how to deal with the translation from "const char * " to "const unsigned char *"?

Discussion in 'C++' started by =?gb2312?B?wNbA1rTzzOzKpg==?=, Jan 29, 2007.

  1. i wrote:
     
    =?gb2312?B?wNbA1rTzzOzKpg==?=, Jan 29, 2007
    #1
    1. Advertisements

  2. =?gb2312?B?wNbA1rTzzOzKpg==?=

    Kaz Kylheku Guest

    At this point you stop wrestling with the C++ bullshit and just use a
    C style cast.

    The good old C style cast will implicitly produce the correct
    combination of reinterpret_cast and static_cast, without the verbiage
    cluttering your code.

    Since no typedef names are involved whose meanings could change, the C
    style cast is no less safe than the new style cast combination, and is
    much more readable.

    (If you use the C cast with abstracted types, like casting some A * to
    some B *, the danger is that someone may redefine what A and B is, and
    the cast will silently continue to work, even though the change may be
    such that the conversion has erroneous consequences).
     
    Kaz Kylheku, Jan 29, 2007
    #2
    1. Advertisements

  3. =?gb2312?B?wNbA1rTzzOzKpg==?=

    Kai-Uwe Bux Guest

    In the code you posted, you do not cast to "const unsigned char *". Try:

    unsigned char const * p = static_cast<unsigned char const *>("abcdg");


    Best

    Kai-Uwe Bux
     
    Kai-Uwe Bux, Jan 29, 2007
    #3

  4. The compiler is right. "unsigned char *" is non-const and string
    literals (like "abcdg") are const.


    If you're casting to a "unsigned char *" you need to ask yourself why.
    Will it be modified after the cast ? (that would be undefined and toast
    your process on some platforms)

    It seems like you have a design problem.
     
    Gianni Mariani, Jan 29, 2007
    #4
  5. =?gb2312?B?wNbA1rTzzOzKpg==?=

    Noah Roberts Guest

    It always intrigues me that such distinction is made in cases like this
    yet this is perfectly ok from a legal point of view:

    char * x = "I'm const";
     
    Noah Roberts, Jan 29, 2007
    #5
  6. Yes, sigh. I think the standard's commitee made a blunder on that one.

    This is due to a rather short sighted view that there was a large legacy
    code base that would break if that was not allowed. At a guess, I think
    this would not be in the standard as an exception if hindsight was
    available.
     
    Gianni Mariani, Jan 29, 2007
    #6
  7. =?gb2312?B?wNbA1rTzzOzKpg==?=

    Grizlyk Guest

    I think, the following expression
    unsigned char * p = static_cast<unsigned char *>( const_cast<char*>
    ("abcdg") );
    is not work because sizeof("type") can be not equal to sizeof("unsigned
    type") in theory,

    Assuming on your target sizeof("type")==sizeof("unsigned type")

    If "p" is _not_ const because there is old library parameter here and you
    are shure, that "p" is always using as "const unsigned char *" you can apply
    C-style cast
    (unsigned char *) ("abcdg")"
    or reinterpret_cast<>
    unsigned char * p = reinterpret_cast<unsigned char *>( const_cast<char*>
    ("abcdg") );

    else you can create a temporary storage

    const char data[]="abcdg";
    char buf[sizeof(data)]; strcpy(buf,data);
    unsigned char * p = reinterpret_cast<unsigned char *>( buf );
     
    Grizlyk, Jan 29, 2007
    #7
  8. =?gb2312?B?wNbA1rTzzOzKpg==?=

    Grizlyk Guest

    And i forget cast via "void*"

    unsigned char * p = static_cast<void*> ("abcdg");
     
    Grizlyk, Jan 29, 2007
    #8
  9. =?gb2312?B?wNbA1rTzzOzKpg==?=

    Grizlyk Guest

    No, can not, i was wrong, better to do like this

    void *const tmp = const_cast<char*>("abcdg");
    unsigned char *p = static_cast<unsigned char*>(tmp);
     
    Grizlyk, Jan 29, 2007
    #9
  10. =?gb2312?B?wNbA1rTzzOzKpg==?=

    Rolf Magnus Guest

    No. At this point you think thrice about letting a pointer to non-const
    point to a string literal, and twice about letting a pointer to unsigned
    char point to a char.
    For the second, there is rarely a reason. For the first, the only reason I
    can think of is to deal with legacy code that isn't const correct.
    No. Rather it produces any combination that will somehow make the target
    type from the supplied object. It doesn't magically know if that's the
    correct one. So better explicitly tell the compiler which conversion you
    actually want.
    Well, with the C style cast, the OP wouldn't have noticed that he let a
    non-const pointer point to something that is const and must not be changed.
    The compiler would have silently accepted the erroneous code.
    It always does silently whatever it takes, so it's always dangerous.
     
