A template class is like a macro, only a whole lot less evil.
Think of a template as a macro. The parameters to the template get substituted into a class (or function) definition, when you define a class (or function) using a template.
The difference is that the parameters have "types" and values passed are checked during compilation, like parameters to functions. The types valid are your regular C++ types, like int and char. When you instantiate a template class, you pass a value of the type you specified, and in a new copy of the template class definition this value gets substituted in wherever the parameter name was in the original definition. Just like a macro.
You can also use the "class" or "typename" types for parameters (they're really the same). With a parameter of one of these types, you may pass a type name instead of a value. Just like before, everywhere the parameter name was in the template class definition, as soon as you create a new instance, becomes whatever type you pass. This is the most common use for a template class; Everybody that knows anything about C++ templates knows how to do this.
Consider this template class example code:
#include <cstdio>
template <int I>
class foo
{
void print()
{
printf("%i", I);
}
};
int main()
{
foo<26> f;
f.print();
return 0;
}It's functionally the same as this macro-using code:
#include <cstdio>
#define MAKE_A_FOO(I) class foo_##I \
{ \
void print() \
{ \
printf("%i", I); \
} \
};
MAKE_A_FOO(26)
int main()
{
foo_26 f;
f.print();
return 0;
}Of course, the template version is a billion times safer and more flexible.