ifstream binary read into unsigned char b[LENGTH]

[Steven T. Hatton]

What is the best way to read data from a file into a fixed size array of
unsigned char? This is a file holding only a SHA1 digest. What I would
really like to do is initialize the ifstream with the buffer allocated to
hold the data, seekg(ios_base: :end) and magically have the data appear in
the buffer. I believe something close to this can be done.

I might allocate the array:

const unsigned LENGTH = 256;
unsigned char b[LENGTH];

Then cast it to a std::streambuf

std::streambuf sb(b[LENGTH]);

But can I construct std::ifstream that uses that same memory to hold the
data from its file? How to I prevent reading past the end of my buffer, or
the end of the file data?
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NOUN:1. Money or property bequeathed to another by will. 2. Something handed
down from an ancestor or a predecessor or from the past: a legacy of
religious freedom. ETYMOLOGY: MidE legacie, office of a deputy, from OF,
from ML legatia, from L legare, to depute, bequeath. www.bartleby.com/61/

[Rolf Magnus]

Re: ifstream binary read into unsigned char b[LENGTH]

Steven T. Hatton wrote:
[color=blue]
> What is the best way to read data from a file into a fixed size array of
> unsigned char? This is a file holding only a SHA1 digest. What I would
> really like to do is initialize the ifstream with the buffer allocated to
> hold the data, seekg(ios_base: :end) and magically have the data appear in
> the buffer.[/color]

Why should a seek read any data?
[color=blue]
> I believe something close to this can be done.
>
> I might allocate the array:
>
> const unsigned LENGTH = 256;
> unsigned char b[LENGTH];
>
> Then cast it to a std::streambuf
>
> std::streambuf sb(b[LENGTH]);[/color]

I don't think that you can be sure that the buffer will contain exactly the
data you want. After all, it's a buffer that's meant for internal use.
[color=blue]
> But can I construct std::ifstream that uses that same memory to hold the
> data from its file?[/color]

The purpose of streams is text formatting. If you want to read binary data,
just use a filebuf directly.
[color=blue]
> How to I prevent reading past the end of my buffer, or the end of the file
> data?[/color]

Your idea seems overly complicated to me. Just use the filebuf::sgetn( )
function to read as many bytes as you want and you're done.

[Steven T. Hatton]

Re: ifstream binary read into unsigned char b[LENGTH]

Rolf Magnus wrote:
[color=blue]
> Steven T. Hatton wrote:
>[color=green]
>> What is the best way to read data from a file into a fixed size array of
>> unsigned char? This is a file holding only a SHA1 digest. What I would
>> really like to do is initialize the ifstream with the buffer allocated to
>> hold the data, seekg(ios_base: :end) and magically have the data appear in
>> the buffer.[/color]
>
> Why should a seek read any data?[/color]

As I understand things, that will (usually?) force the file to be completely
read into memory.
[color=blue]
> Your idea seems overly complicated to me. Just use the filebuf::sgetn( )
> function to read as many bytes as you want and you're done.[/color]

Will that copy the bytes from one location in memory to another? That's
what I want to avoid. This is basically the same thing as move semantics.
Rather than copying the contents of the buffer, I simply want to take
ownership of it.
--
NOUN:1. Money or property bequeathed to another by will. 2. Something handed
down from an ancestor or a predecessor or from the past: a legacy of
religious freedom. ETYMOLOGY: MidE legacie, office of a deputy, from OF,
from ML legatia, from L legare, to depute, bequeath. www.bartleby.com/61/