std::shared_ptr 模板化与非模板化 copy/move 构造函数
std::shared_ptr templated vs. non-templated copy/move constructors
This page shows that std::shared_ptr has a templated and a non-templated version of copy/move constructors ((9) and (10) on the page). Similarly, it also has two versions of copy/move assignment operator (see here).
为什么我们需要非模板版本？单靠模板版本还不够吗？
模板化版本不是复制构造函数。根据标准 §12.8[class.copy]/2 我们有：
A non-template constructor for class X is a copy constructor if its first parameter is of type X&, const X&, volatile X& or const volatile X&, and either there are no other parameters or else all other parameters have default arguments (8.3.6).
同样，根据§12.8[class.copy]/17:
，模板化版本也不是复制赋值运算符
A user-declared copy assignment operator X::operator= is a non-static non-template member function of class X with exactly one parameter of type X, X&, const X&, volatile X& or const volatile X&.
移动构造函数也是如此（在 §12.8[class.copy]/3）：
A non-template constructor for class X is a move constructor if its first parameter is of type X&&, const X&&, volatile X&&, or const volatile X&&, and either there are no other parameters or else all other parameters have default arguments (8.3.6).
并移动赋值运算符，在 §12.8[class.copy]/19:
A user-declared move assignment operator X::operator= is a non-static non-template member function of class X with exactly one parameter of type X&&, const X&&, volatile X&&, or const volatile X&&.
This page shows that std::shared_ptr has a templated and a non-templated version of copy/move constructors ((9) and (10) on the page). Similarly, it also has two versions of copy/move assignment operator (see here).
为什么我们需要非模板版本？单靠模板版本还不够吗？
模板化版本不是复制构造函数。根据标准 §12.8[class.copy]/2 我们有：
A non-template constructor for class X is a copy constructor if its first parameter is of type X&, const X&, volatile X& or const volatile X&, and either there are no other parameters or else all other parameters have default arguments (8.3.6).
同样，根据§12.8[class.copy]/17:
，模板化版本也不是复制赋值运算符A user-declared copy assignment operator X::operator= is a non-static non-template member function of class X with exactly one parameter of type X, X&, const X&, volatile X& or const volatile X&.
移动构造函数也是如此（在 §12.8[class.copy]/3）：
A non-template constructor for class X is a move constructor if its first parameter is of type X&&, const X&&, volatile X&&, or const volatile X&&, and either there are no other parameters or else all other parameters have default arguments (8.3.6).
并移动赋值运算符，在 §12.8[class.copy]/19:
A user-declared move assignment operator X::operator= is a non-static non-template member function of class X with exactly one parameter of type X&&, const X&&, volatile X&&, or const volatile X&&.