13 Templates [temp]

13.4 Template arguments [temp.arg]

13.4.1 General [temp.arg.general]

There are three forms of template-argument, corresponding to the three forms of template-parameter: type, non-type and template.
The type and form of each template-argument specified in a template-id shall match the type and form specified for the corresponding parameter declared by the template in its template-parameter-list.
When the parameter declared by the template is a template parameter pack, it will correspond to zero or more template-arguments.
[Example 1: template<class T> class Array { T* v; int sz; public: explicit Array(int); T& operator[](int); T& elem(int i) { return v[i]; } }; Array<int> v1(20); typedef std::complex<double> dcomplex; // std​::​complex is a standard library template Array<dcomplex> v2(30); Array<dcomplex> v3(40); void bar() { v1[3] = 7; v2[3] = v3.elem(4) = dcomplex(7,8); } — end example]
In a template-argument, an ambiguity between a type-id and an expression is resolved to a type-id, regardless of the form of the corresponding template-parameter.133
[Example 2: template<class T> void f(); template<int I> void f(); void g() { f<int()>(); // int() is a type-id: call the first f() } — end example]
The name of a template-argument shall be accessible at the point where it is used as a template-argument.
[Note 1:
If the name of the template-argument is accessible at the point where it is used as a template-argument, there is no further access restriction in the resulting instantiation where the corresponding template-parameter name is used.
— end note]
[Example 3: template<class T> class X { static T t; }; class Y { private: struct S { /* ... */ }; X<S> x; // OK: S is accessible // X<Y​::​S> has a static member of type Y​::​S // OK: even though Y​::​S is private }; X<Y::S> y; // error: S not accessible — end example]
For a template-argument that is a class type or a class template, the template definition has no special access rights to the members of the template-argument.
[Example 4: template <template <class TT> class T> class A { typename T<int>::S s; }; template <class U> class B { private: struct S { /* ... */ }; }; A<B> b; // error: A has no access to B​::​S — end example]
When template argument packs or default template-arguments are used, a template-argument list can be empty.
In that case the empty <> brackets shall still be used as the template-argument-list.
[Example 5: template<class T = char> class String; String<>* p; // OK: String<char> String* q; // syntax error template<class ... Elements> class Tuple; Tuple<>* t; // OK: Elements is empty Tuple* u; // syntax error — end example]
An explicit destructor call ([class.dtor]) for an object that has a type that is a class template specialization may explicitly specify the template-arguments.
[Example 6: template<class T> struct A { ~A(); }; void f(A<int>* p, A<int>* q) { p->A<int>::~A(); // OK: destructor call q->A<int>::~A<int>(); // OK: destructor call } — end example]
If the use of a template-argument gives rise to an ill-formed construct in the instantiation of a template specialization, the program is ill-formed.
When name lookup for the name in a template-id finds an overload set, both non-template functions in the overload set and function templates in the overload set for which the template-arguments do not match the template-parameters are ignored.
If none of the function templates have matching template-parameters, the program is ill-formed.
When a simple-template-id does not name a function, a default template-argument is implicitly instantiated when the value of that default argument is needed.
[Example 7: template<typename T, typename U = int> struct S { }; S<bool>* p; // the type of p is S<bool, int>*
The default argument for U is instantiated to form the type S<bool, int>*.
— end example]
A template-argument followed by an ellipsis is a pack expansion.
There is no such ambiguity in a default template-argument because the form of the template-parameter determines the allowable forms of the template-argument.

13.4.2 Template type arguments [temp.arg.type]

A template-argument for a template-parameter which is a type shall be a type-id.
[Example 1: template <class T> class X { }; template <class T> void f(T t) { } struct { } unnamed_obj; void f() { struct A { }; enum { e1 }; typedef struct { } B; B b; X<A> x1; // OK X<A*> x2; // OK X<B> x3; // OK f(e1); // OK f(unnamed_obj); // OK f(b); // OK } — end example]
[Note 1:
A template type argument can be an incomplete type ([basic.types]).
— end note]

13.4.3 Template non-type arguments [temp.arg.nontype]

If the type T of a template-parameter ([temp.param]) contains a placeholder type ([dcl.spec.auto]) or a placeholder for a deduced class type ([dcl.type.class.deduct]), the type of the parameter is the type deduced for the variable x in the invented declaration T x = template-argument ;
If a deduced parameter type is not permitted for a template-parameter declaration ([temp.param]), the program is ill-formed.
A template-argument for a non-type template-parameter shall be a converted constant expression ([expr.const]) of the type of the template-parameter.
[Note 1:
If the template-argument is an overload set (or the address of such, including forming a pointer-to-member), the matching function is selected from the set ([over.over]).
— end note]
For a non-type template-parameter of reference or pointer type, or for each non-static data member of reference or pointer type in a non-type template-parameter of class type or subobject thereof, the reference or pointer value shall not refer to or be the address of (respectively):
[Example 1: template<const int* pci> struct X { /* ... */ }; int ai[10]; X<ai> xi; // array to pointer and qualification conversions struct Y { /* ... */ }; template<const Y& b> struct Z { /* ... */ }; Y y; Z<y> z; // no conversion, but note extra cv-qualification template<int (&pa)[5]> struct W { /* ... */ }; int b[5]; W<b> w; // no conversion void f(char); void f(int); template<void (*pf)(int)> struct A { /* ... */ }; A<&f> a; // selects f(int) template<auto n> struct B { /* ... */ }; B<5> b1; // OK, template parameter type is int B<'a'> b2; // OK, template parameter type is char B<2.5> b3; // OK, template parameter type is double B<void(0)> b4; // error: template parameter type cannot be void — end example]
[Note 2:
A string-literal ([lex.string]) is not an acceptable template-argument for a template-parameter of non-class type.
[Example 2: template<class T, T p> class X { /* ... */ }; X<const char*, "Studebaker"> x; // error: string literal object as template-argument X<const char*, "Knope" + 1> x2; // error: subobject of string literal object as template-argument const char p[] = "Vivisectionist"; X<const char*, p> y; // OK struct A { constexpr A(const char*) {} }; X<A, "Pyrophoricity"> z; // OK, string-literal is a constructor argument to A — end example]
— end note]
[Note 3:
A temporary object is not an acceptable template-argument when the corresponding template-parameter has reference type.
[Example 3: template<const int& CRI> struct B { /* ... */ }; B<1> b1; // error: temporary would be required for template argument int c = 1; B<c> b2; // OK struct X { int n; }; struct Y { const int &r; }; template<Y y> struct C { /* ... */ }; C<Y{X{1}.n}> c; // error: subobject of temporary object used to initialize // reference member of template parameter — end example]
— end note]

