'Type non-static const member can't use default assignment operator' - what does this mean?

c++, constants

Solution

Presumably, somewhere in your code you are assigning to a `Member`, something like this:

Member m1, m2;
m1 = m2;

Or you are using it in a context that requires the type to be assignable.

Since you don't provide your own assignment operator overload for `Member`, the implicitly defined defaulted assignment operator would usually kick in. However, since you have a `const` data member, the compiler won't implicitly define one for you. You need to provide it yourself.

This makes sense because, imagine in the code sample I just gave, what should the compiler do to `m1`'s `membershipNo` member? Should it assign `m2`'s `membershipNo` to it? How can it do that if `membershipNo` is `const`? In this case, the compiler just says "Nope, I can't do it."

A defaulted copy/move assignment operator for class `X` is defined as deleted if `X` has:

- a non-static data member of `const` non-class type (or array thereof)

- [...]

As I said, to be able to do assignment with a `Member`, you need to provide an assignment operator of your own:

Member& Member::operator=(const Member& other)
{
  // Copy things from other to this
}

However, herein lies the problem with having `const` members. If you supply your own copy assignment operator as described, and you don't copy over `membershipNo`, then have you really copied the other object? Logically, it makes sense that an object that has any `const` state shouldn't be able to be assigned to.

It is perfectly fine to use a copy constructor however - you only have to make sure you initialise `membershipNo` in the member initialization list:

Member::Member(const Member& other)
  : membershipNo(other.membershipNo)
{
  // ...
}

And then you can do:

Member m1;
Member m2 = m1;

Problem

This class is flagging up the following error: 'non-static const member 'const int Member::membershipNo', can't use default assignment operator'. The strange thing is this code is repeated in another project and works perfectly. Can you help me put it right? Member.h ``` class Member : public Person { public: Member(); Member(int membershipNo); virtual ~Member(); int getMembershipNo() const; private: const int membershipNo; friend class boost::serialization::access; template<class Archive> void serialize(Archive& ar, const unsigned int version) { ar & boost::serialization::base_object<Person>(*this); ar & membershipNo; } }; ``` Member.cpp ``` Member::Member() : membershipNo(0) { Person(); } Member::Member(int memberNo) : membershipNo(memberNo) { Person(); } Member::~Member() { // TODO Auto-generated destructor stub } int Member::getMembershipNo() const { return membershipNo; } ```

Original source