Define operator[]() (array subscription) for template class outside of class definition

Question

I thought this would be easy, but it's not working the way I expected. What is the correct syntax here?

TemplateClass.h

template <typename T> 
class TemplateClass
{
  T & operator[](size_t n);

TemplateClass.cpp

#include "TemplateClass.h"

template <typename T>
T & TemplateClass::operator[](size_t n)
{
  // member declaration not found
}

Are you sure you want to do this? With templates, you normally define the entire thing in the header and don't have a .cpp file at all. — Christian Hackl, Feb 24 '14 at 19:41
Ah, because the linker needs to find the definition when it instantiates the template? Good point. — Sharp Steel Software, Feb 24 '14 at 19:43
This is an FAQ: http://www.parashift.com/c++-faq/templates-defn-vs-decl.html — Christian Hackl, Feb 24 '14 at 19:48
@ChristianHackl There's also a popular post here on SO: [Why can templates only be implemented in the header file?](http://stackoverflow.com/questions/495021/why-can-templates-only-be-implemented-in-the-header-file) — πάντα ῥεῖ, Feb 24 '14 at 19:51

score 7 · Accepted Answer · edited Feb 24 '14 at 19:41

7

You need to provide the whole class name – including template arguments:

template <typename T>
T & TemplateClass<T>::operator[](size_t n)
{
  // ...
}

(Also note that the scope resolution operator is ::, not :.)

edited Feb 24 '14 at 19:41

maxywb

answered Feb 24 '14 at 19:37

Konrad Rudolph

4

... and don't you put the implementation into a separate compilation unit! – πάντα ῥεῖ Feb 24 '14 at 19:46

1 Answers1