部分避免 class 模板参数（从构造函数参数推断）

Question

is there a way to avoid specifying a class template parameter if the type is being used in the constructor's argument?

I have:

template< typename T, typename X >
class myclass {
  myclass( typename X ) {};
  X myfunct() { return X(); };
};

so is there some design pattern to make it possible to instantiate myclass using just

myclass< TType > myclass_object( x_object );

instead of

myclass< TType, XType > myclass_object( x_object );

Thanks to you template programming experts, this stuff sometimes really freaks me out.

Edit: added function with X return type to make situation clearer.

Answer 1

Only if you don't need to know the second type for the rest of your class.

template< typename T >
class myclass {
  template < typename X >
  myclass( typename X ) {};
};

This works because the second template is a function, and for functions the compiler will deduce the template arguments from the types of the regular arguments if it can. In this case it can.

Answer 2

Use a helper function that deduces the argument types and explicitly specifies the class instantiation. See std::make_pair for an example.

Answer 3

Sort answer: no. myclass<X> and myclass<Y> cannot be deduced by construction from Z because Z might convert to either X or Y .

myclass<int> x (123);
myclass<float> x (123);

You need a helper function or a complete type.

template <typename T> myclass<T> make_myclass (const T &);