🚀 go-pugleaf

hvaisane@cs.joensuu.fi (H. Vaisanen) writes:

>If X is a class, what is the difference between constructors

>X(X&) and X(const X&) ?

The first may change the parameter's value, the second may not.  I would
expect the first to be the less common usage.

>Example:

>class Foo {
>public:
>  Foo (Foo &f); // This is non-const
>};


>class Bar {
>  Foo _f;
>public:
>  Foo f() const {return _f;}; // Trouble here!
>};

>generates warnings
>a.C: In method `class Foo Bar::f ()const ':
>a.C:11: warning: conversion to reference of read-only member `_f'


This is correct.  The compiler is warning you that in order to create the
return value of f(), it must create a copy of _f using a copy constructor
that does not guarantee to leave the _f unchanged.  Since f() guarantees
that it does not change _f (or any of the rest of the class).

The reason this is a warning rather than an error is for abstract constness
(where the bitpattern of the object does change, but the semantic value
remains unchanged).


--
Warwick
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