在参数包中调用 base class operator= from derived

Question

我在 C++17 中实现了一个简单的程序，其中我尝试使用参数扩展从派生 class 调用基 class operator=。但是程序不编译。

#include "pch.h"
#include <iostream>

class D
{
    public:
        D()
        {
            std::cout << "D ctror" << std::endl;
        }
        D  &  operator = (const D & other)
        {
            return *this;
        }
};
class C
{
    public:
        C()
        {
            std::cout << "C ctror" << std::endl;
        }
        C  &  operator = (const C & other)
        {
            return *this;
        }
};
class B
{
    public:
        B()
        {
        std::cout << "B ctror" << std::endl;
        }


        B  &  operator = (const B & other)
        {
            std::cout << "operator B" << std::endl;
            return *this;
        }
};


template<typename... Ts> class A: public Ts...
{ 
public:

    A():  Ts()...
    {
    }
    A  &  operator = (const A & other)
    {
        Ts::operator =(other);
        return *this;
    }

};

int main()
{

    A<B,C,D> a1;
    A<B,C,D> a2;
    a1 = a2;
}

使用的工具集是 Visual Studio 2017 (v141)

生成的错误如下

error C3520: '=': parameter pack must be expanded in this context note: while compiling class template member function 'A &A::operator =(const A &)' note: see reference to function template instantiation 'A &A::operator =(const A &)' being compiled note: see reference to class template instantiation 'A' being compiled

Answer 1

您需要扩展参数包。一个漂亮的折叠表达式怎么样：

(Ts::operator=(other), ...);

这将扩展 Ts... 并有效地创建对 operator= 的多个调用，包中的每种类型一个。

在参数包中调用 base class operator= from derived

Call base class operator= from derived in parameter pack

c++

variadic-templates

c++17