13 Templates [temp]

13.7 Template declarations [temp.decls]

13.7.1 Class templates [temp.class]

1
#
A class template defines the layout and operations for an unbounded set of related types.
2
#
[ Example
:
A single class template List might provide an unbounded set of class definitions: one class List<T> for every type T, each describing a linked list of elements of type T.
Similarly, a class template Array describing a contiguous, dynamic array might be defined like this: 
template<class T> class Array {
  T* v;
  int sz;
public:
  explicit Array(int);
  T& operator[](int);
  T& elem(int i) { return v[i]; }
};
The prefix template<class T> specifies that a template is being declared and that a type-name T may be used in the declaration.
In other words, Array is a parameterized type with T as its parameter.
— end example
 ]
3
#
When a member function, a member class, a member enumeration, a static data member or a member template of a class template is defined outside of the class template definition, the member definition is defined as a template definition in which the template-head is equivalent to that of the class template ([temp.over.link]).
The names of the template parameters used in the definition of the member may be different from the template parameter names used in the class template definition.
The template argument list following the class template name in the member definition shall name the parameters in the same order as the one used in the template parameter list of the member.
Each template parameter pack shall be expanded with an ellipsis in the template argument list.
[ Example
: 
template<class T1, class T2> struct A {
  void f1();
  void f2();
};

template<class T2, class T1> void A<T2,T1>::f1() { }    // OK
template<class T2, class T1> void A<T1,T2>::f2() { }    // error

template<class ... Types> struct B {
  void f3();
  void f4();
};

template<class ... Types> void B<Types ...>::f3() { }   // OK
template<class ... Types> void B<Types>::f4() { }       // error

template<typename T> concept C = true;
template<typename T> concept D = true;

template<C T> struct S {
  void f();
  void g();
  void h();
  template<D U> struct Inner;
};

template<C A> void S<A>::f() { }        // OK: template-heads match
template<typename T> void S<T>::g() { } // error: no matching declaration for S<T>

template<typename T> requires C<T>      // ill-formed, no diagnostic required: template-heads are
void S<T>::h() { }                      // functionally equivalent but not equivalent

template<C X> template<D Y>
struct S<X>::Inner { };                 // OK
— end example
 ]
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#
In a redeclaration, partial specialization, explicit specialization or explicit instantiation of a class template, the class-key shall agree in kind with the original class template declaration ([dcl.type.elab]).

13.7.1.1 Member functions of class templates [temp.mem.func]

1
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A member function of a class template may be defined outside of the class template definition in which it is declared.
[ Example
: 
template<class T> class Array {
  T* v;
  int sz;
public:
  explicit Array(int);
  T& operator[](int);
  T& elem(int i) { return v[i]; }
};
declares three member functions of a class template.
The subscript function might be defined like this:
template<class T> T& Array<T>::operator[](int i) {
  if (i<0 || sz<=i) error("Array: range error");
  return v[i];
}
A constrained member function can be defined out of line: 
template<typename T> concept C = requires {
  typename T::type;
};

template<typename T> struct S {
  void f() requires C<T>;
  void g() requires C<T>;
};

template<typename T>
  void S<T>::f() requires C<T> { }      // OK
template<typename T>
  void S<T>::g() { }                    // error: no matching function in S<T>
— end example
 ]
2
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The template-arguments for a member function of a class template are determined by the template-arguments of the type of the object for which the member function is called.
[ Example
:
The template-argument for Array<T>​::​operator[] will be determined by the Array to which the subscripting operation is applied.
Array<int> v1(20);
Array<dcomplex> v2(30);

v1[3] = 7;                              // Array<int>​::​operator[]
v2[3] = dcomplex(7,8);                  // Array<dcomplex>​::​operator[]
— end example
 ]

13.7.1.2 Deduction guides [temp.deduct.guide]

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Deduction guides are used when a template-name appears as a type specifier for a deduced class type ([dcl.type.class.deduct]).
Deduction guides are not found by name lookup.
Instead, when performing class template argument deduction ([over.match.class.deduct]), any deduction guides declared for the class template are considered.
deduction-guide:
	explicit-specifier template-name ( parameter-declaration-clause ) -> simple-template-id ;
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[ Example
: 
template<class T, class D = int>
struct S {
  T data;
};
template<class U>
S(U) -> S<typename U::type>;

struct A {
  using type = short;
  operator type();
};
S x{A()};           // x is of type S<short, int>
— end example
 ]
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#
The same restrictions apply to the parameter-declaration-clause of a deduction guide as in a function declaration ([dcl.fct]).
The simple-template-id shall name a class template specialization.
The template-name shall be the same identifier as the template-name of the simple-template-id.
A deduction-guide shall be declared in the same scope as the corresponding class template and, for a member class template, with the same access.
Two deduction guide declarations in the same translation unit for the same class template shall not have equivalent parameter-declaration-clauses.

13.7.1.3 Member classes of class templates [temp.mem.class]

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A member class of a class template may be defined outside the class template definition in which it is declared.
[ Note
:
The member class must be defined before its first use that requires an instantiation ([temp.inst]).
For example, 
template<class T> struct A {
  class B;
};
A<int>::B* b1;                          // OK: requires A to be defined but not A​::​B
template<class T> class A<T>::B { };
A<int>::B  b2;                          // OK: requires A​::​B to be defined
— end note
 ]

13.7.1.4 Static data members of class templates [temp.static]

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A definition for a static data member or static data member template may be provided in a namespace scope enclosing the definition of the static member's class template.
[ Example
: 
template<class T> class X {
  static T s;
};
template<class T> T X<T>::s = 0;

struct limits {
  template<class T>
    static const T min;                 // declaration
};

template<class T>
  const T limits::min = { };            // definition
— end example
 ]
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An explicit specialization of a static data member declared as an array of unknown bound can have a different bound from its definition, if any.
[ Example
: 
template <class T> struct A {
  static int i[];
};
template <class T> int A<T>::i[4];      // 4 elements
template <> int A<int>::i[] = { 1 };    // OK: 1 element
— end example
 ]

13.7.1.5 Enumeration members of class templates [temp.mem.enum]

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An enumeration member of a class template may be defined outside the class template definition.
[ Example
: 
template<class T> struct A {
  enum E : T;
};
A<int> a;
template<class T> enum A<T>::E : T { e1, e2 };
A<int>::E e = A<int>::e1;
— end example
 ]