C标识符

The identifier we define in our programs may not contain 2 consecutive
underscores,nor can identifier begin with an underscore followed
immediately by an uppercase letter. In addition,identifiers be fined
outside of a function may not begin with an underscore

一切都很好,但是

int _c = 55;                  // outside function starts with _

int main () {

    int _A = 12;              // _ followed by uppercase letter
    cout << _A << endl;

    int __b__ =33;            // 2 consecutive __
    cout << __b__ << endl;

    cout << _c << endl;

}

上面的代码使用以下标志在mac,g 4.7.1上编译完全正常

g++ -pedantic -Wall -Werror -std=c++11 -O3 -funroll-loops -fprefetch-loop-arrays

我错过了什么？