C++ FAQ Celebrating Twenty-One Years of the C++ FAQ!!!
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Section 8:
[8.3] What happens if you return a reference?

The function call can appear on the left hand side of an assignment operator.

This ability may seem strange at first. For example, no one thinks the expression f() = 7 makes sense. Yet, if a is an object of class Array, most people think that a[i] = 7 makes sense even though a[i] is really just a function call in disguise (it calls Array::operator[](int), which is the subscript operator for class Array).

class Array {
  int size() const;
  float& operator[] (int index);

int main()
  Array a;
  for (int i = 0; i < a.size(); ++i)
    a[i] = 7;    // This line invokes Array::operator[](int)