what does it mean when a function is used as left operand in C++

Question

I'm working with NS2 that is in C++ language. I see the following code that I cannot understand it !!!!

ch->size() += IP_HDR_LEN;

thanks for your help...

Answer 1

The method ch->size() returns reference (lvalue) to something which is used in an expression.

For example:

class A{
 int x;
 public:
  int& getX(){ return x;}
};

then it can be used as

A* a= new A;
a->getX() +=5; // which is equivalent to x+=5 or x=x+5, since getX returns reference to 'x', it can be used as LHS of an expression

So for your question:

what does it mean when a function is used as left operand in C++

Here the return value is used as left operand, not the function. The return type is a reference to something, which represents lvalue, and can be used as LHS.

Edit

As pointed out by @dlf, size() can return an object (by reference or by value) of any class which overloads operator +=. If returned by reference, then it is same as the above example ( just int x becomes MyClass x, int& getX() becomes MyClass& getX() ). If returned by value, although one can, but of no use or lead to bad design ( if MyClasss operator += changes some global state).