错误：非静态引用成员 - 不能使用默认赋值运算符

Question

I have a problem where.. Well, I'm not sure. I really don't know what it means. I have used this same paradigm illustrated below, where I create an object within a class and give that object a reference to the class. This design is used as an event handler.

I added a very unrelated piece of code where I loop through a map of objects and re-assign them. It has brought up an issue due to the use of the = operator. But I still don't actually know what it's complaining about.

// Example program
#include <iostream>
#include <string>
#include <map>

class A {  
    public:
        class Handler {
            public:
                Handler(A &a):a(a){}
                virtual void HandleIt(){
                    a.DoThings();   
                }
                A &a;
        };
    
        A():my_handler(*this){}
        Handler my_handler;
        void DoThings(){
            std::cout << "Im doing things";   
        }
};

std::map<std::string, A> my_map;

void ReplaceInMap(A &a){
    std::map<std::string, A>::iterator it;
    for(it = my_map.begin(); it != my_map.end(); ++it){
        it->second = a;
    }
}

int main()
{
    A a;
    A b;
    A c;
    
    my_map.insert(std::make_pair<std::string, A>("A!", a));
    my_map.insert(std::make_pair<std::string, A>("B!", b));
    my_map.insert(std::make_pair<std::string, A>("C!", c));
    
    ReplaceInMap(a);
}

C++ 98

In member function 'A::Handler& A::Handler::operator=(const A::Handler&)':
8:15: error: non-static reference member 'A&A::Handler::a', can't use default assignment operator  
In member function 'A& A::operator=(const A&)': 6:7: 
note: synthesized method 'A::Handler& A::Handler::operator=(const A::Handler&)' first required here   
In function 'void ReplaceInMap(A&)': 29:20: note: synthesized method 'A& A::operator=(const A&)' first required here

http://cpp.sh/4chmu

What does this error mean? What is the problem?

Answer 1

What does this error mean?

It means what it says literally. The assignment operator of A and consequently Handler cannot be used.

Important points:

• References are not assignable.
• The class contains a reference member, and no user defined assignment operator. Therefore the class is not assignable.
• You attempt to assign objects of that class.

What is the problem?

You attempt to assign a non-assignable class.

Your options are:

• Don't use a reference member. Use a pointer or a reference wrapper (standard library has one since C++11) both of which are assignable.
• Don't assign the object.