How to disallow increment operator when operated multiple times on a object?

Question

How do I overload the increment operator so that this code becomes invalid -

Point p(2, 3);
++++p;

while allowing the following -

Point p(2, 3), a(0, 0), b(1, 4);
a = ++p + b;

Like in C this would be invalid -

int a = 2;
++++a;

Answer 1

One way you could do this is to make your operator++() return a const reference. That would prevent subsequent modification of the returned value, as in a 'chained' ++++p;.

Here's an outline version that also includes the required binary addition operator, implemented as a non-member function (as is normal):

#include <iostream>

class Point {
public:
    int x{ 0 }, y{ 0 };
    Point(int a, int b) : x{ a }, y{ b } {}
    const Point& operator++() { ++x; ++y; return *this; }
    Point& operator+=(const Point& rhs) { x += rhs.x; y += rhs.y; return *this; }
};

inline Point operator+(Point lhs, const Point& rhs)
{
    lhs += rhs;
    return lhs;
}

int main()
{
    Point p(2, 3), a(0, 0), b(1, 4);
    a = ++p + b; // OK: The result of the ++p is passed by value (i.e. a copy)
    std::cout << a.x << " " << a.y << "\n";
//  ++++a; // error C2678: binary '++': no operator found which takes a left-hand operand of type 'const Point'
    return 0;
}