Linking error with template classes - c++

Question

I have a class that needs to call an external function in a variety of different contexts. I want to keep things flexible, so I'm using an interface (inspired by the 3rd edition of Numerical Recipes) that should work with functors, function pointers, etc. A simplified example looks like:
class MyClass {
public:
template <class T>
MyClass(T &f_) { f = f_; }
private:
int (*f)(int);
};
int myFn(int i) {
return i % 100;
}
int main() {
MyClass test(myFn);
return 0;
}
So far so good; g++ compiles this without any complaints. In my real application, there's a lot more code so I've got things split up among multiple files. For example,
test2.h:
#ifndef __test2__
#define __test2__
class MyClass {
public:
template <class T>
MyClass(T &f_);
private:
int (*f)(int);
};
#endif
test2.cpp:
#include "test2.h"
template <class T>
MyClass::MyClass(T &f_) {
f = f_;
}
main.cpp:
#include "test2.h"
int myFn(int i) {
return i % 100;
}
int main() {
MyClass test(myFn);
return 0;
}
When I try to compile this using g++ test2.cpp main.cpp, I get the following linking error:
/tmp/ccX02soo.o: In function 'main':
main.cpp:(.text+0x43): undefined reference to `MyClass::MyClass<int ()(int)>(int (&)(int))'
collect2: ld returned 1 exit status
It appears that g++ isn't aware of the fact that I'm also trying to compile test2.cpp. Any ideas about what might be going on here?
Thanks,
--craig

Answer 1

Template classes must have their implementation visible to all translation units that use them, unless they are fully specialized.
That means you have to move the implementation in the header:
//test2.h
#ifndef __test2__
#define __test2__
class MyClass {
public:
template <class T>
MyClass(T &f_);
private:
int (*f)(int);
};
template <class T>
MyClass::MyClass(T &f_) {
f = f_;
}
#endif