What is “template <typename From, typename Tag> struct Alias;” in c++?

Question

The following is the code:

namespace o
{

template <typename From, typename Tag> struct Alias;
template <typename From, typename Tag>
inline std::ostream &operator<<(std::ostream &stream, const Alias<From, Tag> &inst);

template <typename From, typename Tag> struct Alias final
{ ...... } 
}

I was wondering, what is this template <typename From, typename Tag> struct Alias . I know what is the meaning of a template function. Is this a template struct or something like that?

Answer 1

was wondering, what is this template struct Alias. I know what is the meaning of a template function. Is this a template struct or something like that?

That's exactly what it is.

Specifically, that line is a forward declaration, to allow you to use the template struct in a function before you actually define what is in the struct.

The following line(s):

<typename From, typename Tag> struct Alias final { ...... }

actually defines what's in the template struct.

Template structs work similarly to template functions in the sense that the compiler generates a specific case of the struct when it needs it. Like functions, template structs act as a blueprint for the compiler to generate classes from the blueprint when it needs to.