Thread: Does C++ new initialise it's value to zero?

Does C++ new initialise it's value to zero?

Code:

int main()
{
int *i=new int;
cout<<*i;//Will it always  be 0
getch();
return 0;
}

Do you know any compiler which does not do so???

I know of no compiler (or, more accurately, it's library) which offers a guarantee that memory allocated by operator new is implicitly initialised to zero, as providing such a guarantee implies a performance hit (it would be necessary to explicitly initialise the memory). In your example, the value of *i will correspond to whatever happens to be in memory when it is reserved for *i by the use of operator new. And that value is likely to be random junk.

In any event, what you are doing is yielding undefined behaviour (the act of reading an uninitialised value always yields undefined behaviour, and it is necessary to read a value to print it). Your program could just as easily terminate with a system crash as print junk data.

If you want something created with operator new to be initialised, then you have to do it.

Code:

#include <iostream>

int main()
{
     int *i=new int(0);
    std::cout<<*i;//  Now it will always be 0
    return 0;
}

If you are working with user defined types (with a constructor), rather than int, then you can do the initialisation in the default constructor.

I think there's a function called ZeroMemory or something which sets everything to 0 ... I might be wrong though.

ZeroMemory is an MS-Windows specific function. memset() is more portable.

Code:

// allocate 255 integers
int* array = new int[255];
// non-portable use of memset.  
memset(array,0,255 * sizeof(int));
// more portable way to set elements to 0
for(int i = 0; i < 255; i++)
   array[i] = 0;

Ok,It was just my luck that I got garbage value as 0. I have tried several times but you will not always get it as zero.

ZeroMemory is a macro which uses memset().

Code:

#define ZeroMemory(a, b) memset((a), 0, (b))

If you care about portability you're better off defining it yourself or using memset().

Yeah let's just ignore what Prelude said about it and throw all our two cents in.

Well the OP will never again declare uninitialized variables on purpose thanks to this thread.

Originally Posted by citizen

Well the OP will never again declare uninitialized variables on purpose thanks to this thread.

We can live in hope.