Xiaotao Luo Xiaotao Luo - 1 year ago 200
C++ Question

C++ : int* / float* to char*, why get different result using reinterpret_cast?

  • int* to char* :
    int* pNum = new int[1];
    pNum[0] = 57;
    char* pChar = reinterpret_cast< char* >(pNum);

    Result : pChar[0] = '9'; //'9' ASCII value is 57

  • float* to char* :
    float* pFloat = new float[1];
    pFloat[0] = 57; //assign the same value as before
    char* pChar = reinterpret_cast< char* >(pFloat);

Result : pChar[0] = 'a';

Could anyone tell me what is the difference and why ?

Answer Source

Both float and int are data types which are (usually) represented by four bytes:

b1 b2 b3 b4

However, those bytes are interpreted quite differently across the two types - if they wouldn't, there would be hardly any need for two types.

Now if you reinterpret the pointers to pointers-to-char, the result points only to the first byte, as this is the length of a char:

b1 b2 b3 b4
your char* points to here

As said, this first byte has a very different meaning for the two data types, and this is why the representation as a char in general differs.

Application to your example:

The number 57 in float (IEEE754 Single precision 32-bit) is represented in bits as

01000010 01100100 00000000 00000000

In contrast, the representation in a 32-bit integer format is

00000000 00000000 00000000 00111001

Here the number seems to be represented in "big-endian" format, where the most important byte (the one which changes the value of the int the most) comes first. As mentioned by @Jean-FrançoisFabre, in your PC it seems to be the other way round, but nevermind. For both conversions, I used this site.

Now your char* pointers point to the first of those 8-bit-blocks, respectively. And obviously they're different.

Recommended from our users: Dynamic Network Monitoring from WhatsUp Gold from IPSwitch. Free Download