c++: Two ways of declaring multiple templates

Question

I have the following code:

template<typename T> struct Pair3{ T first, second; Pair3() : first(T()), second(T()) {} Pair3(T first, T second) : first(first), second(second) {} Pair3(const Pair3<T>& in) : first(in.first), second(in.second) {} template<typename U> void copyFrom(Pair3<U> in); }; template<typename T> template<typename U> void Pair3<T>::copyFrom(Pair3<U> in){ first = in.first; second = in.second; }

why do I have to write

template<typename T> template<typename U>

before copyFrom implementation and I can't write:

template<typename T, typename U>.

What would that mean? Why would that be wrong? Conversely: why can't I declare struct Pair3 like this:

 template<typename T, typename U> struct Pair3{ T first, second; Pair3() : first(T()), second(T()) {} Pair3(T first, T second) : first(first), second(second) {} Pair3(const Pair3<T>& in) : first(in.first), second(in.second) {} void copyFrom(Pair3<U> in); };

or even:

template<typename T> template<typename U> struct Pair3{ T first, second; Pair3() : first(T()), second(T()) {} Pair3(T first, T second) : first(first), second(second) {} Pair3(const Pair3<T>& in) : first(in.first), second(in.second) {} void copyFrom(Pair3<U> in); };

Thanks!

Dimitrios Bouzas · Accepted Answer · 2015-06-15 21:36:45Z

According to standard N4431 §14.5.2/1 Member templates [temp.mem] (Emphasis Mine):

A template can be declared within a class or class template; such a template is called a member template. A member template can be defined within or outside its class definition or class template definition. A member template of a class template that is defined outside of its class template definition shall be specified with the template-parameters of the class template followed by the template-parameters of the member template.

[Example:

template<class T> struct string { template<class T2> int compare(const T2&); template<class T2> string(const string<T2>& s) { /∗ ... ∗/ } }; template<class T> template<class T2> int string<T>::compare(const T2& s) { }

— end example ]

Defining as:

template<typename T, typename U>.

Would be wrong because this is not the way the C++ standard specifies how member templates of class templates should be defined outside the template class definition.

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