Standard says, we can define static data member template like class template and function template, but how exactly?

Question

The chapter of Templates in the C++03 Standard starts with the following:

A template defines a family of classes or functions.

template-declaration:  
     export_opt template < template-parameter-list > declaration
template-parameter-list:
     template-parameter
     template-parameter-list , template-parameter

The declaration in a template-declaration shall

— declare or define a function or a class, or

— define a member function, a member class or a static data member of a class template or of a class nested within a class template, or

— define a member template of a class or class template.

A template-declaration is a declaration. A template-declaration is also a definition if its declaration defines a function, a class, or a static data member.

So from what I understand reading the bold text is that we can define static data-member-template , in addition to class -template and function-template. But I've never seen static data-member-template. How exactly is it defined? I tried the following, but GCC is not accepting it (ideone):

template<typename T> struct X{};

template<typename T>
struct A
{
   template<typename U> static X<U> data_member;
};

So I started doubting myself if I understood the quotation correctly. What exactly am I missing? Or how exactly can we define static data member template? What does the bold text mean?

This does not *define* a static data member, it merely *declares* one. — Philipp, May 28 '11 at 13:26
Oops.. I got downvote for asking question, and clarifying my doubts. :| — Nawaz, May 28 '11 at 13:29

score 5 · Accepted Answer · answered May 28 '11 at 13:24

5

No, it refers to:

template<typename T> int A<T>::staticDataMember;
//                   ^~~~~~~~~~~~~~~~~~~~~~~~~~~ declaration

If A is a class template like the following

template<typename T>
struct A { static int staticDataMember; };

answered May 28 '11 at 13:24

Johannes Schaub - litb

496,577
130
894
1,212

Ahhh.. this seems to be correct. Johannes, Awesome as always :D... +1. – Nawaz May 28 '11 at 13:25
2

I love the clang underlining :) – David Rodríguez - dribeas May 28 '11 at 13:27
Correct, because in Nawaz’ code no static data member is defined. – Philipp May 28 '11 at 13:27
Can you give me a complete working example? It will be helpful for others as well. – Nawaz May 28 '11 at 13:27

score 0 · Answer 2 · answered May 28 '11 at 13:23

0

You never gave the data_member an actual type.

template<typename T> struct X{};

template<typename T>
struct A
{
   static X<T> data_member;
};

answered May 28 '11 at 13:23

Puppy

144,682
38
256
465

But that declaration is not of the form of `template < template-parameter-list > declaration` as the quotation says. – Nawaz May 28 '11 at 13:24

Standard says, we can define static data member template like class template and function template, but how exactly?

2 Answers2