13.4.4 Template template arguments [temp.arg.template]

A template-argument for a template template-parameter shall be the name of a class template or an alias template, expressed as id-expression.
When the template-argument names a class template, only primary class templates are considered when matching the template template argument with the corresponding parameter; partial specializations are not considered even if their parameter lists match that of the template template parameter.
Any partial specializations associated with the primary class template or primary variable template are considered when a specialization based on the template template-parameter is instantiated.
If a specialization is not visible at the point of instantiation, and it would have been selected had it been visible, the program is ill-formed, no diagnostic required.
[Example 1: template<class T> class A { // primary template int x; }; template<class T> class A<T*> { // partial specialization long x; }; template<template<class U> class V> class C { V<int> y; V<int*> z; }; C<A> c; // V<int> within C<A> uses the primary template, so c.y.x has type int // V<int*> within C<A> uses the partial specialization, so c.z.x has type long — end example]
A template-argument matches a template template-parameter P when P is at least as specialized as the template-argument A.
In this comparison, if P is unconstrained, the constraints on A are not considered.
If P contains a template parameter pack, then A also matches P if each of A's template parameters matches the corresponding template parameter in the template-head of P.
Two template parameters match if they are of the same kind (type, non-type, template), for non-type template-parameters, their types are equivalent ([temp.over.link]), and for template template-parameters, each of their corresponding template-parameters matches, recursively.
When P's template-head contains a template parameter pack ([temp.variadic]), the template parameter pack will match zero or more template parameters or template parameter packs in the template-head of A with the same type and form as the template parameter pack in P (ignoring whether those template parameters are template parameter packs).
[Example 2: template<class T> class A { /* ... */ }; template<class T, class U = T> class B { /* ... */ }; template<class ... Types> class C { /* ... */ }; template<auto n> class D { /* ... */ }; template<template<class> class P> class X { /* ... */ }; template<template<class ...> class Q> class Y { /* ... */ }; template<template<int> class R> class Z { /* ... */ }; X<A> xa; // OK X<B> xb; // OK X<C> xc; // OK Y<A> ya; // OK Y<B> yb; // OK Y<C> yc; // OK Z<D> zd; // OK — end example]
[Example 3: template <class T> struct eval; template <template <class, class...> class TT, class T1, class... Rest> struct eval<TT<T1, Rest...>> { }; template <class T1> struct A; template <class T1, class T2> struct B; template <int N> struct C; template <class T1, int N> struct D; template <class T1, class T2, int N = 17> struct E; eval<A<int>> eA; // OK: matches partial specialization of eval eval<B<int, float>> eB; // OK: matches partial specialization of eval eval<C<17>> eC; // error: C does not match TT in partial specialization eval<D<int, 17>> eD; // error: D does not match TT in partial specialization eval<E<int, float>> eE; // error: E does not match TT in partial specialization — end example]
[Example 4: template<typename T> concept C = requires (T t) { t.f(); }; template<typename T> concept D = C<T> && requires (T t) { t.g(); }; template<template<C> class P> struct S { }; template<C> struct X { }; template<D> struct Y { }; template<typename T> struct Z { }; S<X> s1; // OK, X and P have equivalent constraints S<Y> s2; // error: P is not at least as specialized as Y S<Z> s3; // OK, P is at least as specialized as Z — end example]
A template template-parameter P is at least as specialized as a template template-argument A if, given the following rewrite to two function templates, the function template corresponding to P is at least as specialized as the function template corresponding to A according to the partial ordering rules for function templates.
Given an invented class template X with the template-head of A (including default arguments and requires-clause, if any):
  • Each of the two function templates has the same template parameters and requires-clause (if any), respectively, as P or A.
  • Each function template has a single function parameter whose type is a specialization of X with template arguments corresponding to the template parameters from the respective function template where, for each template parameter PP in the template-head of the function template, a corresponding template argument AA is formed.
    If PP declares a template parameter pack, then AA is the pack expansion PP... ([temp.variadic]); otherwise, AA is the id-expression PP.
If the rewrite produces an invalid type, then P is not at least as specialized as A.