c++ - 为什么 unique_ptr 会重载 reset(pointer p = pointer()) 和 reset(nullptr_t)？

Question

根据http://en.cppreference.com/w/cpp/memory/unique_ptr/reset ,

void reset( pointer ptr = pointer() );

template< class U > 
void reset( U ) = delete;

void reset( std::nullptr_t p );

1) Given current_ptr, the pointer that was managed by *this, performs the following actions, in this order: Saves a copy of the current pointer old_ptr = current_ptr; Overwrites the current pointer with the argument current_ptr = ptr; If the old pointer was non-empty, deletes the previously managed object if(old_ptr != nullptr) get_deleter()(old_ptr).

2) In the specialization for dynamic arrays, std::unique_ptr<T[]>, this template member is provided to prevent using reset() with a pointer to derived (which would result in undefined behavior with arrays).

3) In the specialization for dynamic arrays, std::unique_ptr<T[]>, the third overload is necessary to allow reset to nullptr (which would otherwise be prohibited by the template overload). Equivalent to reset(pointer())

现在reset(nullptr)相当于reset(pointer()) ，为什么后者存在？

如果我想重置一个数组形式的 unique_ptr，为什么我不能只使用 rest(pointer()) ？

Answer 1

template< class U > 
void reset( U ) = delete;

将被选择用于带有 nullptr 参数的调用，如果不是用于

void reset( std::nullptr_t p );

这就是它存在的原因，允许使用 nullptr 进行调用。

---

示例(使用定义的 FIX 编译以抑制编译错误):

#include <cstddef>      // std::nullptr_t

struct S
{
    void reset( char* ) {}

    template< class Type >
    void reset( Type ) = delete;

    #if FIX
    void reset( std::nullptr_t ) {}
    #endif
};

auto main() -> int
{
    S().reset( nullptr );    // Fails when FIX is not defined.
}