bad_alloc

[Toxa]

I'm using VC++ 5.0
As far as I know ANSI C++ standard,
global operator new should throw std::bad_alloc
exception in case of memory lack.
However, it returns 0-pointer.
What's wrong with it?

Sincerely,

--Anton Yevgrafov

[Dave Lorde]

If you use std::new in the <new> header, it should throw a bad_alloc by default.

The default new implementation (not the std::new) returns null, but you can set a new-handler that throws bad_alloc instead. This is described in Article ID: Q167733 "Operator New Doesn't Throw bad_alloc Exception on Failure", as below:

Operator new does call the new handler function after it fails to procure the requestion block of memory, but before it returns the null pointer. An application could install a new handler to throw a bad_alloc exception as follows:


#include <new>
#include <new.h>
int my_new_handler(size_t) {
throw std::bad_alloc();
return 0;
}
int main () {
_PNH _old_new_handler;
_old_new_handler = _set_new_handler(my_new_handler);
/* ... application processing ... */
_set_new_handler(_old_new_handler);
return 0;
}

To call new handler when malloc fails to obtain the requested block of memory, use the _set_new_mode function.
To install the new handler before your global objects are initialized, create a class that sets the new handler in its constructor and installs the old new handler in its destructor. Create a global object of that type and use the init_seg pragma to force this global object to be initialized before any of your global objects. The example below demonstrates this. It also demonstrates the use of _set_new_mode to cause a failed malloc call to generate an exception. Note that to do this, the code below must reside in its own source file. You cannot change the initializations segment more than once per translation unit (source file) with the pragma init_seg.


#include <new>
#include <new.h>
#pragma init_seg(lib)
int my_new_handler(size_t) {
throw std::bad_alloc();
return 0;
}
struct my_new_handler_obj{
_PNH _old_new_handler;
int _old_new_mode;
_tag_g_new_handler_obj() {
_old_new_mode = _set_new_mode(1);
_old_new_handler = _set_new_handler(my_new_handler);
}
~_tag_g_new_handler_obj() {
_set_new_handler(_old_new_handler);
_set_new_mode(_old_new_mode);
}

} _g_new_handler_obj;

Operator new, as implemented by Visual C++ 5.0, ignores the function exception specification. So new(std::nothrow) still generates an exception if your new handler is installed to throw an exception as the examples above demonstrate. To change this behavior, override operator new as follows:

void *__cdecl operator new(size_t cb, const std::nothrow_t&) throw()
{
char *p;
try {
p = new char[cb];
}
catch (std::bad_alloc) {
p = 0;
}
return p;
}

This behavior was by design though not in conformance with the ANSI Draft Working Papers for C++ at the time.

Dave