g++ compile error

I'm a newbie in Ubuntu. I tried to compile a simple "Hello World!" c++ code in Ubuntu 11.04, with that code (in terminal):

gcc -Wall -W -Werror tex.cpp -o tex. 

but compiler returned a lot of errors :

/tmp/ccL8c1p8.o: In function `main':
tex.cpp:(.text+0xa): undefined reference to `std::cout'
tex.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*)'
/tmp/ccL8c1p8.o: In function `__static_initialization_and_destruction_0(int, int)':
tex.cpp:(.text+0x3d): undefined reference to `std::ios_base::Init::Init()'
tex.cpp:(.text+0x42): undefined reference to `std::ios_base::Init::~Init()'
collect2: ld returned 1 exit status
mohrd@mio:~$ gcc -Wall -W -Werror tex.cpp -o tex.
/tmp/ccEke5JP.o: In function `main':
tex.cpp:(.text+0xa): undefined reference to `std::cout'
tex.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*)'
/tmp/ccEke5JP.o: In function `__static_initialization_and_destruction_0(int, int)':
tex.cpp:(.text+0x3d): undefined reference to `std::ios_base::Init::Init()'
tex.cpp:(.text+0x42): undefined reference to `std::ios_base::Init::~Init()'
collect2: ld returned 1 exit status

simple code :

#include <algorithm>
#include <iostream>
using namespace std;

int main ()
{
  std::cout << "Hello World!";
  return 0;
}

Why? and what should I do???

many thanks ...