Cannot convert 'this' pointer from 'const Line' to 'Line &' explanation?

Question

This method:

bool Point::Intersects(const Line& line) const {
    return (line.ContainsPoint(*this, false));
}

causes this error: cannot co

Answer 1

The problem is in fact a simple one:

you have a class A, with a non-const method foo(), you are invoking non-const method foo() through a ref to const A.

const A& a = ...;
a.foo();  // failed

That's what const aimed for: a const variable means, it is declared not going to be changed. While foo() "is going to change itself" (as foo() is a non-const method, which means: "I am legal to change thing inside"), that's why compiler complains: you have a const var (a), but you are going to change its content (thru foo())

The way to solve is straight forward, but you should know which way is correct to do:

1) If you are sure that foo() should be legal to be invoked through const ref etc, you should declare it as a const method: A::foo() const {...}

2) If you know that foo() is not appropriate to make const, you should consider

2.1) review "a" to see if it is more appropriate to make non-const, or

2.2) find another const method in A that did the work.

(There are some other way like using mutable, or const_cast, but that's not the way to go for 99.9% of time. So I didn't mention here)