[temp.over.link] (original) (raw)

13 Templates [temp]

13.7 Template declarations [temp.decls]

13.7.7 Function templates [temp.fct]

It is possible to overload function templates so that two different function template specializations have the same type.

[Example 1:

template<class T> void f(T*);void g(int* p) { f(p); }

template<class T> void f(T);void h(int* p) { f(p); }

— _end example_]

The signature of a function template is defined in [intro.defs].

The names of the template parameters are significant only for establishing the relationship between the template parameters and the rest of the signature.

[Note 1:

Two distinct function templates can have identical function return types and function parameter lists, even if overload resolution alone cannot distinguish them.

template<class T> void f();template<int I> void f(); — _end note_]

When an expression that references a template parameter is used in the function parameter list or the return type in the declaration of a function template, the expression that references the template parameter is part of the signature of the function template.

This is necessary to permit a declaration of a function template in one translation unit to be linked with another declaration of the function template in another translation unit and, conversely, to ensure that function templates that are intended to be distinct are not linked with one another.

[Example 2: template <int I, int J> A<I+J> f(A<I>, A<J>); template <int K, int L> A<K+L> f(A<K>, A<L>); template <int I, int J> A<I-J> f(A<I>, A<J>); — _end example_]

[Note 2:

Most expressions that use template parameters use constant template parameters, but it is possible for an expression to reference a type parameter.

For example, a template type parameter can be used in thesizeof operator.

— _end note_]

Two expressions involving template parameters are consideredequivalentif two function definitions containing the expressions would satisfy the one-definition rule, except that the tokens used to name the template parameters may differ as long as a token used to name a template parameter in one expression is replaced by another token that names the same template parameter in the other expression.

Two unevaluated operands that do not involve template parameters are considered equivalent if two function definitions containing the expressions would satisfy the one-definition rule, except that the tokens used to name types and declarations may differ as long as they name the same entities, and the tokens used to form concept-ids ([temp.names]) may differ as long as the two template-ids are the same ([temp.type]).

[Note 3:

For instance, A<42> and A<40+2> name the same type.

— _end note_]

[Note 4:

The intent is to avoid lambda-expressions appearing in the signature of a function template with external linkage.

— _end note_]

For determining whether two dependent names ([temp.dep]) are equivalent, only the name itself is considered, not the result of name lookup.

[Note 5:

If such a dependent name is unqualified, it is looked up from a first declaration of the function template ([temp.res.general]).

— _end note_]

[Example 3: template <int I, int J> void f(A<I+J>); template <int K, int L> void f(A<K+L>); template <class T> decltype(g(T())) h();int g(int);template <class T> decltype(g(T())) h() { return g(T()); } int i = h<int>(); template <int N> void foo(const char (*s)[([]{}, N)]);template <int N> void foo(const char (*s)[([]{}, N)]);template <class T> void spam(decltype([]{}) (*s)[sizeof(T)]);template <class T> void spam(decltype([]{}) (*s)[sizeof(T)]); — _end example_]

Two potentially-evaluated expressions involving template parameters that are not equivalent arefunctionally equivalentif, for any given set of template arguments, the evaluation of the expression results in the same value.

Two unevaluated operands that are not equivalent are functionally equivalent if, for any given set of template arguments, the expressions perform the same operations in the same order with the same entities.

[Note 6:

For instance, one could have redundant parentheses.

— _end note_]

[Example 4: template<int I> concept C = true;template<typename T> struct A { void f() requires C<42>; void f() requires true; }; — _end example_]

Two template-parameters areequivalentunder the following conditions:

When determining whether types or type-constraint_s_are equivalent, the rules above are used to compare expressions involving template parameters.

If the validity or meaning of the program depends on whether two constructs are equivalent, and they are functionally equivalent but not equivalent, the program is ill-formed, no diagnostic required.

Furthermore, if two declarations A and B of function templates

but do not correspond, the program is ill-formed, no diagnostic required.

[Note 7:

This rule guarantees that equivalent declarations will be linked with one another, while not requiring implementations to use heroic efforts to guarantee that functionally equivalent declarations will be treated as distinct.

For example, the last two declarations are functionally equivalent and would cause a program to be ill-formed: template <int I> void f(A<I>, A<I+10>);template <int I> void f(A<I>, A<I+10>);template <int I> void f(A<I>, A<I+10>);template <int I> void f(A<I>, A<I+11>);template <int I> void f(A<I>, A<I+10>);template <int I> void f(A<I>, A<I+1+2+3+4>);

— _end note_]