Error with generics, pointers and const

[globos]

Hi,

I have a problem with the compilation of some code. It appears when mixing generics with pointers and const.
Here is a dummy sample :

class Object//Our dummy data class
{};

template<class G>//A generic class defining a request
class A
{
public:
  bool has (const G& v) const
  { return true; }

};

class B//Uses A<> with pointers.
{
public:
  bool found (const A<Object*>& a, const Object& v) const
  { return a.has (&v); }
};


int main ()//some test
{
  A<Object*> a;
  Object obj;

  B b;
  b.found (a, obj);

  return 0;
}

The compilation error is :
main-test.cpp(23) : error C2664: 'has' : cannot convert parameter 1 from 'const class Object *' to 'class Object *const & '
Reason: cannot convert from 'const class Object *' to 'class Object *const '
Conversion loses qualifiers

I can't understand why this error occurs, looking at the source code, it seems valid.
What must I do to avoid the problem?

--
Globos

 

[ArkM]

It's a very interesting case.

  bool found (const A<Object*>& a, const Object& v) const
  { return a.has (&v); }

So &v expression yields a pointer to a constant object.
Its type is (const Object*).
We have in template:

  bool has (const G& v) const

We can't modify v (so it's type is Object*const) in has().
In class B we have (const Object*) pointer in a.has(&v)
and trying to pass it to a function which
accepts Object*const argument.
No standard conversion between const* and *const.
Try

  bool found (const A<Object*>& a, const Object& v) const
  { return a.has (const_cast<Object*const>(&v)); }

 

[globos]

ArkM,

I have tried with the const casting, it compiles. But it will be really painful to use. Apparently there does not seem to be other solutions. I have also tried to play with the const, but the current signatures reflect right const semantics, it should stay like this.
I still can't understand the compiler error in :

bool has (const G& v) const

We can't modify v (so it's type is Object*const) in has().

The actual type of v should be `const Object*&' (G = Object*) and not `Object*const'. Why is it not so?

--
Globos

 

[chipperMDW]

The actual type of v should be `const Object*&' (G = Object*) and not `Object*const'. Why is it not so?

The type of [tt]G[/tt] is "pointer to Object."

So the type of [tt]const G[/tt] is "const pointer to Object," not "pointer to const Object."


Try something like this:

template<typename T>
struct PtrConstify
{
  typedef const T Type;
};

template<typename T>
struct PtrConstify<T*>
{
  typedef const T * Type;
};

...

bool has (PtrConstify<G>::Type& v) const

If you need to do stuff like that frequently, check out the TypeTraits facility in the Loki library available at

http://www.moderncppdesign.com.

 

[chipperMDW]

Actually,

bool has (PtrConstify<G>::Type& v) const

probably won't compile...

Better try

bool has ([highlight]typename[/highlight] PtrConstify<G>::Type& v) const

 

[ArkM]

A constant pointer to X has type X*const (see C++ Standard 8.3.1(2)).
A type const X* is a pointer to a constant X.
const X& is a reference to a const X. It's a type of v parameter in template member function.
Now let's substitute...