Reading text from file to unsigned char array, errors while trying to use example [closed]

Question

I'm trying to use example from:

https://stackoverflow.com/a/6832677/1816083 but i have:

invalid conversion from `unsigned char*' to `char*'
initializing argument 1 of `std::basic_istream<_CharT, _Traits>& std::basic_istream<_CharT, _Traits>::read(_CharT*, std::streamsize) [with _CharT = char, _Traits = std::char_traits<char>]' 
invalid conversion from `void*' to `size_t'

in line:

size_t bytes_read = myfile.read((unsigned char *) buffer, BUFFER_SIZE);

That could be part of the reason the answer you link to didn't get a single vote. ;) — NPE
– NPE, Commented Mar 7, 2013 at 8:37
Do you just want the whole file into a linear array with one big fat read? or are you processing this one buffer-size at a time? — WhozCraig
– WhozCraig, Commented Mar 7, 2013 at 8:54
I suggest to flag inappropriate answer, that leads to this question(stackoverflow.com/a/6832677/1816083), for removal, it confuses people — kassak
– kassak, Commented Mar 7, 2013 at 8:54

WhozCraig · Accepted Answer · 2013-03-07 08:59:12Z

3

Firstly, read() takes a char* rather than unsigned char*. Secondly, it does not return the number of characters read.

Instead, try:

myfile.read((char*)buffer, BUFFER_SIZE);
std::streamsize bytes_read = myfile.gcount();

edited Mar 7, 2013 at 8:59

WhozCraig

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answered Mar 7, 2013 at 8:36

NPE

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7 Comments

cerber Over a year ago

invalid conversion from unsigned char*' to char*' maybe some include missing? i'm noob in cpp

kassak Over a year ago

@cerber remove word unsigned from buffer's definition

WhozCraig Over a year ago

@kassak he needs it as an unsigned char array (see the title of the question). He should add a (char *) cast to the first param of the .read() call.

kassak Over a year ago

@WhozCraig yep, missed this, looking only at question's body

cerber Over a year ago

#include <fstream> #include <iostream> #include <vector> #include <string> using namespace std; int main() { const unsigned int BUFFER_SIZE = 1024; unsigned char buffer[BUFFER_SIZE]; //... ifstream myfile("myfile.bin", ios::binary); if (myfile) { size_t bytes_read = myfile.read(( char *) buffer, BUFFER_SIZE); } return 0; } invalid conversion from void*' to size_t'

|

borisbn · Accepted Answer · 2013-03-07 09:08:02Z

1

IMHO the compiler's output is quite enought. It tells you, that you're trying to give unsigned char* to function, that waits char*. BTW, there is even a function name

std::basic_istream<_CharT, _Traits>::read(_CharT*, std::streamsize)
[with _CharT = char ...

If you need unsigned chars buffer[ ... ] then cast it to char*

unsigned char buffer[ BUFFER_SIZE ];
ifstream myfile("myfile.bin", ios::binary);
if (myfile)
{
    myfile.read((char*) buffer, BUFFER_SIZE);
    //          ^^^^^^^
    size_t bytes_read = myfile.gcount();
}

edited Mar 7, 2013 at 9:08

answered Mar 7, 2013 at 8:42

borisbn

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5 Comments

cerber Over a year ago

stackoverflow.com/a/6832677/1816083 , so this example is bad, how to do this properly?

kassak Over a year ago

@cerber And what do you always believe people?)

borisbn Over a year ago

"so this example is bad" - yes. "how to do this properly?" - declare buffer as array of char's or cast it to char* instead of unsigned char*

cerber Over a year ago

but i need uchar array

borisbn Over a year ago

Then cast it to char*. BTW, you choose that answer with negative vote, while there was another answers =)

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