error: LNK2019: unresolved external symbol in Qt

Question

I have two module in Qt
1. SapPackets : lib
2. SapApplication : app
pro file for both module
SapPackets.pro has Qt -= gui
SapApplication.pro has Qt += core gui xml

Target OS is Windows 7

there is a class SapEntity in SapPacket.lib

#ifndef SAPENTITYCLASS_HPP
#define SAPENTITYCLASS_HPP
#include <QString>

namespace Sap
{
    namespace Entity
    {
        class SapEntityClass
        {
            protected:
                unsigned short mush_Id; /* Entity Id */
                QString msz_title; /* Entity Title */
            public:
                SapEntityClass(const unsigned short Id,const QString title);
                unsigned short GetId() const;
                QString GetTitle() const;
        };
    }
}
#endif

implementation file of SapEntity is

#include "SapEntityClass.hpp"
using namespace Sap::Entity;

SapEntityClass::SapEntityClass(const unsigned short Id,const QString title)
:mush_Id(Id),msz_title(title)
{}

inline
unsigned short SapEntityClass::GetId() const
{
     return mush_Id;
}

inline
QString SapEntityClass::GetTitle() const
{
    return msz_title;
}

SapApplication.pro has following line for adding SapPackets.lib

win32:CONFIG(release, debug|release): LIBS += -L$$PWD/../SapPackets_Build/release/ -    lSapPackets
else:win32:CONFIG(debug, debug|release): LIBS += -L$$PWD/../SapPackets_Build/debug/ -lSapPackets
else:unix: LIBS += -L$$PWD/../SapPackets_Build/ -lSapPackets

INCLUDEPATH += $$PWD/../SapPackets
DEPENDPATH += $$PWD/../SapPackets

Main function in SapApplication

#include <iostream>
#include "SapEntityClass.hpp"
using namespace Sap::Entity;
int main(int argc, char *argv[])
{
     SapEntityClass obj(56,"Sample");
     std::cerr<<obj.GetId();
     return 0;
}

Problem: I am getting following error on Compilationa

main.obj:-1: error: LNK2019: unresolved external symbol "public: unsigned short   
__thiscall Sap::Entity::SapEntityClass::GetId(void)const " (?   
GetId@SapEntityClass@Entity@Sap@@QBEGXZ) referenced in function _main

Please help me to resolve this....

Answer 1

Edit:Why do you inline the method in the implementation file? Inlined functions must have visible definition together with their declaration. GCC reports same linker error for inlined method in implementation file, so I think that is your problem - remove inline in method definition or move it to the header.

Old answer: Well you posted only header with Sap::Entity::SapEntityClass::GetId() method declaration. Where is definition? It seems it is not implemented or at least not linked to your application.

Answer 2

This can happen when the linker finds the function definition, but not the implementation, which is most likely compiled into the library.

Try adding

LIBS += -lSapPacket

to your SapApplication.pro file unless it's there already.

This tells your linker that there is a library called SapPacket.lib (on Windows; file extension would be different on other OSs), which contains implementations of some functions.