13 Templates [temp]

13.7 Template declarations [temp.decls]

13.7.2 Class templates [temp.class] Member functions of class templates [temp.mem.func]

A member function of a class template may be defined outside of the class template definition in which it is declared.
[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]; } };
declares three member functions of a class template.
The subscript function can 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]
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 2: 
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]