std::construct_at

 

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未初始化内存算法 uninitialized_copy uninitialized_fill uninitialized_move (C++17) uninitialized_default_construct (C++17) uninitialized_value_construct (C++17) destroy (C++17) destroy_at (C++17) uninitialized_copy_n (C++11) uninitialized_fill_n uninitialized_move_n (C++17) uninitialized_default_construct_n (C++17) uninitialized_value_construct_n (C++17) destroy_n (C++17) construct_at (C++20) 受约束的未初始化内存算法 ranges::uninitialized_copy (C++20) ranges::uninitialized_fill (C++20) ranges::uninitialized_move (C++20) ranges::uninitialized_default_construct (C++20) ranges::uninitialized_value_construct (C++20) ranges::destroy (C++20) ranges::destroy_at (C++20) ranges::uninitialized_copy_n (C++20) ranges::uninitialized_fill_n (C++20) ranges::uninitialized_move_n (C++20) ranges::uninitialized_default_construct_n (C++20) ranges::uninitialized_value_construct_n (C++20) ranges::destroy_n (C++20) ranges::construct_at (C++20) 垃圾收集支持 declare_reachable (C++11)(C++23 前) undeclare_reachable (C++11)(C++23 前) declare_no_pointers (C++11)(C++23 前) undeclare_no_pointers (C++11)(C++23 前) pointer_safety (C++11)(C++23 前) get_pointer_safety (C++11)(C++23 前) 杂项 pointer_traits (C++11) to_address (C++20) addressof (C++11) align (C++11) assume_aligned (C++20) C 库 malloc aligned_alloc (C++17) calloc realloc free 低层内存管理

 

定义于头文件 <memory>
template<class T, class... Args> constexpr T* construct_at( T* p, Args&&... args );		(C++20 起)

在给定地址 p 创建以参数 args... 初始化的 T 对象。此函数模板的特化仅若 ::new(std::declval<void*>()) T(std::declval<Args>()...) 在不求值语境中为良构才参与重载决议。

等价于

return ::new (const_cast<void*>(static_cast<const volatile void*>(p)))
    T(std::forward<Args>(args)...);

除了 construct_at 可用于常量表达式的求值。

在某常量表达式 e 的求值中调用 construct_at 时，参数 p 必须指向用 std::allocator<T>::allocate 获得的存储或生存期始于 e 的求值内的对象。

参数

p	-	指向将在其上构造 T 对象的未初始化存储的指针
args...	-	用于初始化的参数

返回值

p

示例

运行此代码

#include <iostream>
#include <memory>
 
struct S {
    int x;
    float y;
    double z;
 
    S(int x, float y, double z) : x{x}, y{y}, z{z} { std::cout << "S::S();\n"; }
 
    ~S() { std::cout << "S::~S();\n"; }
 
    void print() const {
        std::cout << "S { x=" << x << "; y=" << y << "; z=" << z << "; };\n";
    }
};
 
int main()
{
    alignas(S) unsigned char storage[sizeof(S)];
 
    S* ptr = std::construct_at(reinterpret_cast<S*>(storage), 42, 2.71828f, 3.1415);
    ptr->print();
 
    std::destroy_at(ptr);
}

输出：

S::S();
S { x=42; y=2.71828; z=3.1415; };
S::~S();

参阅

allocate	分配未初始化的存储 (std::allocator<T> 的公开成员函数)
construct [静态]	在分配的存储构造对象 (函数模板)
destroy_at (C++17)	销毁在给定地址的对象 (函数模板)
ranges::construct_at (C++20)	在给定地址创建对象 (niebloid)