converting pointer to const ref

[cppaddict]

I can't figure out why this code won't work, or how to fix it:

list<MyClass> l;
int main() {
for (int i=0;i<5;i++) {
MyClass* myClass = new MyClass;
myClass -> someInitMethod( );
l.push_back(myC lass);
}
}

The compiler errors on "l.push_back(my Class)", saying that it can't convert
MyClass* to const MyClass&.
Two questions:

1. Why can't it do this conversion?
2. How do you make this code work?

thanks,
cppaddict

[David White]

Re: converting pointer to const ref

cppaddict <cppaddict@yaho o.com> wrote in message
news:MoKib.892$ vc4.637@newssvr 27.news.prodigy .com...[color=blue]
> I can't figure out why this code won't work, or how to fix it:
>
> list<MyClass> l;
> int main() {
> for (int i=0;i<5;i++) {
> MyClass* myClass = new MyClass;
> myClass -> someInitMethod( );
> l.push_back(myC lass);
> }
> }
>
> The compiler errors on "l.push_back(my Class)", saying that it can't[/color]
convert[color=blue]
> MyClass* to const MyClass&.
> Two questions:
>
> 1. Why can't it do this conversion?[/color]

Well, there's really no need. If you have a vector of pointers, then you
push back a pointer. If you have a vector of objects (non-pointer objects
that is), then push back an object. Why pass a pointer and expect the
compiler to do a conversion if you can easily pass the correct type?
[color=blue]
> 2. How do you make this code work?[/color]

l.push_back(*my Class);

DW

[John Carson]

Re: converting pointer to const ref

"cppaddict" <cppaddict@yaho o.com> wrote in message
news:MoKib.892$ vc4.637@newssvr 27.news.prodigy .com[color=blue]
> I can't figure out why this code won't work, or how to fix it:
>
> list<MyClass> l;
> int main() {
> for (int i=0;i<5;i++) {
> MyClass* myClass = new MyClass;
> myClass -> someInitMethod( );
> l.push_back(myC lass);
> }
> }
>
> The compiler errors on "l.push_back(my Class)", saying that it can't
> convert MyClass* to const MyClass&.
> Two questions:
>
> 1. Why can't it do this conversion?[/color]

Why should it? C++ is a typed language, which means that variables of
different types are not interchangeable (some limited conversions/casts are
possible, but conversions are not universally available). myClass is a
pointer to a MyClass object, but the list does not store pointers, it stores
MyClass objects.
[color=blue]
> 2. How do you make this code work?[/color]

Either change the list so it stores pointers (i.e., use list<MyClass*> l) or
add objects rather than add pointers to the list (i.e., use
l.push_back(*my Class) ). Whether either option represents the best design is
another issue.


--
John Carson
1. To reply to email address, remove donald
2. Don't reply to email address (post here instead)

[Marcin Vorbrodt]

Re: converting pointer to const ref

"cppaddict" <cppaddict@yaho o.com> wrote in message
news:MoKib.892$ vc4.637@newssvr 27.news.prodigy .com...[color=blue]
> I can't figure out why this code won't work, or how to fix it:
>
> list<MyClass> l;
> int main() {
> for (int i=0;i<5;i++) {
> MyClass* myClass = new MyClass;
> myClass -> someInitMethod( );[/color]

//l.push_back(myC lass);
l.push_back(*my Class); // dereference the pointer
[color=blue]
> }
> }
>
> The compiler errors on "l.push_back(my Class)", saying that it can't[/color]
convert[color=blue]
> MyClass* to const MyClass&.
> Two questions:
>
> 1. Why can't it do this conversion?[/color]

Because there is a type missmatch. Pointer is not a reference. Function
expects a const reference, but instead it gets a pointer... bad.
[color=blue]
> 2. How do you make this code work?[/color]

Look above.
[color=blue]
>
> thanks,
> cppaddict
>
>
>
>
>[/color]

[Jakob Bieling]

Re: converting pointer to const ref

"cppaddict" <cppaddict@yaho o.com> wrote in message
news:MoKib.892$ vc4.637@newssvr 27.news.prodigy .com...[color=blue]
> I can't figure out why this code won't work, or how to fix it:
>
> list<MyClass> l;
> int main() {
> for (int i=0;i<5;i++) {
> MyClass* myClass = new MyClass;
> myClass -> someInitMethod( );
> l.push_back(myC lass);
> }
> }
>
> The compiler errors on "l.push_back(my Class)", saying that it can't[/color]
convert[color=blue]
> MyClass* to const MyClass&.
> Two questions:
>
> 1. Why can't it do this conversion?[/color]

Because you are the programmer.
[color=blue]
> 2. How do you make this code work?[/color]

Change your line to:

l.push_back (*myClass);

You dereference the pointer and have the actual object (and not just the
pointer to it). Now the compiler can create a reference to that object and
pass the reference to push_back ().

hth
--
jb

(replace y with x if you want to reply by e-mail)

[Jonathan Mcdougall]

Re: converting pointer to const ref

cppaddict wrote:

nice name :)
[color=blue]
> I can't figure out why this code won't work, or how to fix it:
>
> list<MyClass> l;
> int main() {
> for (int i=0;i<5;i++) {
> MyClass* myClass = new MyClass;
> myClass -> someInitMethod( );[/color]

someInitMethod( ) ? That is what constructor are for.
[color=blue]
> l.push_back(myC lass);
> }
> }[/color]

Are you sure you want to allocate the objects dynamically? Why not
something like

for (int i=0;i<5;i++)
{
MyClass myClass;
myClass.someIni tMethod();

l.push_back(myC lass);
}
[color=blue]
> The compiler errors on "l.push_back(my Class)", saying that it can't
> convert MyClass* to const MyClass&.[/color]

The list you created accepts MyClass objects and you are giving it a
MyClass*, that is the problem
[color=blue]
> Two questions:
>
> 1. Why can't it do this conversion?[/color]

Because it is invalid.
[color=blue]
> 2. How do you make this code work?[/color]

Either with

list<MyClass*> l;

in which case *do not forget* to delete every pointer in the list, or with
the solution I provided.


Jonathan

[cppaddict]

Re: converting pointer to const ref

thank you everyone for your help.

cpp