C++ - Separate declaration/definition for template function in template class

Question

I am aware that the syntax for declaring a template class method in a header and defining it in a source file goes as so:

myclass.h

template <typename T>
class MyClass {
  public:
    void method(T input);
  private:
    T privVar;
};

myclass.cpp

template <typename T>
void MyClass<T>::method(T input) {
    privVar = input;
}

But, what if the method is also a template? I am adding methods to the basic_string class, and I want to know how to write the implementation for the functions.

MyString.h

template <class _Elem   = TCHAR,
          class _Traits = std::char_traits<_Elem>,
          class _Ax     = std::allocator<_Elem>>
class String
    : public std::basic_string<_Elem, _Traits, _Ax> {
  private:
    // Types for the conversion operators.
    typedef       _Elem* _StrTy;
    typedef const _Elem* _ConstStrTy;

    //...

  public:
        // Conversion operators so 'String' can easily be
        // assigned to a C-String without calling 'c_str()'.
    operator _StrTy() const {
        return const_cast<_StrTy>(this->c_str());
    }

    operator _ConstStrTy() const {
        return this->c_str();
    }

    // ... Constructors ...

    /*------------ Additional Methods ------------*/

    //! Converts a value of the given type to a string.
    template <class _ValTy> static String ConvertFrom(_ValTy val);

    //! Converts a string to the given type.
    template <class _ValTy> static _ValTy ConvertTo(const String& str);
    template <class _ValTy> _ValTy ConvertTo(void) const;

    //! Checks if a string is empty or is whitespace.
    static bool IsNullOrSpace(const String& str);
    bool IsNullOrSpace(void) const;

    //! Converts a string to all upper-case.
    static String ToUpper(String str);
    void ToUpper(void);

    // ...
};

How could I implement template <class _ValTy> static String ConvertFrom(_ValTy val);? Because now not only do I need to specify the class template, but the function template too. I am betting the code I'm about to write isn't valid, but it should show what I am trying to accomplish:

MyString.cpp

template <class _Elem, class _Traits, class _Ax>
template <class _ValTy>
String<_Elem, _Traits, _Ax> String<_Elem, _Traits, _Ax>::ConvertFrom(_ValTy val) {
    // Convert value to String and return it...
}

I am not advanced at all with templates. Not only am I very doubtful that the above is valid, it seems cumbersome to write and not very readable. How would I go about implementing the template methods, and the static template methods which returns its own class type? Because I don't want to define them in the header.

Answer 1

Before I answer your question, let me first say: Don't do this. Extend std::string by using free-functions instead, much like the standard library implements many algorithms. Additionally I'd suggest doing it for ranges rather than strings only but that's more subjective.

Also note that std::string avoids implicit conversions to C-strings not to make your life harder, but to protect your code from a wide variety of obscure bugs that can be caused by unexpected implicit conversions. I would think very long and hard about implementing them. Just think about this: It takes you a few extra moments to type.c_str() once when you write the code, and for the rest of eternity anyone who reads your code will immediately know that it's being used as a C-style string and not as a std::string.

To answer your question, just put the code in the header:

//! Converts a value of the given type to a string.
template <class _ValTy> static String ConvertFrom(_ValTy val)
{
    // Code here
}

Finally note that identifiers starting with underscore+capital letter (and many other things starting with _) are reserved for the compiler and thus all bets are off as to your program's functionality.