是什么导致不调用移动分配运算符?

我正在开发自己的智能指针,遇到了一些奇怪的问题。未调用移动分配运算符。因此,我编写了一个测试类,能够重现这个问题。不会调用移动分配运算符,但会发生复制分配(即使没有复制分配运算符)。

这是我的测试课

#include <utility>
#include <iostream>

struct tag_t {};
constexpr tag_t tag {};

template <typename T>
struct Foo {
  Foo() noexcept
    : val{} {
    std::cout << "Default construct\n";
  }
  template <typename U>
  Foo(tag_t, const U &val) noexcept
    : val{val} {
    std::cout << "Construct " << val << '\n';
  }
  ~Foo() noexcept {
    std::cout << "Destruct " << val << '\n';
  }

  template <typename U>
  Foo(Foo<U> &&other) noexcept
    : val{std::exchange(other.val, U{})} {
    std::cout << "Move construct " << val << '\n';
  }
  template <typename U>
  Foo &operator=(Foo<U> &&other) noexcept {
    std::cout << "Move assign " << other.val << '\n';
    val = std::exchange(other.val, U{});
    return *this;
  }

  T val;
};

这些是测试

int main() {
  {
    Foo<int> num;
    std::cout << "Value " << num.val << '\n';
    num = {tag, 5};
    std::cout << "Value " << num.val << '\n';
  }
  std::cout << '\n';
  {
    Foo<int> num;
    std::cout << "Value " << num.val << '\n';
    num = Foo<int>{tag, 5};
    std::cout << "Value " << num.val << '\n';
  }
  return 0;
}

Default construct
Value 0
Construct 5
Destruct 5
Value 5
Destruct 5

Default construct
Value 0
Construct 5
Move assign 5
Destruct 0
Value 5
Destruct 5

让我困惑的是第一次测试的结果。不会调用移动分配运算符,但会执行复制分配。这导致 5 两次被摧毁。当你试图制作一个智能指针时,这并不理想!