C ++中来自string / int的字节

Question

I'm a beginning user in C++ and I want to know how to do this: How can I 'create' a byte from a string/int. So for example I've:

string some_byte = "202";

When I would save that byte to a file, I want that the file is 1 byte instead of 3 bytes. How is that possible? Thanks in advance, Tim

Answer 1

I would use C++'s String Stream class <sstream> to convert the string to an unsigned char.

And write the unsigned char to a binary file.

so something like [not real code]

std::string some_byte = "202";
std::istringstream str(some_byte);
int val;
if( !(str >> val))
{
  // bad conversion
}

if(val > 255)
{
  // too big
}

unsigned char ch = static_cast<unsigned char>(val);

printByteToFile(ch); //print the byte to file.

Answer 2

In C++, casting to/from strings is best done using string streams:

#include <sstream>
// ...
std::istringstream iss(some_string);
unsigned int ui;
iss >> ui;
if(!iss) throw some_exception('"' + some_string + "\" isn't an integer!");
unsigned char byte = i;

To write to a file, you use file streams. However, streams usually write/read their data as strings. you will have to open the file in binary mode and write binary, too:

#include <fstream>
// ...
std::ofstream ofs("test.bin", std::ios::binary);
ofs.write( reinterpret_cast<const char*>(&byte), sizeof(byte)/sizeof(char) );

Answer 3

The simple answer is...

int value = atoi( some_byte ) ;

There are a few other questions though.

1) What size is an int and is it important? (for almost all systems it's going to be more than a byte)

int size = sizeof(int) ;

2) Is the Endianness important? (if it is look in to the htons() / ntohs() functions)

Answer 4

Use boost::lexical_cast

#include "boost/lexical_cast.hpp"
#include <iostream>

int main(int, char**)
{
    int a = boost::lexical_cast<int>("42");
    if(a < 256 && a > 0)
        unsigned char c = static_cast<unsigned char>(a);

}

You'll find the documentation at http://www.boost.org/doc/libs/1_43_0/libs/conversion/lexical_cast.htm

However, if the goal is to save space in a file, I don't think it's the right way to go. How will your program behave if you want to convert "257" into a byte? Juste go for the simplest. You'll work out later any space use concern if it is relevant (thumb rule: always use "int" for integers and not other types unless there is a very specific reason other than early optimization)

EDIT As the comments say it, this only works for integers, and switching to bytes won't (it will throw an exception). So what will happen if you try to parse "267"? IMHO, it should go through an int, and then do some bounds tests, and then only cast into a char. Going through atoi for example will result extreamly bugs prone.