Left shift a specific no. of 1s specific no. of times

I would like to left shift a specific no. of ones a specific no. of times. I am trying something like this

//Left shift 3 1's by 5. 
int a = 0; 
a = 0b(111 << 5) ;  //Error : unable to find numeric literal operator 'operator""b'        
std::cout << a; //Should be 224

Any suggestions on how I can fix the above ? Ideally I would like something like this

int a = 0;
int noOfOnes = 3;
a = 0b(noOfOnes << 5);

I am not sure how I would accomplish the above in C++ ?