Why gcc and g++ Behave Differently: A Practical Guide to Compiling C and C++ with GCC

GCC and G++ overview

GCC 10.2 (released September 2020) supports many languages; this guide focuses on compiling C and C++ source files.

Driver programs

Both gcc and g++ are front‑ends to the same compiler suite. By default they infer the language from the file extension. The -x option forces a specific language, e.g. gcc -xc file.c or gcc -xc++ file.cpp.

xxx.c → compiled as C

xxx.cpp → compiled as C++

xxx.m → compiled as Objective‑C

xxx.go → compiled as Go

Difference when compiling C code

Example C source (demo.c):

#include <stdio.h>
int main(){
    const char *a = "abc";
    printStr(a);
    return 0;
}
int printStr(const char* str){
    printf(str);
}

Compiling with gcc -xc demo.c succeeds, while g++ demo.c reports errors because the C++ front‑end applies stricter language rules.

Compiling C++ code

Example C++ source (demo.cpp):

#include <iostream>
#include <string>
using namespace std;
int main(){
    string str = "C语言中文网";
    cout << str << endl;
    return 0;
}

Using g++ demo.cpp automatically links the C++ standard library and builds without errors.

Using gcc demo.cpp fails with undefined references (e.g., std::allocator) because gcc does not link -lstdc++ by default.

Compiling C++ with gcc

To compile C++ source with gcc, explicitly request C++ mode and link the C++ runtime:

gcc -xc++ -lstdc++ -shared-libgcc demo.cpp

This command is functionally equivalent to g++ demo.cpp.

Practical recommendation

Use gcc for pure C programs.

Use g++ for C++ programs.

If gcc must be used for C++, add -xc++ -lstdc++ -shared-libgcc (or at least -lstdc++) to obtain the same behavior as g++.