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deduction guides for std::shared_ptr

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

这些 推导指引 为 std::shared_ptr 提供，用于处理隐式推导指引未覆盖的特殊情况。

注意，无法从指针类型推导类模板参数，因为无法区分通过数组形式和非数组形式的 new 所获得的指针。

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
    std::shared_ptr p2{w}; // 此情况下使用了显式推导指南
}