error: use of deleted function

Question

I've been working on some C++ code that a friend has written and I get the following error that I have never seen before when compiling with gcc4.6:

error: use of deleted function

‘GameFSM_<std::array<C, 2ul> >::hdealt::hdealt()’ is implicitly deleted because the default definition would be ill-formed:
uninitialized non-static const member ‘const h_t FlopPokerGameFSM_<std::array<C, 2ul> >::hdealt::h’

Edit: This comes from a part of the code using boost MSM: Boost Webpage

Edit2: There is no = delete() used anywhere in the sourcecode.

Generally speaking, what does this error mean? What should I be looking for when this type of error occurs?

Answer 1

The error message clearly says that the default constructor has been deleted implicitly. It even says why: the class contains a non-static, const variable, which would not be initialized by the default ctor.

class X {
    const int x;
};

Since X::x is const, it must be initialized -- but a default ctor wouldn't normally initialize it (because it's a POD type). Therefore, to get a default ctor, you need to define one yourself (and it must initialize x). You can get the same kind of situation with a member that's a reference:

class X { 
    whatever &x;
};

It's probably worth noting that both of these will also disable implicit creation of an assignment operator as well, for essentially the same reason. The implicit assignment operator normally does members-wise assignment, but with a const member or reference member, it can't do that because the member can't be assigned. To make assignment work, you need to write your own assignment operator.

This is why a const member should typically be static -- when you do an assignment, you can't assign the const member anyway. In a typical case all your instances are going to have the same value so they might as well share access to a single variable instead of having lots of copies of a variable that will all have the same value.

It is possible, of course, to create instances with different values though -- you (for example) pass a value when you create the object, so two different objects can have two different values. If, however, you try to do something like swapping them, the const member will retain its original value instead of being swapped.