Multiple-File Template Implementation

Question

With normal functions, the declaration and definition are often separated across multiple files like so:

// Foo.h

namespace Foo
{
    void Bar();
}

Answer 1

Template code stays in the .hh file. There's no reason to make them separate files, though it is good practice to put the definitions after all the declarations.

When the compiler generates template code, it flags it so that the linker knows that the instantiation of a template in one compilation unit (i.e. .o file) is the exact same code as one in another unit. It will keep one and discard the rest, rather than bailing out with a "multiply defined symbol" error.

This is true of modern compilers with good template support. In the case of GCC, since 2.8. (I know, I wrote a lot of code for gcc 2.7.2.2 before they smartened the linker up and you had to jump through hoops to make templates build right.)