C++ Befriending boost::ptr_map / boost::checked_delete fails

Question

I want to use a boost::ptr_map inside a specific class which stores instances of itself. However, please consider the following example:

#include <boost/checked_delete.hpp>
#include <boost/ptr_container/ptr_map.hpp>


class foo
{
    friend void boost::checked_delete<>(foo*);
    ~foo() {}
};


int main()
{
    boost::checked_delete(new foo);     // OK
    boost::ptr_map<int, foo> foo_map;   // error C2248: 'foo::~foo' : cannot access private member declared in class 'foo'

    return 0;
}

The error happens at the following line

// verify that types are complete for increased safety

template<class T> inline void checked_delete(T * x)
{
    // intentionally complex - simplification causes regressions
    typedef char type_must_be_complete[ sizeof(T)? 1: -1 ];
    (void) sizeof(type_must_be_complete);
    delete x;    // error C2248
}

What exactly is going on here? Shouldn't it work? I assume that the problem is that templates are defined in the compilation unit they are included in and boost::checked_delete is called from another compilation unit in the implementation source of bosst::ptr_map. So, it's not the same function I declared as a friend.

However, is there a workaround for this problem?

Add the exact compiler error that you are getting, it usually contains information on what is going on under the hood that the single error code does not show. — David Rodríguez - dribeas, Jun 15 '11 at 07:35

score 2 · Answer 1 · answered Jun 15 '11 at 07:22

2

Try this syntax when declaring the friend:

template <class T> friend void boost::checked_delete(T*);

answered Jun 15 '11 at 07:22

bshields

3,563
16
16

1

The most explicit (so preferable) syntax would be `friend void boost::checked_delete<>(foo*);` or `friend void boost::checked_delete(foo*);`. That's why I'm using it. But thanks for your approach. – 0xbadf00d Jun 15 '11 at 07:46
The comment is right on target, the syntax in the question befriends a single template instantiation, while the proposed solution befriends all the instantiations, which can be problematic. Consider reading [this](http://stackoverflow.com/questions/4660123/overloading-friend-operator-for-template-class/4661372#4661372) and [this](http://stackoverflow.com/questions/1810753/overloading-operator-for-a-templated-class/1811208#1811208) other answers to different questions. – David Rodríguez - dribeas Jun 15 '11 at 08:13

score 1 · Accepted Answer · answered Jun 15 '11 at 07:23

1

Here is the start of the huge error message* from GCC, which is the start of the chain of instantiations (usually, and in this case):

In file included from main.cpp:1:0: main.cpp: In function 'void boost::checked_delete(T*) [with T = const foo]':

Adding

friend void boost::checked_delete<>(foo const*);

makes the code compile.

(*): 13 lines and 3510 characters for 270 chars/line

answered Jun 15 '11 at 07:23

Luc Danton

34,649
6
70
114

Thanks for your answer. Sorry, for didn't mention that, but I already noticed it. If you add the const modifier it will work, but I thought this could be less portable (I'm using MSVC 10.0). Or is it guaranteed to be const? – 0xbadf00d Jun 15 '11 at 07:44
@FrEEzE2046 It's not so much an issue of portability but of what the code that uses `boost::checked_delete` does: passing pointers to `const` or not to it. So you'll have to check with the docs for `ptr_map` to see if there's any guarantee. – Luc Danton Jun 15 '11 at 07:55
@FrEEze2046: If you go back and read the error message, it will tell you what the library does, which is independent of which compiler you are using. – David Rodríguez - dribeas Jun 15 '11 at 08:09
Wouldn't it be possible to make ´checked_delete´ a friend of foo with all possible modifiers? – 0xbadf00d Jun 15 '11 at 08:17
@FrEEzE2046 No; it's either the template as a whole or select specializations. There's no way to express *cv* qualification as a template specialization (i.e. match any *cv* `foo`) and partial specialization is not allowed for function templates anyway. – Luc Danton Jun 15 '11 at 08:22

C++ Befriending boost::ptr_map / boost::checked_delete fails

2 Answers2