[Qt-interest] Const correctness with QSharedPointer

[Stephen Jackson]

2009/12/7 Brad Howes :
> On Dec 7, 2009, at 3:31 PM, Colin Kern wrote:
>
>> I want to pass a QSharedPointer to a function as a const pointer, so
>> that the compiler will enforce that I don't mutate the object the
>> shared pointer is pointing to.  Can I do this with "void func(const
>> QSharedPointer<A> p);" or does that just enforce that I won't change
>> what the shared pointer is pointing to?
>
>
> I usually use const QSharedPointer<A>& but your declaration is also correct.
>

I'm sorry but both of these options permit mutation of the object
pointed at, as demonstrated by the following code.

The commented out options 1 & 2 both compile, permit the increment and print 5.

The uncommented version is what you need to throw a compile-time error on ++*p;

#include <QtGui>
#include <iostream>

// 1. void foo(const QSharedPointer<int> p)
// 2. void foo(const QSharedPointer<int> & p)
void foo(const QSharedPointer<const int> p)
{
	++*p;
}

int main()
{
	QSharedPointer<int> p(new int);
	*p = 4;
   	foo(p);

	std::cout << *p << "\n";

	return 0;
}

i.e. you need "void func(const QSharedPointer<const A> p)"

-- 
HTH,

Stephen Jackson