Q: How can one encourage his (older) compiler to check new to see automatically if it returns
NULL?
A: His compiler eventually will.
If he contain an old compiler which doesn't auto magically perform the NULL test, he might force the runtime system to do the test through installing a "new handler" function. his "new handler" function might do anything he wish, such as throw an exception, delete some of the objects and return (wherein case operator new will retry the allocation), print a message & abort() the program, etc.
Here's a sample "new handler" which prints a message & throws an exception. The handler is installed by using std::set_new_handler():
#include // To obatin std::set_new_handler
#include // To obtain abort()
#include // To obtain std::cerr
class alloc_error : public std::exception {
public:
alloc_error() : exception() { }
};
void myNewHandler()
{
// this is own handler. It can do anything he wants. throw alloc_error();
}
int main()
{
std::set_new_handler(myNewHandler); // Install "new handler"
...
}
After the std::set_new_handler() line is executed, operator new will call your myNewHandler()
If it run out of memory. It means that new will never return NULL:
Fred* p = new Fred(); // No require to check if p is NULL
Note: If your compiler doesn't support exception handling, you can, as a final resort, change the line throw ...; to:
std::cerr << "try to allocate memory failed!" << std::endl;
abort();
Note: If some global/static object's constructor use new, this won't employ the myNewHandler() function since that constructor will get called before main() starts. Unluckily there's no convenient method to guarantee that the std::set_new_handler() will be called before the first use of new. For instance, even if you put the std::set_new_handler() call in the constructor of global object, still you don't know if the module ("compilation unit") which contains that global object will be elaborated in initial or in last or somewhere in between. Thus you still don't have any guarantee that your call of std::set_new_handler() will happen before any other global's constructor gets invoked.