Re: exercise on reinterpret_cast

From:
David Wilkinson <no-reply@effisols.com>
Newsgroups:
microsoft.public.vc.language
Date:
Tue, 24 Jun 2008 13:14:24 -0400
Message-ID:
<e5rIA3h1IHA.4004@TK2MSFTNGP03.phx.gbl>
Fil wrote:

Hi,

I am trying the following exercise in a book:

//Define an array of int.
//Take the starting address of that array and use static_cast to convert it
into an void*.
//Write a function that takes a void*, a number (indicating a number of
bytes),
//and a value (indicating the value to which each byte should be set) as
arguments.
//The function should set each byte in the specified range to the specified
value.
//Try out the function on your array of int.

#include <iostream>

void func(void* p,short int s,unsigned char v)
{
    unsigned char * pp=reinterpret_cast<unsigned char*>(p);
    for (int i=0;i<s;i++)
    {
        *(pp+i)=v;
    }
}

int main()
{
    unsigned int iA[10];
    for (int i=0;i<10;i++)
    {
        int j;
        std::cout << "gimme some int" << std::endl;
        std::cin >> j;
        iA[i]=j;
    }
    for (int i=0;i<10;i++)
    {
        std::cout << iA[i] << std::endl;
    }
    void * p=static_cast<void *>(&iA);
    std::cout << "gimme a number of bytes to change" << std::endl;
    short i;
    std::cin >> i;
    std::cout << "gimme a value I will force those bytes to" << std::endl;
    char j;
    std::cin >> j;
    func(p,i,j);
    for (int i=0;i<10;i++)
    {
        std::cout << iA[i] << std::endl;
    }
}

I am then expecting that, if I set my whoke array to zero, and I suggest to
change just one byte to 255, I'll have iA[0]=255 and all the other worth 0
(my int are 4 bytes long so when I change 1 byte I just change part of the
first int and shouldn't touch the rest).
But I get 50 for the first element instead.

Can you please tell me where I am wrong?
Thank you


Fil:

What kind of book is this? Doesn't sound very good to me.

Anyway, lotsa things wrong here:

1. Pointer to first element of array is iA, or &iA[0]

2. Don't need to cast to void*

void* p = iA;

3. Can't input 255 to a char value:

unsigned int j;
std::cin >> j;

4. You need to cast back to the original type. Also, casting a void* to a
pointer type should be done with static_cast, not reinterpret cast

unsigned int* pp = static_cast<unsigned int*>(p);

Try it now ...

--
David Wilkinson
Visual C++ MVP

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