Mingw not complaining

I'm using mingw gcc(3.xxx) compiler. In the following code I pass a variable 'q'as 'const' but I later use its private members in the '>>'operator (in the same function) which obviously modify it. but the compiler did'nt complain and the programm compiled. Later during execution when I used the '>>' operation, the program halted when it reached that point. Even though I understood the error and corrected it, what I don't understand is why the compiler did'nt yell at me.
Does someone have a clue?

Code:

//quaternion.h
//no #include iostream as it is included in the main file.
using namespace std; //for the istream and ostream

class Quaternion{
private:
    long double a, b, c, d;
public:
    Quaternion(long double aa=0, long double bb=0,
               long double cc=0, long double dd=0):a(aa), b(bb), c(cc), d(dd){}
    virtual ~Quaternion(){}
    friend ostream& operator <<(ostream& out, const Quaternion& q){
        out<<q.a<<"+"<<q.b<<"i+"<<q.c<<"j+"<<q.d<<"k";
        return out;
    }
    friend istream& operator >>(istream& in, constQuaternion& q){
        in>>q.a>>q.b>>q.c>>q.d;//modifies q's members
        return in;
    }
    Quaternion operator +(const Quaternion q){
        return Quaternion(a+q.a, b+q.b, c+q.c, d+q.d);
    }
    Quaternion operator -(const Quaternion q){
        return Quaternion(a-q.a, b-q.b, c-q.c, d-q.d);
    }
    void set(long double aa=0, long double bb=0,
             long double cc=0, long double dd=0){
        //blah blah
    }
    long double& operator [](unsigned int i){ //0 for 'a' component
       //blah blah
    }
    Quaternion operator *(const Quaternion& q){
        //blahblah
    }
};

Code:

//main.cpp
#include<iostream>
#include"../quaternionfractal/quaternion.h"
using namespace std;

int main(){
Quaternion x, y;
cout<<"x :";
[b]cin>>x; //when here the program halts and terminates without any complaining.
cout<<"y :";
cin>>y;
cout<<"x   :"<<x<<"\n"
    <<"y   :"<<y<<"\n"
    <<"x*y :"<<x*y<<"\n"
    <<"y*x :"<<y*x<<"\n";
return 0;[b]
}

Maybe you don't have all your compiler warnings on.