Re: size of array

From:

Mathieu Mazerolle <mathieu.mazerolle@gmail.com>

Newsgroups:

comp.lang.c++.moderated

Date:

Sun, 5 Oct 2008 20:59:05 CST

Message-ID:

<7d1f69c2-6bda-4d61-aaee-f274b40ac734@b2g2000prf.googlegroups.com>

Perhaps we'd have some new syntax like:
char *p = new char [100];
const size_t n = std::allocation_size(p);

This is what I suggested above: I like the idea! Why did this not
figure into the original design of new[]/delete[]? No one seems to be
able to answer this question!

I could imagine some code like:
     char *p = new char [100];
...
     void f(char *ptr) {
          const size_t n = std::allocation_size(ptr);
          ... do something with whatever ptr points to
     }
But what if we want do this:
     const char x[] = "hello";
     const char *p = x;
     const char *p2 = p;
     f(p2);
What will happen when we call f(char *) above?

My suggestion: undefined behavior. Like all array operations in C++,
the onus of correctness is on the user, not the language.

Given this std::vector seems a wiser choice to me. Or perhaps I
completely misunderstood what you meant. Could you please clarify?

Suggestion:
std::vector <T> p;
std::allocation_size(&p[0]) == p.capacity()

Obviously, this isn't a constraint that should be imposed on
std::vector, but illustrates how an implementation of std::vector
could use such a facility to improve efficiency.

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