RE: [Boost-Users] Why "explicit" in shared_ptr's constructor?

From: Tom Matelich (tmatelich_at_[hidden])
Date: 2003-03-28 12:02:10

the danger comes from code like so:

void foo(boost::shared_ptr<entity> p)
{
...
}

void bar()
{
        entity* e = new entity;
        foo(e);
        delete e;
}

Allowing a pointer to be converted to a shared_ptr implicitly could cause
serious bugs.

You could shorten things with this (untested) function:

template <typename T>
boost::shared_ptr<T> make_sp(T* p)
{
        return boost::shared_ptr<T>(p);
}

> -----Original Message-----
> From: Maurizio Colucci [mailto:seguso_at_[hidden]]
> Sent: Friday, March 28, 2003 8:36 AM
> To: Boost-Users_at_[hidden]
> Subject: [Boost-Users] Why "explicit" in shared_ptr's constructor?
> 
> 
> Hello,
> 
> I am a newbie user of boost. 
> 
> I have a question about boost::shared_ptr. Suppose you have a
> polimorphic list:
> 
> class entity{ ...};
> 
> 
> class simple_entity : public entity{
> public:
>   uint id;
>   simple_entity(uint i) : id(i) {}
> };
> 
> int main(){
>   list<shared_ptr<entity> > l;
>   ...
> 
> 
> I noticed the following code doesn't compile
> 
>   l.push_back(new simple_entity(1));
> 
> too bad, because it was very easy to read, and very similar to the
> behavior if standard pointers (not to speak about similarity to Java
> and C#).  So I am forced to do
> 
>   l.push_back(shared_ptr<entity> (new simple_entity(1)));
> 
> which is less readable and, IMHO, barely tolerable on the long
> run: too much typing.
> 
> It seems that the reason is the explicit keyword in the
> constructor. My question is: why? what was the danger without the
> explicit?
> 
> Thank you very much for any info,
> 
> Maurizio
> 
> 
> 
> 
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