Why use 'g++' instead of 'gcc' to compile *.cc files?

Question

I compiled a library which used g++ instead of gcc. First I thought the source code was written in C++, but I found out later that there was not any C++ code in the *.cc files.

To confirm this, I replaced the g++ in the original makefile with gcc. And I still got the correct program.

What is the explanation? It was not the first time I met such a situation.

Answer 1

It depends on what exactly you changed in the makefile. gcc / g++ is really just a front-end driver program which invokes the actual compiler and / or linker based on the options you give it.

If you invoke the compiler as gcc:

• it will compile as C or C++ based on the file extension (.c, or .cc / .cpp);
• it will link as C, i.e. it will not pull in C++ libraries unless you specifically add additional arguments to do so.

If you invoke the compiler as g++:

• it will compile as C++ regardless of whether or not the file extension is .c or .cc / .cpp;
• it will link as C++, i.e. automatically pull in the standard C++ libraries.

(see the relevant bit of the GCC documentation).

---

Here's a simple program which detects whether or not it has been compiled as C or C++.

(It makes use of the fact that a character constant has the size of an int in C, or a char in C++. sizeof(char) is 1 by definition; sizeof(int) will generally be larger - unless you're using an obscure platform with >= 16-bit bytes, which you're probably not.)

I've called it test.c and copied it as test.cc as well:

File test.c

#include <stdio.h>

int main(void)
{
  printf("I was compiled as %s!\n", sizeof('a') == 1 ? "C++" : "C");
  return 0;
}

Copy

cp test.c test.cc

Compiling and linking test.c with gcc, and test.cc with g++, works as expected:

$ gcc -o test test.c

$ ./test
I was compiled as C!

$ g++ -o test test.cc

$ ./test
I was compiled as C++!

Compiling and linking test.cc with gcc doesn't work: it compiles the code as C++ because the file ends in .cc, but fails at the link stage:

gcc -o test test.cc

Output:

/tmp/ccyb1he5.o:(.eh_frame+0x11): undefined reference to `__gxx_personality_v0'
collect2: ld returned 1 exit status

which we can prove by separately compiling with gcc, and linking with g++ (to pull in the right libraries):

$ gcc -c test.cc

$ g++ -o test test.o

$ ./test
I was compiled as C++!

...gcc has compiled the code as C++ rather than C, because it had a .cc file extension.

Whereas g++ does not compile .c files as plain C:

$ g++ -o test test.c

$ ./test
I was compiled as C++!