Getting a byte value using stringstream

问题

I've got this (incorrect) sample code for getting a value out of stringstream and storing it in a byte-sized variable (it needs to be in a single byte var, not an int):

#include <iostream>
#include <sstream>

using namespace std;

int main(int argc, char** argv)
{
    stringstream ss( "1" );

    unsigned char c;
    ss >> c;

    cout << (int) c << endl;
}

The output when I run this is 49, which is not what I would like to see. Obviously, this is being treated as a char and not simple numeric value. What is the most c++ish way to get c to hold a 1 rather than a 49 when casted to an int?

Thanks!

回答1:

The most C++-ish way is certainly to parse the value properly by reading into another integral type, and then cast to a byte type (since reading into a char will never parse – it will always just read the next character):

typedef unsigned char byte_t;

unsigned int value;
ss >> value;
if (value > numeric_limits<byte_t>::max()) {
    // Error …
}

byte_t b = static_cast<byte_t>(value);

I’ve used unsigned int since that’s the most natural, although unsigned short would of course also work.

回答2:

A char will always do that. You need to read an int (or float or double, etc) or else the wrong 'formatter' will be called.

unsigned char c;
unsigned int i;
ss >> i;
c = i;

回答3:

Subtract '0' from it:

cout << (int) (c - '0') << endl;

'0' has a value of 48, so therefore 49 - 48 = 1

回答4:

stringstream ss( "1" );
unsigned char c;
{
    unsigned int i;
    ss >> i;
    c = i;
}
cout << static_cast<int>(c) << endl;

Will work. You could also do some unsafe pointer stuff but I would just go with the above.

回答5:

That's because string constants in C++ are treated like text.
Two options:

Encode the string using escaped numbers:
- Octal Numbers: \0<d>{1,3}
- Hex Numbers: \0x<d>{2}
std::stringstream( "\01\02\03\04\0xFF");
Or build an array of char and initialize it using numbers:

char data[] = { 0, 1, 2,3 ,4, 255 };

How about:

#include <iostream>
#include <sstream>

using namespace std;

int main(int argc, char** argv)
{
    char x[] = {1,0};
    stringstream ss( x);

    unsigned char c;
    ss >> c;

    cout << (int) c << endl;
}

来源：https://stackoverflow.com/questions/3356661/getting-a-byte-value-using-stringstream

标签

c++

iostream

stringstream