Struct with template variables in C++

Question

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?

score 173 · Accepted Answer · answered Mar 15 '10 at 15:33

173

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; 
};

answered Mar 15 '10 at 15:33

Binary Worrier

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4

+1 for explaining the difference between your code and @monkeyking's. – rcollyer Mar 15 '10 at 16:58
How do you create a constructor with an initializer list for the `struct` if you don't `typedef` it? Make a class instead? [One answer here](https://stackoverflow.com/a/64750247/13132315) basically creates aliases for each type `T`, which isn't ideal. – Breno Jun 02 '22 at 12:42
1

@Breno: Can you ask that in a new question. I haven't written C++ code in 15 or more years now, sorry. – Binary Worrier Jun 02 '22 at 13:28

score 19 · Answer 2 · answered Mar 15 '10 at 15:32

19

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

answered Mar 15 '10 at 15:32

Andrey

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dirkgently · Answer 3 · 2010-03-15T16:58:27.143

9

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;
} ;

edited Mar 15 '10 at 16:58

answered Mar 15 '10 at 15:32

dirkgently

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Hm, did you mean to put that after struct? – GManNickG Mar 15 '10 at 16:47

score 5 · Answer 4 · answered Mar 15 '10 at 15:34

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++).

score 4 · Answer 5 · answered Mar 15 '10 at 16:37

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.

score 3 · Answer 6 · edited Sep 03 '13 at 13:51

3

The syntax is wrong. The typedef should be removed.

edited Sep 03 '13 at 13:51

Amar

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answered Mar 15 '10 at 17:45

webgenius

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score 3 · Answer 7 · answered Mar 15 '10 at 18:06

From the other answers, the problem is that you're templating a typedef. The only "way" to do this is to use a templated class; ie, basic template metaprogramming.

template<class T> class vector_Typedefs {
    /*typedef*/ struct array { //The typedef isn't necessary
        size_t x; 
        T *ary; 
    }; 

    //Any other templated typedefs you need. Think of the templated class like something
    // between a function and namespace.
}

//An advantage is:
template<> class vector_Typedefs<bool>
{
    struct array {
        //Special behavior for the binary array
    }
}

score 2 · Answer 8 · answered Nov 09 '20 at 10:53

2

Looks like @monkeyking is trying it to make it more obvious code as shown below

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

typedef Array<int> iArray;
typedef Array<float> fArray;

answered Nov 09 '20 at 10:53

AlexanderGr8

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Struct with template variables in C++

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