Re: alloca / _alloca / dynamic stack memory

From:

"Doug Harrison [MVP]" <dsh@mvps.org>

Newsgroups:

microsoft.public.vc.language

Date:

Fri, 09 Mar 2007 17:24:34 -0600

Message-ID:

<05r3v25och8ardc5qsabgb9veeb8218st4@4ax.com>

On Fri, 9 Mar 2007 13:40:48 -0600, "Ben Voigt" <rbv@nospam.nospam> wrote:

It seems like you can accomplish the same thing by using a local
std::string variable:

void f(const char* str)
{
std::string s(str);
}

Why would I want to write this as:

void f(const char* str)
{
String* s = new (StackAlloc) String(str);
}

Because the OP's method allows:

void f(int n)
{
String* slist = new (StackAlloc) String[n];
}

but MSVC++ doesn't allow (gcc does):

void f(int n)
{
std::string s[n];
}

Clearly, I was commenting on the scalar example he had just described. He
didn't get to arrays until his next paragraph, which began:

Okay...so what about arrays. No problem... If we go "String* str =
new(StackAlloc) String[N]" we should get this array allocated on the stack
dynamically...for each item in the array, the default constructor is
called...

I acknowledged this in my reply to that part of his message:

This I recognize as a new capability. I'm not sure it would be useful,
since each String is going to have to go to the heap, so you will have
eliminated only one call to the heap manager.

And finally, I ended my message with:

The array usage you described would be useful only for
simple types, so now it sounds like you're asking for dynamic arrays.

It pays to read the whole thing. :)

--
Doug Harrison
Visual C++ MVP