早速はまった

codeはこれ

#include 
using namespace std;

int main()
{
    cout

で出たエラーが。

$ gcc sample.cpp
Undefined symbols for architecture x86_64:
  "std::ios_base::Init::Init()", referenced from:
      __static_initialization_and_destruction_0(int, int)in ccR2BuiH.o
  "std::ios_base::Init::~Init()", referenced from:
      ___tcf_0 in ccR2BuiH.o
  "std::cout", referenced from:
      _main in ccR2BuiH.o
  "std::basic_ostream<char, std::char_traits<char> >& std::operator<< <std::char_traits<char> >(std::basic_ostream<char, std::char_traits<char> >&, char const*)", referenced from:
      _main in ccR2BuiH.o
ld: symbol(s) not found for architecture x86_64
collect2: ld returned 1 exit status

で、

$ gcc -lstdc++ sample.cpp 
$ ls
a.out       helloworld.cpp  sample.cpp
$ ./a.out 
ようこそC++へ! 
C++をはじましょう!

これで解決した。