Chapter 139: Common compile/linker errors (GCC)

Section 139.1: undefined reference to `***'

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

Section 139.2: error: '***' was not declared in this scope

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.

Section 139.3: fatal error: ***: No such file or directory

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