Problem when compiling c++ Singleton

Question

When I’m trying to compile a singleton I always get the following return message:

g++ -g -Wall pkg-config --cflags stage -fPIC pkg-config --libs stage -c -Wall Connection.cpp cc Connection. o -o Connection /usr/lib/gcc/x86_64-linux-gnu/6/.. /.. /.. /x86_64-linux-gnu/Scrt1.o: in function _start': (.text+0x20): referência indefinida paramain' Connection. o: in function Connection::Connection()': /home/orion/Documentos/Workspace/AngoritmoTCC/Aplicacao-Tcc-Refazer/Connection.cpp:16: referência indefinida paravtable for Connection' Connection. o: in function Connection::getInstance()': /home/orion/Documentos/Workspace/AngoritmoTCC/Aplicacao-Tcc-Refazer/Connection.cpp:20: referência indefinida paraConnection:'sinstance' /home/Orion/Documents/Workspace/Angoritmotcc/Application-Tcc-Redo/Connection.cpp:21: undefined reference to operator new(unsigned long)' /home/orion/Documentos/Workspace/AngoritmoTCC/Aplicacao-Tcc-Refazer/Connection.cpp:21: referência indefinida paraConnection:'sinstance' /home/Orion/Documents/Workspace/Angoritmotcc/Application-Tcc-Redo/Connection.cpp:22: undefined reference to Connection::sinstance' /home/orion/Documentos/Workspace/AngoritmoTCC/Aplicacao-Tcc-Refazer/Connection.cpp:21: referência indefinida paraOperator delete(void*, unsigned long)' Connection. o:(.data.DW.ref.__gxx_personality_v0[DW.ref.__gxx_personality_v0]+0x0): undefined reference to `__gxx_personality_v0' collect2: error: Ld returned 1 Exit status : Recipe for target 'Connection' failed make: *** [Connection] Error 1

This way, I can’t compile, and I need to turn it into a file. So, to run using a library. Can someone help me?

Edited

Connection.cpp file

Connection::Connection() {
}

static Connection *Connection::getInstance() {
    if (!Connection::sinstance)
        Connection::sinstance = new Connection();
    return Connection::sinstance;
}

File Connection. h

#ifndef CONNECTION_H
#define CONNECTION_H

class Connection {
public:
    static Connection *getInstance();
    Connection();
    virtual ~Connection();


private:
    static Connection *sinstance;
};

#endif /* CONNECTION_H */

Main.cpp

int main(){
    ControllerPrincipal *controller = new ControllerPrincipal();
}

Makefile

COMMON_DIR = ../common

run: all

all: coordination.so createScenario

createScenario: createScenario.cpp
    $(CXX) createScenario.cpp -o createScenario

coordination.so: ControllerPrincipal.o main.o Connection
    $(CXX) $(CXXFLAGS) $(LINKFLAGS) main.o ControllerPrincipal.o 
Connection.o -o coordination.so -shared

ControllerPrincipal.o: ControllerPrincipal.cpp ControllerPrincipal.h
    $(CXX) $(CXXFLAGS) $(LINKFLAGS) -c ControllerPrincipal.cpp

main.o: main.cpp
    $(CXX) $(CXXFLAGS) $(LINKFLAGS) -c main.cpp

Connection.o: Connection.cpp Connection.h
    $(CXX) $(CXXFLAGS) $(LINKFLAGS) -c -Wall Connection.cpp


clean:
    @rm -f *.o *.so  server createScenario

reset:
    rm -rf nRobos*

Answer 1

Basically, you are trying to compile a file to become an executable. It is not being compiled for library (neither static nor dynamic), using an object file as an intermediate.

Analyzing the executed commands:

g++ Connection.cpp -c

Basically being told to the compiler g++ to do partial file compilation Connection.cpp, resulting therefore in the object file Connection.o.

cc Connection.o -o Connection

cc is usually a nickname for the gcc. When used to pass object files and no flag indicating the desired compile target, it generates a single executable file containing all the information of the object files passed as argument. The output is usually the file a.out, but while using -o Connection, you are directing the exit to Connection.

To the gcc (and g++), the executable must necessarily have a function main described in any of the included object files. The absence of this function causes an error when linking the executable.

In your case, Connection seems to be a class to be imported into the project, it does not seem to be where the function main. When using the command cc Connection.o -o Connection, you wouldn’t be passing the function main, generating the error described above.

Since the code wasn’t posted, that’s as much as I can deduce based on past information.

---

I also recommend using the g++ to make the final build, so that the standard C++ library is also included for Runtime

Answer 2

I decided, the way the code looks is this.

static Connection& getInstance(int numbRobot) {
    static Connection instance(numbRobot); // Guaranteed to be destroyed.
    // Instantiated on first use.
    return instance;
}