    Rolf Magnus, Jan 29, 2007
    #10
  11. Gianni Mariani a écrit :
    The standard comitee didn't want to constraint compilers with putting
    the array in a rw area.
    The compiler is free to put in ro and save some copy time. The array can
    even be shared among other part of the program.

    That is a reason why casting away the consteness is undefined in this
    case and should not be done.

    Michael
     
    Michael DOUBEZ, Jan 29, 2007
    #11
  12. =?gb2312?B?wNbA1rTzzOzKpg==?=

    Rolf Magnus Guest

    What does this have to do with the choice to allow letting a pointer to
    non-const point to string literals without a cast?
    Casting away constness isn't undefined. Only writing to an object that was
    initially declared const is.
     
    Rolf Magnus, Jan 29, 2007
    #12
  13. Michael DOUBEZ a écrit :
    In fact, it is legacy code that allowed this writing because string use
    to be defaulted as char[] and automaticaly casted to char*.
    And the standard changed the type of string to const char[] and thus
    strings can now only be automatically casted into const char*.

    Michael
     
    Michael DOUBEZ, Jan 29, 2007
    #13
  14. Rolf Magnus a écrit :
    Why would you be allowed to cast it to not const if not to be able to
    modify it ?
    And if it is done automatically, there is the risk you modify the string
    without remebering it is const.
    Yes, writing is undefined; that is what I meant.

    Michael
     
    Michael DOUBEZ, Jan 29, 2007
    #14
  15. =?gb2312?B?wNbA1rTzzOzKpg==?=

    Andre Kostur Guest

    So? That code is casting pointers-to-object. The pointer size won't
    change, regardless of the size of the pointed to object.
     
    Andre Kostur, Jan 29, 2007
    #15
  16. =?gb2312?B?wNbA1rTzzOzKpg==?=

    Grizlyk Guest

    I am not shure (i have no standard) that pointers for all types must have
    fixed sizeof() (as "int" for example) or unsigned type must have the same
    sizeof() as signed type.

    For example there are memory models, where "sizeof(char *_far)==4", but
    "sizeof(char *)==2".

    Let pointers have fixed sizeof() and sizeof("type")!=sizeof("unsigned
    type"). Here pointer arithmetic can be changed, because value of next
    pointer (++ptr) is depending from size of object pointer point to.

    For example:

    int *i=0;
    ++i; //value of i is 4

    char *c=0;
    ++c; //value of c is 1

    //assuming sizeof(int)!=sizeof(unsigned)
    unsigned *u=0;
    ++u; //value of u is 8
     
    Grizlyk, Jan 29, 2007
    #16
  17. =?gb2312?B?wNbA1rTzzOzKpg==?=

    Andre Kostur Guest

    sizeof(char * _far) is a non-standard type, and thus is implementation-
    dependant. All Standard pointers-to-object have the same size.
    Pointer arithmetic has nothing to do with sizeof(T*). But is related to
    sizeof(T).
     
    Andre Kostur, Jan 29, 2007
    #17
  18. =?gb2312?B?wNbA1rTzzOzKpg==?=

    Noah Roberts Guest

    I think you've missed the point. The compiler will not complain about
    code that does this:

    char * x = "hello";
    x[2] = 't';

    You'll only get a warning when you try to run the program...if
    then...maybe only when you create your release compile with its
    optimizations...maybe a customer will find it.

    The standard requires that this is ok. It is undefined, even in C where
    it comes from, but there can be no diagnostic. Most people would like
    to get the usual error about attempting to assign const to non-const
    without a cast, but you don't get that in this one special case. Allows
    for all sorts of unfortunate errors.
     
    Noah Roberts, Jan 29, 2007
    #18
  19. =?gb2312?B?wNbA1rTzzOzKpg==?=

    Daniel T. Guest

    I think, the following expression
    unsigned char * p = static_cast<unsigned char *>( const_cast<char*>
    ("abcdg") );
    is not work because sizeof("type") can be not equal to sizeof("unsigned
    type") in theory,[/QUOTE]

    AFAIK, the above is incorrect. sizeof( "unsigned type" ) always equals
    sizeof( "type" )
     
    Daniel T., Jan 29, 2007
    #19
  20. =?gb2312?B?wNbA1rTzzOzKpg==?=

    Rolf Magnus Guest

    Well, but using a pointer for accessing an object of a size different from
    the pointed-to type might lead to trouble if you aren't prepared for it.

    Btw: Different pointer types might also have different sizes.
     
    Rolf Magnus, Jan 29, 2007
    #20
    1. Advertisements

Ask a Question

Want to reply to this thread or ask your own question?

You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.