error: use of deleted function

Question

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.

You are using a function, which is marked as deleted.
Eg:

int doSomething( int ) = delete;

The =delete is a new feature of C++0x. It means the compiler should immediately stop compiling and complain "this function is deleted" once the user use such function.

If you see this error, you should check the function declaration for =delete.

To know more about this new feature introduced in C++0x, check this out.

gcc 4.6 supports a new feature of deleted functions, where you can write

hdealt() = delete;

to disable the default constructor.

Here the compiler has obviously seen that a default constructor can not be generated, and =delete'd it for you.

I encountered this error when inheriting from an abstract class and not implementing all of the pure virtual methods in my subclass.

In the current C++0x standard you can explicitly disable default constructors with the delete syntax, e.g.

MyClass() = delete;

Gcc 4.6 is the first version to support this syntax, so maybe that is the problem...