3

I am trying to do a forward declaration of a struct in c++ that has a typename. Something like this is entirely valid:

typedef struct foo foo;

struct foo{
    int f;
};

My struct just instead has a typename, so I tried this:

template <typename T>
typedef struct mV<T> mV;

template <typename T>
struct mV{
   //contents of struct
};

However, I then get the errors a typedef cannot be a template, explicit specialization of undeclared template struct and redefinition of 'mV' as different kind of symbol. How can I go about fixing that?

Sam Moldenha
  • 463
  • 2
  • 11
  • 3
    do you mean forward declaration? – 463035818_is_not_an_ai Feb 12 '22 at 20:11
  • 2
    You don't need those struct typedefs in C++, that's a C thing. Just do `struct foo;` (similarly with the template). – HolyBlackCat Feb 12 '22 at 20:11
  • 1
    Not clear why you're defining an alias for something that matches its existing name. But here it is anyway: `template using mV = struct mV;` – Raymond Chen Feb 12 '22 at 20:27
  • Something like `typedef struct foo foo;` is entirely valid but also entirely unnecessary (in parts); This is not C where you'd need to write `struct foo`, if you don't do the typedef; in C++ `struct foo;` is sufficient to allow for a use of `foo` as a type name without the use of an additional `struct` keyword... – fabian Feb 12 '22 at 20:28

1 Answers1

6

You're describing a forward- declaration . (A future is something completely different in modern C++).

typedef aliasing structure tags is not needed, and rarely desired, in C++. Instead you simply declare the class type and be done with it.

// typedef struct mV mV;  // not this
struct mV;                // instead this

The same goes for templates

template<class T>
struct mV;

If you need/want to attach an alias to your template type, you still can do so via using

template<class T>
struct mV;

template<class T>
using MyAliasNameHere = mV<T>;

Armed with all of that, and heading off what I surmise you'll be discovering in short order, you'll probably need to read this as well: Why can templates only be implemented in header files?. Something tells me that's about to become highly relevant.

WhozCraig
  • 65,258
  • 11
  • 75
  • 141