问题:

I would like to be able to do this:

X<int> type_0;
X<int> type_1;

and I would like for type_0 and type_1 to be two different types. How can I do it?

回答1:

template < typename T, int I > class X; 

X<int, __LINE__ > x0; 
X<int, __LINE__ > x1;

x0 and x1 will be different types as will any other declarations like this if they are not on the same line in the file.

回答2:

You'll need to parameterize on another thing (e.g. an integer?). For example, define X as template <typename T, int N> struct X {...}; and use X<int,0> type_0; X<int,1> type_1. If template parameters match, they are the same type.

回答3:

Make a class that inherits from the X template class, like this:

template <int I>
class type_i : public X<int>
   {
   };

typedef type_i<0> type_0;
typedef type_i<1> type_1;

回答4:

You could use ordinal tags:

template <typename T, int Tag> class X { ... };

typedef X<int, 0> type_0;
typedef X<int, 1> type_1;

Alternatively, you could use inheritance:

class type_0 : X<int> { ... };
class type_1 : X<int> { ... };

But this suffers from some difficulties, such as the need to forward constructor parameters and hazards with mixing assignment semantics and inheritance.