Dynamic array and void pointers

Question

I have quite peculiar problem. I want initialize an array pointed by a void pointer to which memory is allocated using new as shown below.

const int ARRAY_SIZE = 10;
void InitArray()
{
    int *ptrInt = new int[ARRAY_SIZE];
    for(int i=0; i<ARRAY_SIZE;i++)
    {
         ptrInt[i] = 1; //OK
    }

    void *ptrVoid = new int[ARRAY_SIZE];
    for(int i=0; i<ARRAY_SIZE;i++)
    {
         *(int*)ptrVoid[i] = 1; //Culprit  : I get a compiler error here 
                                //(error C2036: 'void *' : unknown size)
    }
}

Now, I want to initialize the elements of this array which is pointed by ptrVoid with say 1. How do I go about it? With this code I get a compiler error as shown in the code(I am using VS 2010). Any suggestions?

Why do it all at once in the expression? Just get yourself a new intptr, and assign it the value of your voidptr. There are no extra points for the most cryptic code! — Bo Persson
– Bo Persson, Commented Mar 23, 2011 at 17:58

Carl Norum · Accepted Answer · 2011-03-23 00:51:53Z

9

You have an order of operations problem (and an extra *). Try this inside your second loop:

((int *)ptrVoid)[i] = 1;

answered Mar 23, 2011 at 0:51

Carl Norum

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1 Comment

Carl Norum Over a year ago

@Chris - yes it is. The [] array index operator occurs first - that's where the error comes from. The fact that he has an extra dereference is also an error, but not the one causing the compiler message.

Lightness Races in Orbit · Accepted Answer · 2011-03-23 00:54:06Z

2

*(int*)ptrVoid[i] is *((int*)(ptrVoid[i])), and you're dereferencing too many times (the [] does a dereference).

Write ((int*)ptrVoid)[i] (or, better, static_cast<int*>(ptrVoid)[i]) then re-consider your use of void* at all.

answered Mar 23, 2011 at 0:54

Lightness Races in Orbit

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2 Comments

Prasad Dixit Over a year ago

I wish I could avoid using void pointer but its use was necessary as I don't know what type of data I will have store using that pointer before hand and I don't want to use templates as it was slowing down the speed. I am using it in a very low level function which is called zillion times to read data from file.

Lightness Races in Orbit Over a year ago

@user672160: Templates do not affect execution speed. They are a compile-time construct only. Also, speed cannot be slowed down or sped up: an object has a speed that can be decreased or increased, making the object slow down or speed up, respectively.

Jon · Accepted Answer · 2011-03-23 00:52:16Z

1

You just need to parenthesize correctly and cast the void* to an int*, so that the compiler knows how many bytes to offset when you index it with [i].

for(int i=0; i<ARRAY_SIZE;i++)
{
     ((int*)ptrVoid)[i] = 1;
}

answered Mar 23, 2011 at 0:52

Jon

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Comments

Keith · Accepted Answer · 2011-03-23 02:34:40Z

0

How about:

int* ptrVoidAsInt = new int[ARRAY_SIZE];
for(int i=0; i<ARRAY_SIZE;i++)
{
     ptrVoidAsInt[i] = 1;                               
}
void* ptrVoid = ptrVoidAsInt;

But, one has to wonder what the meaning of either a void array or 1 initialised data is. Is this really an array of int or some other type that is going to be passed as a void* and then recast back to a typed array?

answered Mar 23, 2011 at 2:34

Keith

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