title: g++ output: file not recognized: File format not recognized
body: <p>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:</p>
  
  <pre><code>$ 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
  </code></pre>
  
  <p>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</p>