问 unique_ptr ：编译错误？

#include<memory>
using namespace std;

int  main()
{
    std::unique_ptr<int> p1(new int(5));
    return 0;
}

$ gcc main.cpp 
main.cpp: In function ‘int main()’:
main.cpp:6:2: error: ‘unique_ptr’ was not declared in this scope
main.cpp:6:13: error: expected primary-expression before ‘int’
main.cpp:6:13: error: expected ‘;’ before ‘int’

$ gcc --version
gcc (Ubuntu/Linaro 4.6.1-9ubuntu3) 4.6.1