C++ "smart pointer" template that auto-converts to bare pointer but can't be explicitly deleted

Question

I am working in a very large legacy C++ code base which shall remain nameless. Being a legacy code base, it passes raw pointers around all over the place. But we are gradually trying to modernize it and so there are some smart pointer templates as well. These smart pointers (unlike, say, Boost's scoped_ptr) have an implicit conversion to the raw pointer, so that you can pass one of them into a routine that takes a raw pointer without having to write .get(). A big downside of this is that you can also accidentally use one in a delete statement, and then you have a double free bug, which can be a real pain to track down.

Is there a way to modify the template so that it still has the implicit conversion to the raw pointer, but causes a compile error if used in a delete statement? Like this:

#include <my_scoped_ptr>

struct A {};
extern void f(A*);

struct B
{
    scoped_ptr<A> a;

    B();
    ~B();
};

B::B()
    : a(new A)
{
    f(a); // this should compile
}

B::~B()
{
    delete a; // this should NOT compile
}

Answer 1

The Standard says

The operand shall have a pointer type, or a class type having a single conversion function (12.3.2) to a pointer type. If the operand has a class type, the operand is converted to a pointer type by calling the above-mentioned conversion function, and the converted operand is used in place of the original operand for the remainder of this section.

You can (ab)-use the absence of overload resolution by declaring a const version of the conversion function. On a conforming compiler that's enough to make it not work anymore with delete:

struct A {
  operator int*() { return 0; }
  operator int*() const { return 0; }
};

int main() {
  A a;
  int *p = a; // works
  delete a; // doesn't work
}

Results in the following

[js@HOST2 cpp]$ clang++ main1.cpp
main1.cpp:9:3: error: ambiguous conversion of delete expression of type 'A' to a pointer
  delete a; // doesn't work
  ^      ~
main1.cpp:2:3: note: candidate function            
  operator int*() { return 0; }
  ^
main1.cpp:3:3: note: candidate function             
  operator int*() const { return 0; }
  ^
1 error generated.

---

On compilers that are less conforming in that regard (EDG/Comeau, GCC) you can make the conversion function a template. delete does not expect a particular type, so this would work:

template<typename T>
operator T*() { return /* ... */ }

However, this has the downside that your smartpointer is now convertible to any pointer-type. Although the actual conversion is still typechecked, but this won't rule out conversions up-front but rather give a compile time error much later. Sadly, SFINAE does not seem to be possible with conversion functions in C++03 :) A different way is to return a private nested type pointer from the other function

struct A {
  operator int*() { return 0; }

private:
  struct nested { };
  operator nested*() { return 0; }
};

The only problem now is with a conversion to void*, in which case both conversion functions are equally viable. A work-around suggested by @Luther is to return a function pointer type from the other conversion function, which works with both GCC and Comeau and gets rid of the void* problem while having no other problems on the usual conversion paths, unlike the template solution

struct A {
  operator int*() { return 0; }

private:
  typedef void fty();
  operator fty*() { return 0; }
};

Notice that these workarounds are only needed for compilers that are not conforming, though.