如何从函数运算符（x，y）返回矢量元素的引用

Question

I have a class template which contains a vector of T as a protected member variable. I want to overload the operator() so that it returns a reference of the vector's element in line y and column x.

When i declare the operator() function as:

template <class T>
T & ArrayT<T>::operator()(unsigned int x, unsigned int y)const{
    return buffer[y*width + x];
}

i get C2440 error: 'return': cannot convert from 'const_Ty' to 'T&'

If i declare the operator function as:

template <class T>
const T & ArrayT<T>::operator()(unsigned int x, unsigned int y)const{
    return buffer[y*width + x];
}

then my code compiles but if for example i create a derived templeted class where T is float and i write (obj is an object of the derived class with a member variable vector of floats):

float f=obj(i,j); 
f=pow(f,2);

then nothing seems to happen. The float in the position i,j inside the vector does not change so assume that i dont really work on a reference, because if a reference was returned by the operator () the above lines should change the element in position (i,j) right?

I am new to c++ and i understand i might be doing some very silly mistakes here, but please any kind of help woulb be welcome.

Answer 1

Declare the operator without const , like this:

template <class T>
T & ArrayT<T>::operator()(unsigned int x, unsigned int y) {
    return buffer[y*width + x];
}

Access elements like this:

float & f = obj(i, j);

---

We usually provide const operator (as OP did):

template <class T>
const T & ArrayT<T>::operator()(unsigned int x, unsigned int y) const {
    return buffer[y*width + x];
}

float f = obj(i, j);          // f is copy of current i,j value 

const float & f = obj(i, j);  // f references i,j value, i.e. we can
                              // observe future modifications