g++ output: file not recognized: File format not recognized

Question

I am trying to build program with multiple files for the first time. I have never had any problem with compliling program with main.cpp only. With following commands, this i

Answer 1

This is wrong:

 g++ -c src/CNumber.cpp src/CNumber.h -o src/CNumber.o

You shouldn't "compile" .h files. Doing so will create precompiled header files, which are not used to create an executable. The above should simply be

 g++ -c src/CNumber.cpp -o src/CNumber.o

Similar for compiling the other .cpp files

Answer 2

I ran into this error in building something - it turned out to be due to a previous build failing while compiling a source file to an .o file - that .o file was incomplete or corrupted, so when I tried another build it gave this error on that file.

The solution was to just delete the .o file (or run make clean, if you have a makefile with that target).

Answer 3

Try putting all of the following files in one directory:

example.cpp:

#include<iostream>
#include<string>

#include "my_functions.h"

using namespace std;

int main()
{
    cout << getGreeting() << "\n";

    return 0;
}

my_functions.cpp:

#include<string>
using namespace std;

string getGreeting()
{
    return "Hello world";
}

my_functions.h:

#ifndef _MY_FUNCTIONS_H
#define _MY_FUNCTIONS_H

#include<string>
using namespace std;

string getGreeting();

#endif

Then issue these commands:

$ g++ example.cpp my_functions.cpp -o myprogram
~/c++_programs$ ./myprogram
Hello world