C++ FAQ Celebrating Twenty-One Years of the C++ FAQ!!!
(Click here for a personal note from Marshall Cline.)
Section 35:
35.1 What's the idea behind templates?
35.2 What's the syntax / semantics for a "class template"?
35.3 What's the syntax / semantics for a "function template"?
35.4 How do I explicitly select which version of a function template should get called?
35.5 What is a "parameterized type"?
35.6 What is "genericity"?
35.7 My template function does something special when the template type T is int or std::string; how do I write my template so it uses the special code when T is one of those specific types?
35.8 Huh? Can you provide an example of template specialization that doesn't use foo and bar?
35.9 But most of the code in my template function is the same; is there some way to get the benefits of template specialization without duplicating all that source code?
35.10 All those templates and template specializations must slow down my program, right?
35.11 So templates are overloading, right?
35.12 Why can't I separate the definition of my templates class from its declaration and put it inside a .cpp file?
35.13 How can I avoid linker errors with my template functions? Updated!
35.14 How does the C++ keyword export help with template linker errors? Updated!
35.15 How can I avoid linker errors with my template classes? Updated!
35.16 Why do I get linker errors when I use template friends?
35.17 How can any human hope to understand these overly verbose template-based error messages?
35.18 Why am I getting errors when my template-derived-class uses a nested type it inherits from its template-base-class?
35.19 Why am I getting errors when my template-derived-class uses a member it inherits from its template-base-class?
35.20 Can the previous problem hurt me silently? Is it possible that the compiler will silently generate the wrong code?
35.21 How can I create a container-template that allows my users to supply the type of the underlying container that actually stores the values?
35.22 Follow-up to previous: can I pass in the underlying structure and the element-type separately?
35.23 Related: all those proxies must negatively reflect on the speed of my program. Don't they?
[35.13] How can I avoid linker errors with my template functions? Updated!

This answer will be updated due to C++11 extern template. Watch this space for updates in the near future!!

Tell your C++ compiler which instantiations to make while it is compiling your template function's .cpp file.

As an example, consider the header file foo.h which contains the following template function declaration:

// File "foo.h"
template<typename T>
extern void foo();
Now suppose file foo.cpp actually defines that template function:
// File "foo.cpp"
#include <iostream>
#include "foo.h"

template<typename T>
void foo()
{
  std::cout << "Here I am!\n";
}
Suppose file main.cpp uses this template function by calling foo<int>():
// File "main.cpp"
#include "foo.h"

int main()
{
  foo<int>();
  ...
}
If you compile and (try to) link these two .cpp files, most compilers will generate linker errors. There are two solutions for this. The first solution is to physically move the definition of the template function into the .h file, even if it is not an inline function. This solution may (or may not!) cause significant code bloat, meaning your executable size may increase dramatically (or, if your compiler is smart enough, may not; try it and see).

The other solution is to leave the definition of the template function in the .cpp file and simply add the line template void foo<int>(); to that file:

// File "foo.cpp"
#include <iostream>
#include "foo.h"

template<typename T> void foo()
{
  std::cout << "Here I am!\n";
}

template void foo<int>();
If you can't modify foo.cpp, simply create a new .cpp file such as foo-impl.cpp as follows:
// File "foo-impl.cpp"
#include "foo.cpp"

template void foo<int>();
Notice that foo-impl.cpp #includes a .cpp file, not a .h file. If that's confusing, click your heels twice, think of Kansas, and repeat after me, "I will do it anyway even though it's confusing." You can trust me on this one. But if you don't trust me or are simply curious, the rationale is given earlier.