Making a generic function in C++

I try to make a generic function in C++, that creating an array of a type which depends on usage (when calling it), like array of int or char.

This is the code in my header file:

template<class T&g开发者_StackOverflow社区t;
T** makeArray(...);

This is in the .cpp file:

template<class T>
T** ClassA::makeArray(...){
    //...
}

And this is how I call it in the main file:

char** charArr = cg.makeArray<char>(...);
//...
int** intArr = cg.makeArray<int>(...);

But when compiling, I get these errors:

undefined reference to `char** ClassA::makeArray<char>( ...
undefined reference to `int** ClassA::makeArray<int>( ...

What do the errors mean and how do I fix it?

You have to include the entire body of a C++ template function in the header.

This is because of the way that C++ templates work: template parameter substitution happens only when you actually use the template, and the entire template definition has to be visible at that time. Another way of looking at it is to say that you cannot compile and link to a templateFunction<T>—you can only compile a template function once it's been instantiated as a templateFunction<char> or whatever. For this reason, you cannot put template function definitions in .cpp files, but rather have to put them in .h files.

The compiler needs to have access to the entire template definition (not just the signature) in order to generate code for each instantiation of the template, so you need to move the definitions of the functions to your header.

This is called the inclusion model. Also read this FAQ

In general, you cannot have template definitions in a source file; they should be in the header file so that the compiler can see them.

This comes up over and over again here, so I can't be bothered to give the full explanation again; I recommend you read this from the C++ FAQ: Why can't I separate the definition of my templates class from its declaration and put it inside a .cpp file?.

[Incidentally, why are you returning a pointer-to-pointer? A pointer should be enough...]

If your code is using the template on a diferent cpp file, the template should go on the header file.