Why do I get "undefined reference" errors compiling a simple C++ program with gcc?

I try to compile c++ in Ubuntu. I write my code in Gedit, it is the simple hello world project. I go to terminal to run it with gcc helloworld.cc and this message pops up:

/tmp/ccy83619.o: In function `main':
helloworld.cc:(.text+0xa): undefined reference to `std::cout'
helloworld.cc:(.text+0xf): undefined reference to `std::basic_ostream<char, std::char_traits<char> >& std::operator<< <std::char_traits<char> >(std::basic_ostream<char, std::char_traits<char> >&, char const*)'
/tmp/ccy83619.o: In function `__static_initialization_and_destruction_0(int, int)':
helloworld.cc:(.text+0x3d): undefined reference to `std::ios_base::Init::Init()'
helloworld.cc:(.text+0x4c): undefined reference to `std::ios_base::Init::~Init()'
collect2: error: ld returned 1 exit status

What does it mean and where do I go from here?

1 Answer

C++ programs need to be linked with the C++ standard library. Although you could link the standard library manually i.e. gcc -o hello hello.cpp -lstdc++, it's not generally done like that. Instead, you should use g++ in place of gcc, which links libstdc++ automatically.

Ex. given

$ cat hello.cpp
#include <iostream>
int main(void) { std::cout << "Hello world" << std::endl; return 0; }

then

$ gcc -o hello hello.cpp
/tmp/ccty9cjF.o: In function `main':
hello.cpp:(.text+0xa): undefined reference to `std::cout'
hello.cpp:(.text+0xf): undefined reference to `std::basic_ostream<char, std::char_traits<char> >& std::operator<< <std::char_traits<char> >(std::basic_ostream<char, std::char_traits<char> >&, char const*)'
hello.cpp:(.text+0x14): undefined reference to `std::basic_ostream<char, std::char_traits<char> >& std::endl<char, std::char_traits<char> >(std::basic_ostream<char, std::char_traits<char> >&)'
hello.cpp:(.text+0x1c): undefined reference to `std::ostream::operator<<(std::ostream& (*)(std::ostream&))'
/tmp/ccty9cjF.o: In function `__static_initialization_and_destruction_0(int, int)':
hello.cpp:(.text+0x4a): undefined reference to `std::ios_base::Init::Init()'
hello.cpp:(.text+0x59): undefined reference to `std::ios_base::Init::~Init()'
collect2: error: ld returned 1 exit status

whereas

g++ -o hello hello.cpp
$ ./hello
Hello world