template class and free function namespace problems with implementation

问题

I have a class defined like this (trivial includes and preprocessor wrappers, etc. are omitted, but they are there):

In Matrix.h

namespace matrixmanip {
template<typename T>
class Matrix {
    public:
        void someFunc() const;
    //...
};

template<typename T>
void Matrix<T>::someFunc() const {
    //...
}
} // namespace matrixmanip

In bitTransform.h

#include "Matrix.h"

namespace matrixmanip {
Matrix<char> bitExpand(const Matrix<char> &mat);
} // namespace matrixmanip

In bitTransform.cpp

#include "bitTransform.h"

using namespace matrixmanip;

Matrix<char> bitExpand(const Matrix<char> &mat) {
    mat.someFunc();
    //...
}

In tester.cpp

#include "Matrix.h"
#include "bitTransform.h"

matrixmanip::Matrix<char> A ... // construct a character matrix, details of which are unimportant here
matrixmanip::Matrix<char> B = matrixmanip::bitExpand(A);

However, when I compile and link like this:

g++ -c tester.cpp
g++ -c bitTransform.cpp
g++ -Wall -O2 tester.o bitTransform.o -o tester

I get an undefined reference error, specifically

/tmp/ccateMEK.o: In function `main':
tester.cpp:(.text+0xbf9): undefined reference to `matrixmanip::bitExpand(matrixmanip::Matrix<char> const&)'
collect2: error: ld returned 1 exit status

Why am I getting this error? It seems to me that my namespace resolution is okay and my linkage is fine...

回答1:

bitExpand defines a free function in global namespace, not in matrixmanip namespace. using directive does not move definitions into the namespace being used. You should put definition in proper namespace directly:

namespace matrixmanip
{
    Matrix<char> bitExpand(const Matrix<char> &mat)
    {
        mat.someFunc();
       //...
    }
}

来源：https://stackoverflow.com/questions/53578238/template-class-and-free-function-namespace-problems-with-implementation

标签

c++

linker

namespaces

undefined-reference