Question

Lets say I have the following:

class MyClass
{
    // ...
}

void doSomething (MyClass instance)
{
    // Is instance passed by reference or by value (copied)?
}

void main ()
{
    MyClass instance = MyClass();

    doSomething(instance);
}

In doSomething() is instance passed by reference? Or is the class duplicated in memory? Or something else?

Answer 1

This is passed by value

void doSomething (MyClass instance)

This is passed by reference

void doSomething (MyClass& instance)

This is passed by const reference

void doSomething (const MyClass& instance)

Also MyClass doesn’t need to by constructed by assignment.
So:

MyClass mc=MyClass();

is effectively the same as:

MyClass mc; //no parens needed for default constructor (no args).

EDIT:
This is passed by const reference to a const function, a const function can be called on a const object as it guarantees not to modify the object state.

void doSomething (const MyClass& instance) const

Const correctness is considered good practice in C++ unlike many less strict languages.

see me:

http://en.wikipedia.org/wiki/Const-correctness

http://www.gotw.ca/gotw/006.htm