In VC++10 the following example fails with error C2027: "use of undefined type 'X'". However g++ 4.6 compiles it just fine.
template<class T>
class C
{
static const size_t size = sizeof(T);
};
class X : public C<X> { };
So which one is right? And how would I do this so that it works on mainstream compilers?
It's not a huge deal though, cause VC++ still allows sizeof(T) inside member functions of C. I just have to repeat some long type definitions which is annoying.
EDIT: I realize my example was bad because what I really wanted to do was to use the size as a compile time constant, in this way:
template<size_t size> class C2 { };
template<class T>
class C
{
typedef C2<sizeof(T)> A;
};
class X : public C<X> { };
Both compilers reject this so I assume it's probably not possible, but like I said I can still use sizeof inside functions. I was just hoping I wouldn't have to repeat the typedef inside every function.
template<size_t size> class C2 { };
template<class T>
class C
{
void foo() { typedef C2<sizeof(T)> A; }
};
class X : public C<X> { };
The static member cannot be initialized in the class itself, because the type T
is being defined, and is not complete yet.
However, you can initialize it outside the class as:
template<class T>
class C
{
static const size_t size;
};
template<typename T>
const size_t C<T>::size = sizeof(T); //initialization of the static member
It should compile fine: http://ideone.com/6sNgN
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