From Jason Turner

[temp.class.spec.mfunc]

C++20 → C++23 3.6 KB 88 lines
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- #### Members of class template specializations <a id="temp.class.spec.mfunc">[[temp.class.spec.mfunc]]</a>
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-
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- The template parameter list of a member of a class template partial
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- specialization shall match the template parameter list of the class
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- template partial specialization. The template argument list of a member
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- of a class template partial specialization shall match the template
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- argument list of the class template partial specialization. A class
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- template partial specialization is a distinct template. The members of
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- the class template partial specialization are unrelated to the members
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- of the primary template. Class template partial specialization members
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- that are used in a way that requires a definition shall be defined; the
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- definitions of members of the primary template are never used as
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- definitions for members of a class template partial specialization. An
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- explicit specialization of a member of a class template partial
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- specialization is declared in the same way as an explicit specialization
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- of the primary template.
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-
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- [*Example 1*:
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-
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- ``` cpp
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- // primary class template
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- template<class T, int I> struct A {
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- void f();
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- };
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-
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- // member of primary class template
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- template<class T, int I> void A<T,I>::f() { }
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-
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- // class template partial specialization
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- template<class T> struct A<T,2> {
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- void f();
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- void g();
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- void h();
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- };
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-
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- // member of class template partial specialization
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- template<class T> void A<T,2>::g() { }
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-
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- // explicit specialization
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- template<> void A<char,2>::h() { }
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-
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- int main() {
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- A<char,0> a0;
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- A<char,2> a2;
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- a0.f(); // OK, uses definition of primary template's member
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- a2.g(); // OK, uses definition of partial specialization's member
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- a2.h(); // OK, uses definition of explicit specialization's member
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- a2.f(); // error: no definition of f for A<T,2>; the primary template is not used here
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- }
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- ```
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-
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- — *end example*]
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-
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- If a member template of a class template is partially specialized, the
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- member template partial specializations are member templates of the
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- enclosing class template; if the enclosing class template is
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- instantiated ([[temp.inst]], [[temp.explicit]]), a declaration for
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- every member template partial specialization is also instantiated as
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- part of creating the members of the class template specialization. If
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- the primary member template is explicitly specialized for a given
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- (implicit) specialization of the enclosing class template, the partial
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- specializations of the member template are ignored for this
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- specialization of the enclosing class template. If a partial
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- specialization of the member template is explicitly specialized for a
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- given (implicit) specialization of the enclosing class template, the
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- primary member template and its other partial specializations are still
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- considered for this specialization of the enclosing class template.
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-
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- [*Example 2*:
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-
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- ``` cpp
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- template<class T> struct A {
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- template<class T2> struct B {}; // #1
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- template<class T2> struct B<T2*> {}; // #2
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- };
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-
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- template<> template<class T2> struct A<short>::B {}; // #3
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-
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- A<char>::B<int*> abcip; // uses #2
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- A<short>::B<int*> absip; // uses #3
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- A<char>::B<int> abci; // uses #1
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- ```
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-
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- — *end example*]
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-