Wednesday, October 12, 2011

char const * -- string OR ptr-to-single-char?(my take

see also my blog - http://bigblog.tanbin.com/2011/09/qq-char-reflex.html
see http://www.codeguru.com/forum/archive/index.php/t-227977.html and http://cboard.cprogramming.com/c-programming/106063-difference-between-const-char-*-char-const-*.html

First thing first -- q(const char*) is better written as q(char const *) which reads left-to-right "non-const pointer to const char"

Q: If this is a common C-string, is the entire string const? Some say The const is on the first char only.
%%A:You can advance this (non-const) pointer through an array of chars, but you can't edit _any_ of the chars using this (const!) pointer - like changing 'X' to 'Y'.
 
Q: Do we need to explicitly remove constness of a q(char const *)?
A: Yes. Constness radiates left. Use const_cast to remove const
A: I saw this in the Macqurie coding test.

char const * ptr
- often used to represent a c-string, but technically a pointer to a single char. The real thing in memory could be a char-array or could be a single char. You can't tell. If you don't know the length of the array, then you don't know where the array ends, even if there's a continuous stream of 9999999999 chars.
- This pointer can't be used to state-edit the char, but the char is possibly state-editable by other pointers.

Q: how long is this c-string?
A: you can't tell. The original thingy
- could be a single char + null
- could be a single char without a null
- could be nothing but the null char
- could be a regular null-terminating c-string
- could be a bunch or chars without a null. This ptr may advance forever without a null character.
- or this pointer may point to a heap location already deallocated - dangling pointer
- or this pointer may be uninitialized

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