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Old 01-22-2011, 10:42 PM   #1
girlsmakegraves
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Post Error when converting C to C++


Hello,

I'm working on a project where I am converting a C file to C++. I have worked out 99% of the errors but there is still one lingering one that I cant seem to figure out.

I get the following error when compiling with g++
Code:
mscp.cpp:749: error: invalid conversion from ‘const void*’ to ‘move*’
mscp.cpp:750: error: invalid conversion from ‘const void*’ to ‘move*’
The offending lines are in this function:

Code:
static int cmp_move(const void *ap, const void *bp)
{
        struct move *a = ap;
        struct move *b = bp;

        if (a->prescore < b->prescore) return -1;
        if (a->prescore > b->prescore) return 1;
        return a->move - b->move; /* this makes qsort deterministic */    
}
I would rather if someone not directly gave me the answer but rather pushed me in the right direction.

What I can tell is that it is trying to convert from one datatype to another which was allowed in c but is invalid in cpp.
 
Old 01-22-2011, 11:07 PM   #2
a4z
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you may want to read this
http://www.cplusplus.com/doc/tutorial/typecasting/
 
Old 01-22-2011, 11:58 PM   #3
girlsmakegraves
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Thank you for that article. It cleared up a lot of my confusion. So from what I understood from that is that C++ requires some sort of type casting before assigning void* (void pointers) to other data types.

So I did this:
Code:
static int cmp_move(const void *ap, const void *bp)
{
        struct move *a = (short*)ap;
        struct move *b = (short*)bp;

        if (a->prescore < b->prescore) return -1;
        if (a->prescore > b->prescore) return 1;
        return a->move - b->move; /* this makes qsort deterministic */    
}
but now I'm receiving the following errors:

Code:
mscp.cpp:749: error: cannot convert ‘short int*’ to ‘move*’ in initialization
mscp.cpp:750: error: cannot convert ‘short int*’ to ‘move*’ in initialization
I don't exactly understand how I went wrong here. Any help would be appreciated.
 
Old 01-23-2011, 01:12 AM   #4
Sergei Steshenko
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Quote:
Originally Posted by girlsmakegraves View Post
Thank you for that article. It cleared up a lot of my confusion. So from what I understood from that is that C++ requires some sort of type casting before assigning void* (void pointers) to other data types.

So I did this:
Code:
static int cmp_move(const void *ap, const void *bp)
{
        struct move *a = (short*)ap;
        struct move *b = (short*)bp;

        if (a->prescore < b->prescore) return -1;
        if (a->prescore > b->prescore) return 1;
        return a->move - b->move; /* this makes qsort deterministic */    
}
but now I'm receiving the following errors:

Code:
mscp.cpp:749: error: cannot convert ‘short int*’ to ‘move*’ in initialization
mscp.cpp:750: error: cannot convert ‘short int*’ to ‘move*’ in initialization
I don't exactly understand how I went wrong here. Any help would be appreciated.

If you look at your

Code:
struct move *a = (short*)ap;
, you'll see that 'a' is pointer to 'move' struct while because of '(short*)' '(short*)ap' is pointer to 'short'. So, by your own construction, figuratively speaking you try to assign pointer to oranges to pointer to apples, and C++ justly objects. It objects because dereferencing pointer apples being actually pointer to oranges will yield unpredicted and quite possibly crashing/crushing results.
 
Old 01-23-2011, 02:55 AM   #5
girlsmakegraves
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Registered: Dec 2005
Posts: 5

Original Poster
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I realized I had to make it (move*) not (short*)

and that worked.

Thanks for all the assistance. I appreciate the reading provided too.
 
Old 01-23-2011, 03:55 AM   #6
Nominal Animal
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Location: Finland
Distribution: Xubuntu, CentOS, LFS
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Also note you really should mark a and b const, too. It allows the compiler to optimize code more aggressively, but it also helps you catch programming errors -- say, if in a later version you tried to modify something pointed by a or b. Because you cast it to a non-const type, the compiler will allow it, but the users of the function would assume otherwise because the content at ap and bp is marked const. (With aggressive optimization, the compiler may trust the declaration, and assume the values are not changed. This may result in very strange behaviour, very difficult bugs to pinpoint.)

Since you already realized the problem, let me push you a bit further. This is how you really should write the function:
Code:
static int cmp_move(const void *const ap, const void *const bp)
{
    const struct move *const a = (const struct move *const)ap;
    const struct move *const b = (const struct move *const)bp;

    if (a->prescore < b->prescore) return -1;
    if (a->prescore > b->prescore) return 1;
    return a->move - b->move;
}
The above version adds the const qualifier to ap and bp, to a and b, and to data pointed to by a and b.

Here's the three lines that help me get the const qualifiers right:
Code:
const char *s = ...; /* Data pointed to by s is constant. s++ is allowed, s[0]++ is not allowed. */
char *const s = ...; /* Pointer s is constant. s++ is not allowed, s[0]++ is allowed. */
const char *const s = ...; /* Pointer s and the data it points to are both constant. Neither s++ nor s[0]++ is allowed. */
Hope this helps you write better code,
Nominal Animal

Last edited by Nominal Animal; 03-21-2011 at 06:02 AM.
 
  


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