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std::allocate_shared

STD：：分配[医]共享

Defined in header ​​
template< class T, class Alloc, class... Args > shared_ptr<T> allocate_shared( const Alloc& alloc, Args&&... args ​(since C++11)

构造类型的对象。T并将其包装在std::shared_ptr使用args的构造函数的参数列表。T对象的构造方式就像由表达式构造一样。::new(pv) T(std::forward<Args>(args)...)，在哪里pv是内部的void*指向适于保存类型对象的存储的指针。T存储空间通常大于sizeof(T)为了对共享指针的控制块和T对象。

所有内存分配都使用alloc，它必须满足Allocator所需经费。

大std::shared_ptr此函数调用的构造函数启用shared_from_this具有指向新构造的类型对象的指针。T...

参数

alloc-The Allocator to use.
args...-list of arguments with which an instance of T will be constructed.

返回值

std::shared_ptr类型实例的T...

例外

引发的异常。Alloc::allocate()的构造函数T如果抛出异常，则此函数没有任何效果。

注记

就像std::make_shared，此函数通常只执行一次分配，并将两个T对象和标准建议的分配内存块%28中的控制块(但不需要此要求)，所有已知实现都执行此%29。一份alloc作为控制块的一部分存储，以便一旦共享和弱引用计数达到零时，就可以使用它来释放它。

与STD：：Shared不同[医]PTR构造函数，STD：：[医]Shared不接受单独的自定义删除器：提供的分配器用于销毁控制块，而T对象，并对它们的共享内存块进行去分配。

std::shared_ptr支持C++17%29的数组类型%28 A，但是std::allocate_shared不会的。支持此功能。助推：：分配[医]共享...

构造者使shared_from_this用指针ptr类型U*意味着它决定U具有明确和可访问的基类，该基类是std::enable_shared_from_this，如果是这样，构造函数将计算语句：

二次

if (ptr != nullptr && ptr->weak_this.expired())
  ptr->weak_this = std::shared_ptr<std::remove_cv_t<U>>(*this,
                                  const_cast<std::remove_cv_t<U>*>(ptr)

二次

何地weak_this是隐藏的可变的std::weak_ptr成员std::shared_from_this.给弱者的任务[医]该成员不是原子成员，并且与对同一对象的任何可能并发访问相冲突。这确保了将来调用shared_from_this()将与shared_ptr由此原始指针构造函数创建。

试验ptr->weak_this.expired()在上面的解说代码中，确保弱[医]如果它已经指示了所有者，则不会重新分配。从C++17开始，就需要进行此测试。

另见

(constructor)constructs new shared_ptr (public member function)
make_sharedcreates a shared pointer that manages a new object (function template)

 © cppreference.com

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http：//en.cppreference.com/w/cpp/Memory/Shared[医]PTR/分配[医]共享

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