Re: This HAS to be UB...
"James Kanze" <james.kanze@gmail.com> wrote in message
news:7878ab49-834f-4bbc-b687-efdd8f31f1f3@z66g2000hsc.googlegroups.com...
On Oct 2, 9:52 pm, "Chris M. Thomasson" <n...@spam.invalid> wrote:
Keep in mind that I am a C programmer; well, anyway here is
the C++ program...
It looks to me like you're attacking some fairly tricky stuff.
You'd probably be better of starting with something simpler if
you're still learning C++. However...
I was exploring the feature in C++ delete operator in which the size of the
allocation is returned along with the pointer to allocated memory. One could
create heavily optimized custom memory allocator using that important piece
of information.
______________________________________________________________________
#include <cstdio>
#include <cstdlib>
#include <new>
struct custom_allocator {
static void* allocate(std::size_t size)
throw(std::bad_alloc()) {
That should doubtlessly be:
throw( std::bad_alloc )
What you've said is that the only exception type which will
escape from your function is a pointer to a function returning
an std::bad_alloc and taking no arguments. I really don't think
you meant to say that you're going to throw pointers to
functions.
That was definitely a typo/error on my part.
In practice, exception specifications are not really that
useful, except when they're empty. (It's very important in
certain cases to know that a function cannot throw any
exceptions, but it's rarely useful to know that it can't throw
certain types of exceptions.)
I thought it would be prudent to give the overloaded operator new an
exception specification of `std::bad_alloc'. Also, I wanted to give an empty
specification to the overload of operator delete. As to how useful it is...
Well, I don't quite know.
void* const mem = ::operator new(size);
std::printf("custom_allocator::allocate(%p, %lu)\n",
(void*)mem, (unsigned long)size);
return mem;
}
static void deallocate(void* const mem, std::size_t size)
throw() {
std::printf("custom_allocator::deallocate(%p, %lu)\n",
(void*)mem, (unsigned long)size);
::operator delete(mem);
}
};
template<typename T>
struct allocator_base {
static void* operator new(std::size_t size)
The static isn't really necessary: allocation and deallocation
member functions (operator new and operator delete) are always
static, whether you declare them so or not. (On the other hand,
it doesn't hurt.)
Its a habit of mine. Also, using printf in C++ is another habit.
throw(std::bad_alloc()) {
return custom_allocator::allocate(size);
}
static void* operator new[](std::size_t size)
throw(std::bad_alloc()) {
return custom_allocator::allocate(size);
}
static void operator delete(void* mem)
Just curious: since you require the size in delete[], why don't
you require it here? Derivation can mean that the size isn't a
constant, e.g.:
class Base : public allocator_base< Base >
{
// ...
} ;
class Derived : public Base
{
// ...
} ;
Base* p = new Derived ;
// ...
delete p ;
(This supposes, of course, that Base has a virtual destructor.)
[...]
______________________________________________________________________
On GCC I get the following output:
custom_allocator::allocate(00246C50, 2234)
custom_allocator::deallocate(00246C50, 2234)
custom_allocator::allocate(00247760, 11174)
custom_allocator::deallocate(00247760, 11174)
On MSVC 8 I get:
custom_allocator::allocate(00362850, 2234)
custom_allocator::deallocate(00362850, 2234)
custom_allocator::allocate(00366B68, 11170)
custom_allocator::deallocate(00366B68, 2234)
Are they both right due to UB? WTF is going on? GCC seems to
be accurate at least... DAMN!
Well, there's no undefined behavior. You're program seems
perfectly legal and well defined to me. It looks like a bug in
VC++, see ?12.5/5:
It definitely looks like a bug is MSVC++. I get erroneous behavior on
versions 6 through 9.
When a delete-expression is executed, the selected
deallocation function shall be called with the address
of the block of storage to be reclaimed as its first
argument and (if the two-parameter style is used) the
size of the block as its second argument.
And I can't think of any way of interpreting "the size of the
block" to mean anything other than the size requested in the
call to operator new.
I thought that MSVC was crapping out because `allocator_base' was a
template. So I created another little test which hopefully has all the bugs
fixed:
__________________________________________________________________________
#include <cstdio>
#include <cstdlib>
#include <new>
struct custom_allocator {
static void* allocate(std::size_t size)
throw(std::bad_alloc) {
void* const mem = std::malloc(size);
if (! mem) {
throw std::bad_alloc();
}
std::printf("custom_allocator::allocate(%p, %lu)\n",
(void*)mem, (unsigned long)size);
return mem;
}
static void deallocate(void* const mem, std::size_t size)
throw() {
if (mem) {
std::printf("custom_allocator::deallocate(%p, %lu)\n",
(void*)mem, (unsigned long)size);
std::free(mem);
}
}
};
struct allocator_base {
void* operator new(std::size_t size)
throw(std::bad_alloc) {
return custom_allocator::allocate(size);
}
void* operator new [](std::size_t size)
throw(std::bad_alloc) {
return custom_allocator::allocate(size);
}
void operator delete(void* mem, std::size_t size)
throw() {
custom_allocator::deallocate(mem, size);
}
void operator delete [](void* mem, std::size_t size)
throw() {
custom_allocator::deallocate(mem, size);
}
};
template<std::size_t T_size>
class buf : public allocator_base {
char mem[T_size];
public:
virtual ~buf() throw() {}
};
class buf2 : public buf<1234> {
char mem2[1000];
};
int main() {
buf<1024>* b1 = new buf<1024>;
delete b1;
buf2* b2 = new buf2;
delete b2;
b2 = new buf2[5];
delete [] b2;
return 0;
}
__________________________________________________________________________
On every version of GCC I have, I get the following output on a 32-bit
machine:
custom_allocator::allocate(00246C50, 1028)
custom_allocator::deallocate(00246C50, 1028)
custom_allocator::allocate(002472A8, 2240)
custom_allocator::deallocate(002472A8, 2240)
custom_allocator::allocate(002472A8, 11204)
custom_allocator::deallocate(002472A8, 11204)
On every version of MSVC, I get:
custom_allocator::allocate(00365B28, 1028)
custom_allocator::deallocate(00365B28, 1028)
custom_allocator::allocate(00362850, 2240)
custom_allocator::deallocate(00362850, 2240)
custom_allocator::allocate(00366FA8, 11204)
custom_allocator::deallocate(00366FA8, 2240)
Well, MSVC has a fairly nasty bug indeed. Anyway, what do you think James?