Can't copy from char pointer to short

Question

I have a char pointer that holds the following information:

char* data = "22";

I want to copy it from the char into a short variable. I have the following code:

short BinarySerializer::getMessageTypeID(char* data)
{
    short* messageTypeID;
    memcpy( messageTypeID, data, sizeof( messageTypeID ) );
    return *messageTypeID;
}

I don't know why, but when I'm printing it I get: 12850.

This is how I'm printing it:

short temp = msg->messageTypeID;
cout << "Message ID is: " << temp << endl;

Thanks in advance

_"I want to copy it from the char into a short variable."_ No, you want to **convert** it to a `short` value most probably. That's a big difference. — πάντα ῥεῖ, Jan 11 '15 at 10:24
No, I want to copy. because in the future I will have more data on the char array (pointer). — Shaul Zuarets, Jan 11 '15 at 10:25
@ShaulZuarets So clarify please what exactly you expect to be output for `temp`. — πάντα ῥεῖ, Jan 11 '15 at 10:28
I expect the output to be 22. Even if the char array is "22sdfsd". — Shaul Zuarets, Jan 11 '15 at 10:32
@ShaulZuarets As mentioned _copying_ isn't the right approach to do so. You want to parse numbers from a [`std::stringstream`](http://en.cppreference.com/w/cpp/io/basic_stringstream). — πάντα ῥεῖ, Jan 11 '15 at 10:36
What I really want to do is to take a char array and extract from it short, int and some more variables. I thought memcpy is the right approach. I Would be glad to hear about more approaches.. Thanks! — Shaul Zuarets, Jan 11 '15 at 10:48

Vlad from Moscow · Answer 1 · 2015-01-11T11:02:38.407

Maybe you mean something from the following

#include <iostream>
#include <sstream>
#include <numeric>
#include <cstring>

struct BinarySerializer
{
    short getMessageTypeID( const char *data ) const;
    short getMessageTypeID1( const char *data ) const;
};

short BinarySerializer::getMessageTypeID( const char *data ) const
{
    short messageTypeID = 0;
    std::istringstream is( data );

    is >> messageTypeID;

    return messageTypeID;
}

short BinarySerializer::getMessageTypeID1( const char *data ) const
{
    short messageTypeID;

    messageTypeID = std::accumulate( data, data + std::strlen( data ), ( short)0,
                                     []( short acc, char c ) 
                                     { 
                                        return 10 * acc + ( c - '0' ); 
                                     } );

    return messageTypeID;
}

int main()
{
    const char *s = "22";

    std::cout << BinarySerializer().getMessageTypeID( s ) << std::endl;
    std::cout << BinarySerializer().getMessageTypeID1( s ) << std::endl;
}

The output is

22
22

If the data can look like "22sdfsd". then you need to add code that will find the first non-digit value.

Also you could use function std::strtol and check whether the obtained value is in the range for values of type short.

For example (without checking whether the obtained value is valid for type short)

short BinarySerializer::getMessageTypeID( const char *data ) const
{
    short messageTypeID = 0;

    messageTypeID = ( short )strtol( data, nullptr, 10 );   

    return messageTypeID;
}

score 1 · Accepted Answer · edited May 23 '17 at 12:30

Let's take a look at the memory, assuming size of short is twice the size of char (not always the case!):

char array with "22" string is in memory:

50 50

so basically

00110010 00110010

when you memcpy it to a short, you get a single number, consisting of two bytes:

00110010 00110010

which, translated to decimal, means 12850.

Now, I think you really want to convert the "22" ascii table into "22" value. In such case, you may refer to

Convert string to int C++

In case you really need support for strings like "23123dsdwdwqdwqd" (which is rarely a case), you may copy a substring of required size and work on the substring.

Keep in mind that only thing guaranteed by C++ standard when it comes to variable sizes is that

sizeof(char) <= sizeof (short) <= sizeof(int) <= sizeof(long)

So if you rely on the fact that sizeof(short) = 2 * sizeof(char) then you're not portable at all.

Can't copy from char pointer to short

2 Answers2