question about "new" with pointers

hi

i'm trying to understand how to use new and while i was experimenting I came accross something i dont understand...

here is a quick example:

Code:

int main()
{
	int* var1;
	int* var2;
	
	var1 = new int[1];
	var2 = new int[1];
	
	var1[1] = 2;
	var2[100] = 3;
	var1[100] = 3;

	printf("var1[1] = %d, var2[100] = %d\n", var1[1], var2[100]);
	printf("var1[100] = %d\n", var1[100]);
	return 0;
}

why does the compiler let me give var1 and var2 values at array index 100 if I only made a new integer the size of 2 array indexes ([1])? I thought the "new" was like calling malloc just a little nicer

here is the output f the program aswell

var1[1] = 2, var2[100] = 3
var1[100] = 3
Press any key to continue

whats even more weird is that its actually being set to the right values 1 even tho i didnt allow that much space memory to be used when i used new!!!...now I really don get it..

thanx for your help in advance,
Boomba

You create two arrays of size 1, not 2. That means that all of your code exhibits undefined behavior because it accesses data past the bounds of the array.

var1 = new int[1]; // Array of size 1
var1[0] = 2; // OK.
var1[1] = 2; // Undefined behavior.
var1[100] = 2; // Undefined behavior.

The reason it appears to work is because undefined behavior means the compiler can make the code do anything - maybe crash, maybe work, maybe print garbage. In reality, new only allocates the number of objects that you specify when calling it. In this case, that is 1.

P.S. Don't forget:
delete [] var1;
delete [] var2;

> now I really don get it
Sooner or later, you'll find yourself in this mess
Memory issue with new[] and delete[]

Small simple programs seldom fall over with undefined behaviour. It's only much later when your programs reach some kind of critical mass that all hell breaks loose and you have problem after problem trying to figure out what is actually going on.