Bytes to integer

[zozoa]

Hello all.
I use to program in VB,but decided recently to try also C++.
I read the learning tutorial here which was very helpful.
My question is,how can I convert bytes values into integer.
I read a file using char[] and I want to convert specific bytes to integer.
For example,to convert the bytes from char[57] to char[60] into integer.

In VB there is a function bitconverter.ToInt32(array() as byte,start_pos as integer).

Thanks for helping.

[Duthomhas]

Typically C people just write it themselves.

Here's one:

template <typename IntegerType>
IntegerType bitsToInt( IntegerType& result, const unsigned char* bits, bool little_endian = true )
  {
  result = 0;
  if (little_endian)
    for (int n = sizeof( result ); n >= 0; n--)
      result = (result << 8) +bits[ n ];
  else
    for (unsigned n = 0; n < sizeof( result ); n++)
      result = (result << 8) +bits[ n ];
  return result;
  }

Hope this helps.

[Mitsakos]

Can you please give a small example on how to call this function?

Thank you.

[PWWWWR]

I think that you want to do readInt32() like in VB, right?

Some bytes in a file are just one value (one byte long):

int Int32 = (int)myByte;

Simple brace conversion...

But since Int32's are 4 bytes long....

byte MyBytes[4];  //set values to this also.
int Int32 = 0;

Int32 = (Int32 << 8) + MyBytes[3];
Int32 = (Int32 << 8) + MyBytes[2];
Int32 = (Int32 << 8) + MyBytes[1];
Int32 = (Int32 << 8) + MyBytes[0];

Now your Int32 is set to the combination of those 4 bytes!

Bytes can appear as something like (34, 45, 13, 30), but are a very large number in Int32 form. For this example it's actually equal to...

504180002 (try it!)


Also, a 'byte' value in C++ is an unsigned char:

typedef unsigned char byte;

[zozoa]

Thanks Mayflower,that is exactly what I have been looking for.

[Zaita]

@mayflower: That doesn't cover both endian types. I would suggest using Duoas's solution.