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Mac Forum / Programming / CodeWarrior / July 2003Not sure why this is an error
I have a function which has one argument defined as const unsigned char ** (a list of pointers to Pascal strings) When I pass a pointer to a Str255 or even when I pass an actual unsigned char **, the compiler gives me this error: Error : illegal implicit conversion from 'unsigned char **' to 'const unsigned char **' When I remove the 'const' from the function, it compiles fine. Why would the 'const' prevent me from passing the unsigned char **?  Signaturegbeggs@Canada.com http://www3.sympatico.ca/~gbeggs/ > I have a function which has one argument defined as > [quoted text clipped - 7 lines] > Error : illegal implicit conversion from 'unsigned char **' to > 'const unsigned char **' a pointer to a const-unsigned-char-pointer is not the same as a pointer to an unsigned-char-pointer It isn't even default convertable to one. You need to read up on the meaning of the keyword "const" in C++ in Scott Meyers' and Herb Sutter's books (and so do I.) > a pointer to a const-unsigned-char-pointer is not the same as > a pointer to an unsigned-char-pointer > > It isn't even default convertable to one. You need to read up on > the meaning of the keyword "const" in C++ in Scott Meyers' and > Herb Sutter's books (and so do I.) I don't think of what I have as a pointer-to-const-unsigned-char-pointer. I was thinking of it as a const-pointer-to-pointer-to-unsigned-char. If that makes any sense. What would be the best way to define an unsigned char ** to be read-only then? (So that I could pass an unsigned char ** without problems) Signaturegbeggs@Canada.com http://www3.sympatico.ca/~gbeggs/ > I don't think of what I have as a > pointer-to-const-unsigned-char-pointer. [quoted text clipped - 4 lines] > What would be the best way to define an unsigned char ** to be read-only > then? (So that I could pass an unsigned char ** without problems) Read it right to left. char * pointer to char char const * pointer to constant character const char * pointer to character constant == char const * char * const constant pointer to character char const * const constant pointer to constant character const char * const constant pointer to character constant == char const * const char * * pointer to pointer to character char const * * const char * * pointer to pointer to constant character char * const * pointer to constant pointer to character char * * const constant pointer to pointer to character char const * const * const char * const * pointer to constant pointer to constant character char const * * const const char * * const constant pointer to pointer to constant character char * const * const constant pointer to constant pointer to character char const * const * const const char * const * const constant pointer to constant pointer to constant character conar char * volatile & const constant reference to volatile pointer to constant character You get the idea. So, if you want const pointer to pointer to unsigned char, you want unsigned char * * const However, that's almost certainly not what you mean, because you probably mean a pointer to a const pointer to unsigned char. "const pointer" means the pointer itself can't change, whereas I suspect you mean "pointer to const" which means that the pointee can't change, so that would be unsigned char * const * hth meeroh > You get the idea. > [quoted text clipped - 8 lines] > > unsigned char * const * Wow Thanks :) That's a bit of an eye opener. I've always found the way C defines types a bit confusing when it gets beyond the trivial. Signaturegbeggs@Canada.com http://www3.sympatico.ca/~gbeggs/ |
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