c++ - Template class with virtual member: linker error -

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• what undefined reference/unresolved external symbol error , how fix it? 27 answers

consider following code. abstract, generic class; b both implements , specializes it. code seems trivially correct me, reason end strange linker errors.

template<typename t> class {     public:         virtual void f(); };  class b : public a<int> {     public:         void f() {}; };  int main(int argc, char** argv) {     auto b = new b();     return 0; } 

gcc output:

/tmp/ccxg2z8a.o:(.rodata._ztv1aiie[_ztv1aiie]+0x10): undefined reference `a<int>::foo()' collect2: error: ld returned 1 exit status 

clang output:

/tmp/l2-2a09ab.o: in function `main': l2.cpp:(.text+0x35): undefined reference `operator new(unsigned long)' /tmp/l2-2a09ab.o:(.rodata._zti1aiie[_zti1aiie]+0x0): undefined reference `vtable __cxxabiv1::__class_type_info' /tmp/l2-2a09ab.o:(.rodata._zti1b[_zti1b]+0x0): undefined reference `vtable __cxxabiv1::__si_class_type_info' /tmp/l2-2a09ab.o:(.rodata._ztv1aiie[_ztv1aiie]+0x10): undefined reference `a<int>::foo()' clang: error: linker command failed exit code 1 (use -v see invocation) 

from gcc output, i'll assume function called foo instead of f.

the problem class a not abstract, because haven't declared method such. in way:

virtual void foo() = 0; 

but forgot = 0, linker doesn't know method abstract, , therefore looking function body not there.