copy assignment with ref attributes doesnt work

If you ever make a class or struct that has a member reference, there are important things you need to know.

Consider the following working code:

#include <vector>

class X {
public:
    int val;  
    X(int val_param) : val(val_param) {}
};

int main() {
   int i = 0, j = 1; 
   X x(i);
   X y(j);
   x = y;
   return 0;
}

This works because the compiler automatically generates a member-wise copy assignment operator. That would look something like this:

X& operator=(const X& other) {
    this->val = other.val;
    return *this;
}

The copy assignment simply copies val from other into this->val, which is valid.

The Problem with References

Now consider a small change where X has a reference to an int:

#include <vector>

class X {
public:
    int& ref;  
    X(int& ref_param) : ref(ref_param) {}
};

int main() {
   int i = 0, j = 1; 
   X x(i);
   X y(j);
   x = y;
   return 0;
}

If the compiler were to generate a copy assignment operator for this version, it would look like this:

X& operator=(const X& other) {
    this->ref = other.ref;  // Error: References cannot be reassigned!
    return *this;
}

This fails because references cannot be reassigned after they are initialized. The reference this->ref is permanently bound to whatever int it was initialized with and cannot be redirected to another int.

Issues with std::vector

If you don't catch this issue early, more problems will arise, especially when using std::vector. Consider this:

#include <vector>

class X {
public:
    int& ref;  
    X(int& ref_param) : ref(ref_param) {}
};

int main() {
   int i = 5; 
   X x(i);
   std::vector<X> mcs;
   mcs.push_back(x);
   mcs.erase(mcs.begin());
   return 0;
}

Here’s why this is a problem:

1. mcs.push_back(x) attempts to copy x into the vector. But since X has a reference member, this can cause unexpected behavior, especially if the reference goes out of scope.
2. mcs.erase(mcs.begin()) triggers a shift in elements, invoking the copy assignment operator, which we already established is ill-formed.

Solutions

1. Use Pointers Instead of References

#include <vector>

class X {
public:
    int* ptr;  
    X(int& ref_param) : ptr(&ref_param) {}
    
    X& operator=(const X& other) {
        this->ptr = other.ptr;  // Reassigning pointer is valid
        return *this;
    }
};

int main() {
   int i = 5, j = 10;
   X x(i);
   X y(j);
   x = y;  // Now works, as we are just copying a pointer

   std::vector<X> mcs;
   mcs.push_back(x);
   mcs.erase(mcs.begin()); // Now safe

   return 0;
}

Now, ptr can be reassigned, avoiding the reference issue. However, you must ensure that the pointed-to object remains valid.

2. Use std::reference_wrapper

#include <vector>
#include <functional>

class X {
public:
    std::reference_wrapper<int> ref;  
    X(int& ref_param) : ref(ref_param) {}
};

int main() {
   int i = 5, j = 10;
   X x(i);
   X y(j);
   x = y;  // Works because reference_wrapper can be reassigned

   std::vector<X> mcs;
   mcs.push_back(x);
   mcs.erase(mcs.begin()); // Now safe

   return 0;
}

This approach allows you to store references without encountering reassignment errors.

Conclusion

When using references in a class, be mindful of:

• The compiler-generated copy assignment operator will not work if your class contains a reference.
• Using references in std::vector can lead to subtle issues due to element shifting and reassignments.
• Prefer using pointers or std::reference_wrapper when you need reassignable references.