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

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 is the result:
$ g++ -c src/CNumber.cpp src/CNumber.h -o src/CNumber.o
$ g++ -c src/CExprPart.cpp src/CExprPart.h -o src/CExprPart.o
$ g++ -c src/CExpr.cpp src/CExpr.h -o src/CExpr.o
$ g++ -c src/main.cpp -o src/main.o
$ g++ src/CNumber.o src/CExprPart.o src/CExpr.o src/main.o -o execprogram
src/CNumber.o: file not recognized: File format not recognized
collect2: error: ld returned 1 exit status
What could cause such error and what should I do with it?
Using Linux Mint with gcc (Ubuntu/Linaro 4.7.2-2ubuntu1).
Thank you

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