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Why is this reinterpret_cast wrong?

Posted on 2013-01-15
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359 Views
Last Modified: 2013-01-17
ConstItems.hh
class ConstItems {
         const int getAsInt() const;

          private:
            string mValue;    
 }

ConstItems.cpp
#include "ConstItems.hh"
const int ConstItems::getAsInt() const
{
    int intVal = reinterpret_cast<int>(mValue.c_str());

    return intValue;
}

This is the compiler error:
error: cast from ‘const char*’ to ‘int’ loses precision


Why? Don't ger this.
0
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Question by:prain
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7 Comments
 
LVL 40

Expert Comment

by:evilrix
ID: 38780441
The standard doesn't guarantee that you can cast from a pointer to an int. This is what it actually says:

"A pointer can be explicitly converted to any integral type large enough to hold it."

In other words, the standard doesn't guarantee that an int is large enough to store a pointer. For example, on 64 bit platforms int is still quite often 32 bit and so could no be large enough to store a 64 bit pointer.

It's also worth pointing out that the only safe thing you can so with a point cast to an integral type is to cast it back to the original pointer. Any other usage is implementation defined and may or may not work as you expect (or hope).
0
 
LVL 22

Expert Comment

by:ambience
ID: 38780558
Use ptrdiff_t for safe and portable storage of simple pointers.

ptrdiff_t ConstItems::getAsInt() const
{
    ptrdiff_t intVal = reinterpret_cast<ptrdiff_t>(mValue.c_str());
    return intValue;
}
0
 
LVL 86

Accepted Solution

by:
jkr earned 100 total points
ID: 38780600
Do you want the pointer value (which I somwhow doubt) or the integer value of the string (if any)? In the latter case, you might be better off with

#include <sstream>
#include "ConstItems.hh"
const int ConstItems::getAsInt() const
{
    stringstream ss(mValue);
    int intVal;

    ss >> intVal;

   if(ss.fail()) {

       // conversion failed, not a number. Return sth. appropriate
    }

    return intValue;
}

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Author Closing Comment

by:prain
ID: 38783219
Thanks
0
 
LVL 40

Expert Comment

by:evilrix
ID: 38784274
prain,

Just so you know, the comment that actually answered the question you asked was http:#a38780441. Generally, you should accept the comment that answered your question. Of course, if another comment was also useful there is no reason you can't split the points.

As it stands, I have no objection to leaving things as they are, but please bear that in mind for the future. Experts are motivated by the points you award them and if you skip over the actual answer to the question you asked you'll soon end up de-motivating experts.

Thanks and good luck with your project.
0
 
LVL 34

Expert Comment

by:sarabande
ID: 38786134
to add to above comments:

you may easily find out whether pointer size exceeds int size by the following:

size_t sz_cc   =  sizeof(const char *) ;
size_t sz_int  =  sizeof(int);
std::cout << "size of const char pointer is " << (int)sz_cc << std::endl;
std::cout << "size of int is " << (int)sz_int << std::endl;

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you could assign any pointer without cast to a void *. the reverse action would need a reinterpret_cast:

void * voidptr = reinterpret_cast<int>(mValue.c_str());
...
std::string strval;
strval = reinterpret_cast<const char *>voidptr;

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note, usage of void pointers is rareley necessary in c++. you would do it only if an existing c interface requires it. in case of a string you much better would use a fixed-sized char array for transfering the string to any which cannot use a std::string.

Sara
0
 

Author Comment

by:prain
ID: 38787208
Thanks Sara.
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