Qt and CMake fails with duplicate symbols - c++

Question

I have 3 files in my c++/qt project and I'm using CMake. I'm trying to compile it Here are some code:
CMakeLists contains:
cmake_minimum_required(VERSION 3.8)
project(untitled)
set(CMAKE_CXX_STANDARD 14)
set(CMAKE_PREFIX_PATH /Users/username/Qt/5.9.2/clang_64/)
set(CMAKE_INCLUDE_CURRENT_DIR ON)
set(CMAKE_AUTOMOC ON)
find_package(Qt5Core)
find_package(Qt5Network)
set(SOURCE_FILES main.cpp server.cpp)
add_executable(untitled ${SOURCE_FILES})
target_link_libraries(${PROJECT_NAME} Qt5::Core)
target_link_libraries(${PROJECT_NAME} Qt5::Network)
Main.cpp contains:
#include <iostream>
#include <QCoreApplication>
#include <QtDebug>
#include "server.cpp"
int main(int argc, char *argv[])
{
QCoreApplication app(argc, argv);
MyTcpServer server;
return app.exec();
}
and finally server.cpp contains:
#include <QObject>
#include <QTcpSocket>
#include <QTcpServer>
#include "server.moc"
class MyTcpServer : public QObject
{
Q_OBJECT
public:
explicit MyTcpServer(QObject *parent = 0);
public slots:
void slotNewConnection();
void slotServerRead();
void slotClientDisconnected();
private:
QTcpServer * mTcpServer;
QTcpSocket * mTcpSocket;
};
MyTcpServer::MyTcpServer(QObject *parent) : QObject(parent)
{
...
}
void MyTcpServer::slotNewConnection()
{
...
}
void MyTcpServer::slotServerRead()
{
...
}
void MyTcpServer::slotClientDisconnected()
{
mTcpSocket->close();
}
I'm trying to compile my project with CMake, and when I run the CMake, I have this problems :
duplicate symbol __ZN11MyTcpServer18qt_static_metacallEP7QObjectN11QMetaObject4CallEiPPv in:
CMakeFiles/untitled.dir/main.cpp.o
CMakeFiles/untitled.dir/server.cpp.o
...
duplicate symbol __ZN11MyTcpServer16staticMetaObjectE in:
CMakeFiles/untitled.dir/main.cpp.o
CMakeFiles/untitled.dir/server.cpp.o
ld: 13 duplicate symbols for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
Telling me that there is a duplicate symbol.
How to resolve this?

Answer 1

It is better not to use #include for .cpp files. It is good practice to split definition and declaration into different files.
(One exception from this is the private declaration in case of PIMPL pattern.)
In case you want to avoid the split because you have only small pieces of code use a header file and implement your methods within the definition of the class.
In case a library is implemented: Do not install the header file in case your class must not accessible from outside.

Answer 2

The simple fix for your case would be fo#include "server.cpp" from your cpp as mentioned in comments (it is rarely needs to be done and virtually never is a good thing).
And the second thing would be to move the #include "server.moc" line to the end of the cpp file. This is important because this file contains implementation of some member functions injected with Q_OBJECT and the class needs to be defined before implementing member functions outside of its body.