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Templates [C++] - Why doesn't this work?

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Started by c4c0d3m0n October 10, 2008 09:49 AM

11 comments, last by Zahlman 15 years, 6 months ago

c4c0d3m0n

100

Author

October 10, 2008 09:49 AM

#include <boost/shared_ptr.hpp>

class Parent
{
  public:
    virtual void foo();
    int bar;
};

class Child : public Parent
{
  public:
    void foo() { bar++; };
    int bar;
};



template <Parent T>
boost::shared_ptr<T>
create()
{
    boost::shared_ptr<T> pointer( new T );
    return pointer;
}



int main()
{
    boost::shared_ptr<Parent> pointer = create<Child>();
    return 0;
}

temp.cpp:19: error: ‘class Parent’ is not a valid type for a template constant parameter

I always thought I could create a template of whatever I want, not just a class.. Could someone please explain why this is not allowed? What I basically want is a template that only allows classes inherited from Parent. How would I go on about this?

SiCrane

11,840

October 10, 2008 09:53 AM

Ok, there are two problems. The first problem is that non-type template parameters need to be either integral constants or a pointer or reference to an entity with external linkage, so you need to have a Parent * or Parent &. The second problem is that you aren't using it as a non-type template parameter, since you're trying to use Child as the template parameter. Basically, the way you're using it you should replace Parent with class or typename.

soconne

105

October 10, 2008 09:55 AM

Quote:Original post by c4c0d3m0n
*** Source Snippet Removed ***
temp.cpp:19: error: ‘class Parent’ is not a valid type for a template constant parameter
I always thought I could create a template of whatever I want, not just a class.. Could someone please explain why this is not allowed?

What I basically want is a template that only allows classes inherited from Parent. How would I go on about this?

You need to change your code to:

template <class T>boost::shared_ptr<T> create(){   boost::shared_ptr<T> pointer(new T);   return pointer;}int main){...}

Author Freeworld3Dhttp://www.freeworld3d.org

c4c0d3m0n

100

Author

October 10, 2008 10:01 AM

Thanks for the quick replies!

@SiCrane:
I am not sure what you mean by "pointer or reference to an entity with external linkage" I do see now why the code wouldn't work anyway; Child is not of type Parent (the second issue you addressed).

@soconne:
The problem with that code is that it would also allow classes that are not inherited from Parent. How can I make it so only classes inherited from Parent are allowed as template parameter?

SiCrane

11,840

October 10, 2008 10:05 AM

The simplest way to restrict it to child classes of your Parent class would be to add a BOOST_STATIC_ASSERT() that Parent is a base of the template type. boost::type_traits' is_base_of would probably be the most straightforward way to ensure that.

c4c0d3m0n

100

Author

October 10, 2008 10:15 AM

Hmm.. I seem to be very terible at finding good boost documentation through google, but I found an example on a mailing list. Still, I can't quite get it to work...

template <class T>boost::shared_ptr<T>create(){    BOOST_STATIC_ASSERT(( boost::is_base_of<T, Parent>::value ));    boost::shared_ptr<T> pointer( new T );    return pointer;}

rob@rob-laptop:~/Desktop$ g++ temp.cpptemp.cpp: In function ‘boost::shared_ptr<T> create() [with T = Child]’:temp.cpp:34:   instantiated from heretemp.cpp:24: error: invalid application of ‘sizeof’ to incomplete type ‘boost::STATIC_ASSERTION_FAILURE<false>’

SiCrane

11,840

October 10, 2008 10:19 AM

I think you want <Parent, T> not <T, Parent>

c4c0d3m0n

100

Author

October 10, 2008 10:23 AM

In that case I get funky errors about the vtables in Parent ... oO

rob@rob-laptop:~/Desktop$ g++ temp.cpp/tmp/ccWNjx4L.o: In function `Parent::Parent()':temp.cpp:(.text._ZN6ParentC2Ev[Parent::Parent()]+0x4): undefined reference to `vtable for Parent'/tmp/ccWNjx4L.o:(.rodata._ZTI5Child[typeinfo for Child]+0x8): undefined reference to `typeinfo for Parent'collect2: ld returned 1 exit status

edit:
If I remove all the contents of the classes, it seems to work. Maybe something wasnt kosher with the using no::brain inheritance up there

SiCrane

11,840

October 10, 2008 10:27 AM

That probably has something to do with the fact you never provide a definition for Parent::foo().

soconne

105

October 10, 2008 01:50 PM

Quote:Original post by c4c0d3m0n
@soconne:
The problem with that code is that it would also allow classes that are not inherited from Parent. How can I make it so only classes inherited from Parent are allowed as template parameter?

Oooohh, sorry, I should have read your post more carefully. One way to solve it is to use the dynamic_cast function.

class Parent { };class Child : public Parent { };template <typename P, typename T>boost::shared_ptr<T> create(){    T* t = new T();    if (dynamic_cast<P*>(t) != 0)        return boost::shared_ptr(t);    else        // throw error or return 0}

Author Freeworld3Dhttp://www.freeworld3d.org

Templates [C++] - Why doesn't this work?

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