Template explicit instantiation (definition and declaration) in gcc

Question

Suppose there is a code like this:

template <typename T>
CLASS_KEY1 X{};

PREFIX template CLASS_KEY2 X<int>;

where CLASS_KEY1, CLASS_KEY2 and PREFIX are macros. CLASS_KEY1 and CLASS_KEY2 may be expanded to class, struct or union keywords. PREFIX may be expanded to empty set of characters or extern keyword.

Here is the table which shows when such code compiles (Yes - compiles, No - does not compile) for all combinations of macros values (compiler gcc-4.8.1, option -std=c++11):

PREFIX                                        extern  extern  extern
CLASS_KEY1\CLASS_KEY2 class   struct  union   class   struct  union
class                 Yes     Yes?    No      Yes     Yes?    No
struct                Yes?    Yes     No      Yes?    Yes     No
union                 No      No      Yes     No      No      Yes

Is it a bug in gcc or standard requirement (strange cases are labeled with question marks)? What about other compilers?

Appart from the encapsulation policy, there is no difference between `struct` and `class`. I bet that the retained policy is the one anounced where the class is defined. — Morwenn, Feb 25 '14 at 14:14
However, clang++ produces a warning when you try to do so: `warning: class template 'X' was previously declared as a struct template`. Both compilers apply the encapsulation policy annoucned where the class is defined. — Morwenn, Feb 25 '14 at 14:21

score 2 · Accepted Answer · answered Feb 25 '14 at 14:34

Section 7.1.6.3 (Elaborated type specifiers) of the C++11 standard says :

The class-key or enum keyword present in the elaborated-type-specifier shall agree in kind with the declaration to which the name in the elaborated-type-specifier refers. This rule also applies to the form of elaborated-type-specifier that declares a class-name or friend class since it can be construed as referring to the definition of the class. Thus, in any elaborated-type-specifier, the enum keyword shall be used to refer to an enumeration (7.2), the union class-key shall be used to refer to a union (Clause 9), and either the class or struct class-key shall be used to refer to a class (Clause 9) declared using the class or struct class-key.

So, the behavior you're seeing is allowed.

Template explicit instantiation (definition and declaration) in gcc

1 Answers1