How to convert char* to unsigned short in C++

Question

I have a char* name which is a string representation of the short I want, such as "15" and need to output this as unsigned short unitId to a binary file. This cast must also be cross-platform compatible.

Is this the correct cast: unitId = unsigned short(temp);

Please note that I am at an beginner level in understanding binary.

Answer 1

I assume that your char* name contains a string representation of the short that you want, ie "15" .

Do not cast a char* directly to a non-pointer type. Casts in C don't actually change the data at all (with a few exceptions)--they just inform the compiler that you want to treat one type into another type. If you cast a char* to an unsigned short , you'll be taking the value of the pointer (which has nothing to do with the contents), chopping off everything that doesn't fit into a short , and then throwing away the rest. This is absolutely not what you want.

Instead use the std::strtoul function, which parses a string and gives you back the equivalent number:

unsigned short number = (unsigned short) strtoul(name, NULL, 0);

(You still need to use a cast, because strtoul returns an unsigned long . This cast is between two different integer types, however, and so is valid. The worst that can happen is that the number inside name is too big to fit into a short --a situation that you can check for elsewhere.)

Answer 2

#include <boost/lexical_cast.hpp>

unitId = boost::lexical_cast<unsigned short>(temp);

Answer 3

To convert a string to binary in C++ you can use stringstream.

#include <sstream>

. . .

int somefunction()
{
    unsigned short num;
    char *name = "123";
    std::stringstream ss(name);

    ss >> num;

    if (ss.fail() == false)
    {
        // You can write out the binary value of num.  Since you mention
        // cross platform in your question, be sure to enforce a byte order.
    }
}

Answer 4

that cast will give you (a truncated) integer version of the pointer, assuming temp is also a char*. This is almost certainly not what you want (and the syntax is wrong too). Take a look at the function atoi, it may be what you need, eg unitId = (unsigned short)(atoi(temp)); Note that this assumes that (a) temp is pointing to a string of digits and (b) the digits represent a number that can fit into an unsigned short

Answer 5

Is the pointer name the id, or the string of chars pointed to by name ? That is if name contains "1234" , do you need to output 1234 to the file? I will assume this is the case, since the other case, which you would do with unitId = unsigned short(name) , is certainly wrong.

What you want then is the strtoul() function.

char * endp
unitId = (unsigned short)strtoul(name, &endp, 0);
if (endp == name) {
     /* The conversion failed. The string pointed to by name does not look like a number. */
}

Be careful about writing binary values to a file; the result of doing the obvious thing may work now but will likely not be portable.

Answer 6

If you have a string (char* in C) representation of a number you must use the appropriate function to convert that string to the numeric value it represents.

There are several functions for doing this. They are documented here: http://www.cplusplus.com/reference/clibrary/cstdlib