不完整类型的标准容器

Question

i'm having a little problem with containers and incomplete types.

I have this code:

template<typename T>
class IncompleteType
{ /*Class Definition*/ }

On another class i want to use a vector of incomplete types. This class serializes c-style structs to strings, so the behaviour is the same, no matter what type i'm supplying.

However, when i do this:

std::vector<IncompleteType>

or this:

std::vector<IncompleteType*>

The compiler complains that i'm not supplying argument list for template class.

What i want to do is something like this:

IncompleteType<Type1> a;
IncompleteType<Type2> b;

std::vector<IncompleteType*> vector;

vector.push_back(&a);
vector.push_back(&b);

Is there a way to have a container to incomplete types?

Is this the wrong way to achieve polymorphism?

Thanks!

Answer 1

You do not have an incomplete type - you've got a class template there. It often catches out people new to C++, but instantiations of that template with different types are (to the compiler/type system) entirely unrelated .

The template itself also isn't a "base class" of the instantiations - it cannot be used without supplying it's template type arguments.

As you're talking about "serializing to strings" I wonder if you're looking for the streaming operators instead - specifically streaming to string streams (see eg https://docs.microsoft.com/en-us/cpp/standard-library/overloading-the-output-operator-for-your-own-classes ).

What happens there is that you overload the << operator for your own types and the type of stream you want, eg

std::ostringstream& operator<<(std::ostringstream& i_stream, const MyType& i_val) {
  i_stream << "( " << i_val.GetValue1() << ", " << i_val.GetValue2() << " )" << std::endl;
  return i_stream;
}

Then you can stream any kind of object into a string stream, as long as you've overloaded the stream operator for it.