g++ can't find this method even though it's included - c++

Question

I am using FANN (http://leenissen.dk/fann/wp/download/). Both headers are available in the archive that you can download there (I hope I don't create too much trouble for you).
/*
* File: main.cpp
* Author: johannsebastian
*
* Created on November 26, 2013, 8:50 PM
*/
#include "../FANN-2.2.0-Source/src/include/doublefann.h"
#include "../FANN-2.2.0-Source/src/include/fann_cpp.h"
#include <cstdlib>
#include <iostream>
using namespace std;
using namespace FANN;
/*
*
*/
//Remember: fann_type is double!
int main(int argc, char** argv) {
neural_net * N = new neural_net;
const unsigned int myLayerArray[4] = {1,2,2,1};
const unsigned int numLayers = 4;
N->create_standard(3,1,2,1);
cout<<"Easy!";
return 0;
}
g++ says
"/usr/bin/make" -f nbproject/Makefile-Debug.mk QMAKE= SUBPROJECTS= .build-conf
make[1]: Entering directory `/home/johannsebastian/Desktop/heatmap3d/BACHnet/BACHnet'
"/usr/bin/make" -f nbproject/Makefile-Debug.mk dist/Debug/GNU-Linux-x86/bachnet
make[2]: Entering directory `/home/johannsebastian/Desktop/heatmap3d/BACHnet/BACHnet'
mkdir -p build/Debug/GNU-Linux-x86
rm -f "build/Debug/GNU-Linux-x86/main.o.d"
g++ -c -g -MMD -MP -MF "build/Debug/GNU-Linux-x86/main.o.d" -o build/Debug/GNU-Linux-x86/main.o main.cpp
mkdir -p dist/Debug/GNU-Linux-x86
g++ -o dist/Debug/GNU-Linux-x86/bachnet build/Debug/GNU-Linux-x86/main.o
build/Debug/GNU-Linux-x86/main.o: In function `FANN::neural_net::destroy()':
/home/johannsebastian/Desktop/heatmap3d/BACHnet/BACHnet/../FANN-2.2.0-Source/src/include/fann_cpp.h:915: undefined reference to `fann_get_user_data'
/home/johannsebastian/Desktop/heatmap3d/BACHnet/BACHnet/../FANN-2.2.0-Source/src/include/fann_cpp.h:919: undefined reference to `fann_destroy'
build/Debug/GNU-Linux-x86/main.o: In function `FANN::neural_net::create_standard_array(unsigned int, unsigned int const*)':
/home/johannsebastian/Desktop/heatmap3d/BACHnet/BACHnet/../FANN-2.2.0-Source/src/include/fann_cpp.h:979: undefined reference to `fann_create_standard_array'
collect2: error: ld returned 1 exit status
make[2]: *** [dist/Debug/GNU-Linux-x86/bachnet] Error 1
make[2]: Leaving directory `/home/johannsebastian/Desktop/heatmap3d/BACHnet/BACHnet'
make[1]: *** [.build-conf] Error 2
make[1]: Leaving directory `/home/johannsebastian/Desktop/heatmap3d/BACHnet/BACHnet'
make: *** [.build-impl] Error 2
BUILD FAILED (exit value 2, total time: 773ms)
Can you help me fix this? I think I might have some basic compiler config wrong or something.

Answer 1

The compiler has compiled your code. The includes are really work. (But you don't have to specify full path there, just the name of the header, if the library is installed correctly).
This is the linker (ld) who can't find the method.
The linker problems about whether you have library with the method definition and give it to the linker (in its options) when you call it.
You use make file, so you have to specify options (something like -lFANN) and path to the library there (like -L/path/path/path. The path you also can specified in $LD_LIBRARY_PATH system variable, then you don't need -L option). Also you have to install (compile) the library itself before to use it. You have installation guide here.
The guide works for me:
Download the FANN-2.2.0-Source.
Install it with cmake:
[08:42:24]~/Downloads$ cd FANN-2.2.0-Source/
[08:42:27]~/Downloads/FANN-2.2.0-Source$ ls
CMakeLists.txt README.txt bin datasets src
COPYING.txt VS2010 cmake examples
[08:42:27]~/Downloads/FANN-2.2.0-Source$ cmake .
-- The C compiler identification is GNU 4.7.3
-- The CXX compiler identification is GNU 4.7.3
-- Checking whether C compiler has -isysroot
-- Checking whether C compiler has -isysroot - yes
-- Checking whether C compiler supports OSX deployment target flag
-- Checking whether C compiler supports OSX deployment target flag - yes
-- Check for working C compiler: /opt/local/bin/gcc
-- Check for working C compiler: /opt/local/bin/gcc -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Checking whether CXX compiler has -isysroot
-- Checking whether CXX compiler has -isysroot - yes
-- Checking whether CXX compiler supports OSX deployment target flag
-- Checking whether CXX compiler supports OSX deployment target flag - yes
-- Check for working CXX compiler: /<...>g++
-- Check for working CXX compiler: /<...>g++ -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- FANN is used as APPLICATION_NAME
-- Configuring done
-- Generating done
-- Build files have been written to: /Users/ikulakov/Downloads/FANN-2.2.0-Source
[08:43:57]~/Downloads/FANN-2.2.0-Source$ make
Scanning dependencies of target doublefann
[ 25%] Building C object src/CMakeFiles/doublefann.dir/doublefann.c.o
Linking C shared library libdoublefann.dylib
[ 25%] Built target doublefann
Scanning dependencies of target fann
[ 50%] Building C object src/CMakeFiles/fann.dir/floatfann.c.o
Linking C shared library libfann.dylib
[ 50%] Built target fann
Scanning dependencies of target fixedfann
[ 75%] Building C object src/CMakeFiles/fixedfann.dir/fixedfann.c.o
Linking C shared library libfixedfann.dylib
[ 75%] Built target fixedfann
Scanning dependencies of target floatfann
[100%] Building C object src/CMakeFiles/floatfann.dir/floatfann.c.o
Linking C shared library libfloatfann.dylib
[100%] Built target floatfann
[08:44:02]~/Downloads/FANN-2.2.0-Source$ sudo make install

Answer 2

In your case, not the Compiler, but the Linker fails to find the function.
The compiler is what you satisfy with including the files containing the declarations of the functions you use, and specifying the directory for the included files.
The linker wants to see the actual library containing the compiled definition of the code.
You probably have to specify -lfann on the linker command line (for the exact name to use after -l, refer to what the library is actually called - if it is called libfann.so, use above parameter).
In case the library is installed in the default directories, thish should be enough; otherwise you might also need the -L parameter pointing to the path containing the library (and the library would have to be compiled first, of course).