Still trying to get a grip on Boost with some toy problems. I have a very simple program that multiplies two vectors. It worked as a single file program, but doesn't work as a multi file program. I'm finding it difficult to deal with impressive looking templates in the compiler error messages. This is on Linux, g++, Debian testing. 09-prob.h: ======= #pragma once #include #include #include using namespace boost::numeric::ublas; // n-vector outer product: |u> typename vector_matrix_binary_traits >::result_type mult_vector_vector( E1 bra, E2 ket ); 09-prob.cpp: ========= #include "09-prob.h" template typename vector_matrix_binary_traits >::result_type mult_vector_vector( E1 bra, E2 ket ) { return outer_prod(bra, ket); } 09-driver.cpp: ========== #include "09-prob.h" using std::cout; using std::endl; int main( void ) { using boost::numeric::ublas::vector; // Use Boost's vector, not STL's. vector<double> u(3), v(3); for( unsigned i = 0; i < u.size(); ++i ) { u(i) = static_cast<double>(i); v(i) = u(i) * u(i); } // Test mult_vector_vector // cout << mult_vector_vector(u, v) << endl; return 0; } And the error message: g++ -g3 -Wall -Wextra -Waggregate-return -Wpointer-arith -Weffc++ -Wcast-qual -Wcast-align -Wmissing-declarations -Wredundant-decls -Wwrite-strings -Winline -c -o 09-driver.o 09-driver.cpp g++ 09-prob.o 09-driver.o -o 09-prob 09-driver.o: In function `main': /home/p/Baruch/numla/hw1/09-driver.cpp:29: undefined reference to `boost::numeric::ublas::vector_matrix_binary_traits >, boost::numeric::ublas::vector >, boost::numeric::ublas::scalar_multiplies >::value_type, boost::numeric::ublas::vector >::value_type> >::result_type mult_vector_vector >, boost::numeric::ublas::vector > >(boost::numeric::ublas::vector >, boost::numeric::ublas::vector >)' collect2: ld returned 1 exit status make: *** [09-prob] Error 1 Since 09-driver.o and 09-prob.o are being linked together, I'm guessing there's something wrong with the function signature? Can some kind soul give me a few hints on how to make this work? Thanks! ~