问题
Can I use forward declaration for template class?
I try:
template<class que_type>
class que;
int main(){
que<int> mydeque;
return 0;
}
template<class que_type>
class que {};
I get:
error: aggregate 'que<int> mydeque' has incomplete type and cannot be defined.
回答1:
Forward declaration of a class should have complete arguments list specified. This would enable the compiler to know it's type.
When a type is forward declared, all the compiler knows about the type is that it exists; it knows nothing about its size, members, or methods and hence it is called an Incomplete type. Therefore, you cannot use the type to declare a member, or a base class, since the compiler would need to know the layout of the type.
You can:
1. Declare a member pointer or a reference to the incomplete type.
2. Declare functions or methods which accepts/return incomplete types.
3. Define functions or methods which accepts/return pointers/references to the incomplete type.
Note that, In all the above cases, the compiler does not need to know the exact layout of the type & hence compilation can be passed.
Example of 1:
class YourClass;
class MyClass
{
YourClass *IncompletePtr;
YourClass &IncompleteRef;
};
回答2:
This is not a template issue. You cannot use a type as a by-value variable unless it has been fully defined.
回答3:
No. At the point of instantiation, the complete definition of the class template must be seen by the compiler. And its true for non-template class as well.
来源：https://stackoverflow.com/questions/7111252/forward-declaration-of-class-using-templates