is_trivially_destructible
Definition type_traits:1539
std::iterator_traits
Traits class for iterators.
Definition stl_iterator_base_types.h:180