'const Obj' 和 'Obj &' 是怎么回事？

Question

I want to write a class that is compatible for std::set, so I overload the "less than" operator like this. It works.

bool Segment::SVertex::operator<(const SVertex &rhs) const
{
    return id < rhs.id;
}

However, since I write Java more than C++, 'rhs.id' looks very uncomfortable to me. So I change it to 'rhs.getId()' where getId() is just a normal getter function:

long SVertex::getId(){return id;}

This turns out to be a compile time error: can't convert "this" pointer from "const Segment::SVertex" to "Segment::SVertex &"

(note that my VS2008 is not English and I translated the error message, so the line above is not necessarily exact)

I only know that '&' denotes passing by reference and 'const' forbids any changes. I don't exactly understand what's going on behind the scene.

Answer 1

the problem is your getter function isn't exactly 'normal'; your getter should be

long SVertex::getId() const
{return id;}

Note the const at the end: it tells the compiler/users that this method does not modify the class' internals. So, it's possible to use it in your operator < which is also const. Const member functions can only call other member functions if they are also const; Non-const member functions can call any.

Answer 2

Your getter must be constant:

long SVertex::getId() const {return id;}

This signals that it doesn't change the object.

Answer 3

Your rhs is const , so you are not allowed to change it. Your getId on the other hand doesn't promise not to change this - it also needs to be const:

int SVertex::getId() const { return id; }