Error message "undefined reference to template function passed as template parameter"

Question

When I pass a template function as a template parameter of a base class, the linker complains that it cannot link the function:

#include <stdio.h>  template<int I> inline int identity() {return I;} //template<> inline int identity<10>() {return 20;}  template<int (*fn)()> class Base { public:     int f() {         return fn();     } };  template<int Val> class Derived : public Base<identity<10> > { public:     int f2() {         return f();     } };  int main(int argc, char **argv) {     Derived<10> o;     printf("result: %d\n", o.f2());     return 0; } 

Results in:

$ g++ -o test2 test2.cpp && ./test2 /tmp/ccahIuzY.o: In function `Base<&(int identity<10>())>::f()': test2.cpp:(.text._ZN4BaseIXadL_Z8identityILi10EEivEEE1fEv[_ZN4BaseIXadL_Z8identityILi10EEivEEE1fEv]+0xd): undefined reference to `int identity<10>()' collect2: error: ld returned 1 exit status 

If I comment out the specialization, then the code compiles and links as expected. Also, if I inherit from Base<identity<Val> > instead of Base<identity<10> >, the code works as I expect.

Try here: http://coliru.stacked-crooked.com/a/9fd1c3aae847aaf7

What do I miss?

Answer 1

It seems the problem is a gcc error: the code compiles and links with clang, icc, and the EDG frontend. A potential work-around not changing any of the uses would be the use of a class template identity instead of a function:

template<int I> struct identity {     operator int() { return I; } };  template<typename fn> class Base { public:     int f() {         return fn();     } };