Some issues with using sizeof() in template definitions

From:

Sylvester Hesp <s.hespNO@SPAMoisyn.nl>

Newsgroups:

microsoft.public.vc.language

Date:

Wed, 08 Aug 2007 17:57:42 +0200

Message-ID:

<mn.44357d78c484a49f.75415@SPAMoisyn.nl>

I have two issues with VC++ (2005 Pro SP1), which might or might not be
related, but both have to do with using the sizeof inside template
definitions.

The first one just baffles me, it makes no sense at all. I was creating
an allocator to be used with a std::map. When using sizeof inside my
allocator, such as to assign to an enum value defined in the class, the
compiler complains about errors in <xtree>. Here's the code:

-------------------------------------
#include <memory>
#include <map>

// my allocator, stripped to the bare minimum
template<class T> struct MyAlloc : public std::allocator<T>
{
    typedef std::allocator<T> MyBase;

    template<class U> struct rebind
    {
        typedef MyAlloc<U> other;
    };

    enum { foo = sizeof(T) };

    MyAlloc() { }
    MyAlloc(const MyAlloc & other) : MyBase(other) { }
    template<class U> MyAlloc(const MyAlloc<U> & other) : MyBase(other)
{ }
};

int main()
{
    std::map<int, int, std::less<int>, MyAlloc<std::pair<const int,
int> > > m;
}
-------------------------------------

If I compile this, I get these errors:

1>main.cpp
1>c:\...\xtree(39) : error C2061: syntax error : identifier '_Genptr'
1> c:\...\main.cpp(14) : see reference to class template
instantiation 'std::_Tree_nod<_Traits>::_Node' being compiled
1> with
1> [
1>
_Traits=std::_Tmap_traits<int,int,std::less<int>,MyAlloc<std::pair<const
int,int>>,false>
1> ]
1> c:\...\xtree(35) : see reference to class template
instantiation 'MyAlloc<T>' being compiled
1> with
1> [
1>
T=std::_Tree_nod<std::_Tmap_traits<int,int,std::less<int>,MyAlloc<std::pair<const
int,int>>,false>>::_Node
1> ]
1> c:\...\xtree(68) : see reference to class template
instantiation 'std::_Tree_nod<_Traits>' being compiled
1> with
1> [
1>
_Traits=std::_Tmap_traits<int,int,std::less<int>,MyAlloc<std::pair<const
int,int>>,false>
1> ]
1> c:\...\xtree(94) : see reference to class template
instantiation 'std::_Tree_ptr<_Traits>' being compiled
1> with
1> [
1>
_Traits=std::_Tmap_traits<int,int,std::less<int>,MyAlloc<std::pair<const
int,int>>,false>
1> ]
1> c:\...\xtree(112) : see reference to class template
instantiation 'std::_Tree_val<_Traits>' being compiled
1> with
1> [
1>
_Traits=std::_Tmap_traits<int,int,std::less<int>,MyAlloc<std::pair<const
int,int>>,false>
1> ]
1> c:\...\map(82) : see reference to class template
instantiation 'std::_Tree<_Traits>' being compiled
1> with
1> [
1>
_Traits=std::_Tmap_traits<int,int,std::less<int>,MyAlloc<std::pair<const
int,int>>,false>
1> ]
1> c:\...\main.cpp(23) : see reference to class template
instantiation 'std::map<_Kty,_Ty,_Pr,_Alloc>' being compiled
1> with
1> [
1> _Kty=int,
1> _Ty=int,
1> _Pr=std::less<int>,
1> _Alloc=MyAlloc<std::pair<const int,int>>
1> ]
1>c:\...\xtree(46) : error C2146: syntax error : missing ';' before
identifier '_Left'
1>c:\...\xtree(46) : error C4430: missing type specifier - int assumed.
Note: C++ does not support default-int
1>c:\...\xtree(46) : error C4430: missing type specifier - int assumed.
Note: C++ does not support default-int
1>c:\...\xtree(47) : error C2146: syntax error : missing ';' before
identifier '_Parent'
1>c:\...\xtree(47) : error C4430: missing type specifier - int assumed.
Note: C++ does not support default-int
1>c:\...\xtree(47) : error C4430: missing type specifier - int assumed.
Note: C++ does not support default-int
1>c:\...\xtree(48) : error C2146: syntax error : missing ';' before
identifier '_Right'
1>c:\...\xtree(48) : error C4430: missing type specifier - int assumed.
Note: C++ does not support default-int
1>c:\...\xtree(48) : error C4430: missing type specifier - int assumed.
Note: C++ does not support default-int

(I put the ellipses in the paths to keep things more readable).

The funny thing is, as soon as I remove the enum definition in MyAlloc,
everything compiles fine. Also, if I do not use sizeof(T) to calculate
the value for foo, it works fine as well.

My other issue occured when trying to call a memberfunction on an
object when it exists, or do nothing otherwise. I was using SFINAE for
that:

-------------------------------------
#include <iostream>

template<unsigned> struct MemberCheck
{
    MemberCheck(int) { }
};

template<class T>
void callFoo(T * t, MemberCheck<sizeof(&T::foo)> = 0)
{
    t->foo();
}

void callFoo(void *)
{
}

struct A
{
    void foo() { std::cout << "foo()" << std::endl; }
};

struct B
{
};

int main()
{
    A a;
    B b;
    callFoo(&a);
    callFoo(&b);
}
-------------------------------------

This gives me the following error:
1>c:\...\main.cpp(31) : error C2070: 'overloaded-function': illegal
sizeof operand

Comeau accepts both code snippets just fine. I haven't tested other
compilers though... And there's nothing in the C++ standard saying that
using sizeof in a template function declaration is prohibited.

Are these known issues, or should I file a bugreport at Microsoft
Connect? And if so, are these related, or are they seperate?

Regards,

Sylvester Hesp