exception handling ( memory allocation)

[manzoor]

#include <iostream>
#include <new>

using std::cout;
using std::endl;
using std::bad_alloc;

int main()
{
	char* pdata = 0;
	size_t count = ~static_cast<size_t>(0) / 3; // if you divide it by two it'll work
	cout << endl
		 << count
		 << endl << count / 3 << endl;
	try
	{
		pdata = new char[count];
		cout << "Memory allocated." << endl;
	}
	catch(bad_alloc &ex)
	{
		cout << "Memory allocation failed." << endl
			<< "The information from the exception object is: "
			<< ex.what() << endl;
	}

	delete[] pdata;
	return 0;
}

Look at the bold part of my code,

When I divide it by 3, the program doesn't responds, but when divide it by 2 the program works and throws an exception but not in the case of 3.

Why?

[matsp]

If you really want to figure out how much memory you can allocate, I would suggest some method like this:

#include <iostream>
#include <new>

using std::cout;
using std::endl;
using std::bad_alloc;


size_t absdiff(size_t a, size_t b)
{
    
    if (a > b)
	return a-b;
    return b-a;
}

int main()
{
    char *pdata;

    size_t max = ~0;
    size_t min = 1024;
    size_t size;
    while(absdiff(min,max) > 1)
    {
	size = (min + max) / 2;
 	try
	{
	    cout << "trying size " << size << " min=" << min << " Max=" 
		 << max << endl;
	    pdata = new char[size];
	    cout << "Memory allocated." << endl;
	    min = size;
	    delete pdata;
	}
	catch(bad_alloc &ex)
	{
	    cout << "Memory allocation failed." << endl
		 << "The information from the exception object is: "
		 << ex.what() << endl;
	    max = size;
	}
    }
    cout << "Max size allocation = " << size << endl;
    return 0;
}

--
Mats

[manzoor]

Well what I wanna know is that why the code provided by me doesn't works,

It throws an exception when you divide it by 2
but the programs stops when you try to divide count by 3


since the number divided by 2 is larger than if that number is divided by 3

so if it can throw an excption to a larger number then why a not smaller number ?


I want to know whats causing that program not to respond.

[phantomotap]

O_o

When you see local 318 attempting to control a burning house do you blame the firemen for the fire?

Soma

[anon]

Works for me in either case (doesn't throw).

Perhaps your memory is rather fragmented and the OS has to do a lot of work to provide a contiguous block, so it just appears to be not responding?

[matsp]

Well what I wanna know is that why the code provided by me doesn't works,

It throws an exception when you divide it by 2
but the programs stops when you try to divide count by 3


since the number divided by 2 is larger than if that number is divided by 3

so if it can throw an excption to a larger number then why a not smaller number ?


I want to know whats causing that program not to respond.

I have no idea. Running the code you posted unmodified and compiled with gcc, it works fine.

--
Mats