Struct with template variables in C++

I'm playing around with templates. I'm not trying to reinvent the std::vector, I'm trying to get a grasp of templateting in C++.

Can I do the following?

template <typename T>
typedef struct{
  size_t x;
  T *ary;
}array;

What I'm trying to do is a basic templated version of:

typedef struct{
  size_t x;
  int *ary;
}iArray;

It looks like it's working if I use a class instead of struct, so is it not possible with typedef structs?

Solution 1:

The problem is you can't template a typedef, also there is no need to typedef structs in C++.

The following will do what you need

template <typename T> 
struct array { 
  size_t x; 
  T *ary; 
}; 

Solution 2:

template <typename T>
struct array {
  size_t x;
  T *ary;
};

Solution 3:

You don't need to do an explicit typedef for classes and structs. What do you need the typedef for? Further, the typedef after a template<...> is syntactically wrong. Simply use:

template <class T>
struct array {
  size_t x;
  T *ary;
} ;

Solution 4:

You can template a struct as well as a class. However you can't template a typedef. So template<typename T> struct array {...}; works, but template<typename T> typedef struct {...} array; does not. Note that there is no need for the typedef trick in C++ (you can use structs without the struct modifier just fine in C++).

Solution 5:

The Standard says (at 14/3. For the non-standard folks, the names following a class definition body (or the type in a declaration in general) are "declarators")

In a template-declaration, explicit specialization, or explicit instantiation the init-declarator-list in the dec-laration shall contain at most one declarator. When such a declaration is used to declare a class template, no declarator is permitted.

Do it like Andrey shows.