Re: baffled by 'new' operator with array type.

From:

Ian Collins <ian-news@hotmail.com>

Newsgroups:

comp.lang.c++

Date:

Sat, 06 Jul 2013 21:46:24 +1200

Message-ID:

<b3q7bgFmks6U2@mid.individual.net>

Ray Dillinger wrote:

I have the following type declarations, which I thought were quite simple;

const uint16_t SUBTREES = 64;
typedef struct stringnode *strpt; // a strpt is a pointer to a struct stringnode.
typedef strpt branchtype [SUBTREES]; // a branchtype is an array of SUBTREES strpt.

and the following function, which also seems simple. (I have added the line numbers
at the front so you can see which lines the error messages refer to).

132: // allocate and return a subtree, copied from argument within the (modular) range specified
133: branchtype *branchcopy(const branchtype * const subtrees, const int start, const int end){

General point, making the int parameters const is unnecessary.

134: branchtype *newbranch = new(branchtype);
135: int count;
136: for (count = start; count != end; count = (count + 1) % SUBTREES)
137: newbranch[count] = subtrees[count];
138: return(newbranch);
139: }

This function is supposed to allocate a new branchtype (via a pointer to it which is
named newbranch) and then copy a subrange of an existing branchtype (whose address
it gets via its argument subtrees) to it. The subrange copied is intended to be
'modular' in that if 'end' is less than 'start' it copies a subrange from 'start'
to the end of the array, then continues from the beginning of the array to 'end'.

When compiling this function, I get the following errors and I don't understand
what I've done wrong.

string3.cpp:133:41: error: cannot convert ?stringnode**? to ?stringnode* (*)[64]? in initialization

I think you are assuming using a typedef for an array makes it something
other than an array.

string3.cpp:136:38: error: invalid array assignment

Here you are trying to assign one array to another which is something
the language doesn't support.

Your should consider using std::vector rather than naked arrays and
pointers to them.
--
Ian Collins