This error happens if a unknown object is used.

Variables

Not compiling:

#include <iostream>

int main(int argc, char *argv[])
{
    {
        int i = 2;
    }

    std::cout << i << std::endl; // i is not in the scope of the main function

    return 0;
}

Fix:

#include <iostream>

int main(int argc, char *argv[])
{
    {
        int i = 2;
        std::cout << i << std::endl;
    }

    return 0;
}

Functions

Most of the time this error occurs if the needed header is not included (e.g. using std::cout without #include <iostream>)

Not compiling:

#include <iostream>

int main(int argc, char *argv[])
{
    doCompile();

    return 0;
}

void doCompile()
{
    std::cout << "No!" << std::endl;
}

Fix:

#include <iostream>

void doCompile(); // forward declare the function

int main(int argc, char *argv[])
{
    doCompile();

    return 0;
}

void doCompile()
{
    std::cout << "No!" << std::endl;
}

Or:

#include <iostream>

void doCompile() // define the function before using it
{
    std::cout << "No!" << std::endl;
}

int main(int argc, char *argv[])
{
    doCompile();

    return 0;
}

Note: The compiler interprets the code from top to bottom (simplification). Everything must be at least declared (or defined) before usage.

This linker error happens, if the linker can't find a used symbol. Most of the time, this happens if a used library is not linked against.

qmake:

LIBS += nameOfLib

cmake:

TARGET_LINK_LIBRARIES(target nameOfLib)

g++ call:

g++ -o main main.cpp -Llibrary/dir -lnameOfLib

One might also forget to compile and link all used .cpp files (functionsModule.cpp defines the needed function):

g++  -o binName main.o functionsModule.o

The compiler can't find a file (a source file uses #include "someFile.hpp").

qmake:

INCLUDEPATH += dir/Of/File

cmake:

include_directories(dir/Of/File)

g++ call:

g++ -o main main.cpp -Idir/Of/File