Undefined template function reference

Question

I have three files. The contents of main.cpp:

#include<iostream> #include<QString> #include "util.h" int main() { using Util::convert2QString; using namespace std; int n =22; QString tmp = convert2QString<int>(n); return 0; } 

util.h

 namespace Util { template<class T> QString convert2QString(T type , int digits=0); } 

util.cpp

 namespace Util { template<class T> QString convert2QString(T type, int digits=0) { using std::string; string temp = (boost::format("%1%") % type).str(); return QString::fromStdString(temp); } } 

When I try to compile these files with the following command, I get an undefined reference error link

 vickey@tb:~/work/trash/template$ g++ main.cpp util.cpp -lQtGui -lQtCore -I. -I/usr/local/Trolltech/Qt-4.8.0/include/QtCore -I/usr/local/Trolltech/Qt-4.8.0/include/QtGui -I/usr/local/Trolltech/Qt-4.8.0/include /tmp/cca9oU6Q.o: In function `main': main.cpp:(.text+0x22): undefined reference to `QString Util::convert2QString<int>(int, int)' collect2: ld returned 1 exit status 

Is there something wrong with declaring or implementing a template? why M am I getting these binding errors :?

Answer 1

An implementation of a non-specialized template should be visible to the translation unit that uses it.

The compiler should be able to see the implementation in order to generate code for all the specializations in your code.

This can be achieved in two ways:

1) Move the implementation inside the header.

2) If you want to save it separately, move it to another header, which you will include in your original header:

util.h

 namespace Util { template<class T> QString convert2QString(T type , int digits=0); } #include "util_impl.h" 

util_impl.h

 namespace Util { template<class T> QString convert2QString(T type, int digits=0) { using std::string; string temp = (boost::format("%1") % type).str(); return QString::fromStdString(temp); } } 

Answer 2

You have 2 ways:

• convert2QString in util.h.

• Manually create convert2QString using int in util.cpp and define this specialization as an extern function in util.h

util.h

 namespace Util { template<class T> QString convert2QString(T type , int digits=0); extern template <> QString convert2QString<int>(int type , int digits); } 

util.cpp

  namespace Util { template<class T> QString convert2QString(T type, int digits) { using std::string; string temp = (boost::format("%1") % type).str(); return QString::fromStdString(temp); } template <> QString convert2QString<int>(int type , int digits); }