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deduction guides for std::weak_ptr

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std::weak_ptr
定义于头文件 <memory>
template < class T >
weak_ptr ( std:: shared_ptr < T > ) - > weak_ptr < T > ;
(C++17 起)

为 std::weak_ptr 提供了一个 推导指引 ，以处理隐式推导指引未覆盖的边缘情况。

示例

运行此代码 
#include <memory>
int main()
{
    auto p = std::make_shared<int>(42);
    std::weak_ptr w{p}; // 此情况下使用了显式推导指南
}