C99 dynamic array

[kwikius]

Does anyone know what a C99 dynamic array is, and if it will be
useable in C++?

regards
Andy Little

[Jeff Schwab]

kwikius wrote:

Does anyone know what a C99 dynamic array is

A variable-length array, i.e. an array whose size can be set by a
run-time variable.

<C>
#include <stdlib.h>

void f(size_t z) {

typedef enum some_t { some_value } some_type;

some_type a[5] = { some_value }; // always ok
// some_type b[z] = { some_value }; // always illegal
some_type c[z]; // ok in c99 only
}
</C>

, and if it will be useable in C++?

Not AFAIK, although C++0x will incorporate some features of C99. The
std::vector is preferable in most respects, anyway. The down-side of
vector is that it uses dynamic storage, which may incur more performance
overhead than automatic (stack-based) storage. Chances are excellent
that whatever you're doing, std::vector is a better choice than a raw array.

[kwikius]

On Feb 13, 4:52*am, Jeff Schwab <j...@schwabcen ter.comwrote:

kwikius wrote:

Does anyone know what a C99 dynamic array is

A variable-length array, i.e. an array whose size can be set by a
run-time variable.

<C>
#include <stdlib.h>

void f(size_t z) {

* * * * typedef enum some_t { some_value } some_type;

* * * * some_type a[5] = { some_value }; // always ok
// * * *some_type b[z] = { some_value }; // always illegal
* * * * some_type c[z]; // ok in c99 only}

</C>

<...>

The std::vector is preferable in most respects, anyway. *The down-side of
vector is that it uses dynamic storage, which may incur more performance
overhead than automatic (stack-based) storage.

Ah ha yes, but does the dynamic array actually just look like a
std::vector underneath (use heap allocation) or is there some other
magic.

Its a sort of holy grail isnt it, that you can create a dynamically
sized array without the heap, so if C99 dynamic array does use another
method to allocate, would be interesting.

(my guess is that it woud have same performance as std::vector. There
is no "magic")

regards
Andy Little

[Jeff Schwab]

kwikius wrote:

On Feb 13, 4:52 am, Jeff Schwab <j...@schwabcen ter.comwrote:

>kwikius wrote:

>>Does anyone know what a C99 dynamic array is

A variable-length array, i.e. an array whose size can be set by a
run-time variable.

<C>
#include <stdlib.h>

void f(size_t z) {

typedef enum some_t { some_value } some_type;

some_type a[5] = { some_value }; // always ok
// some_type b[z] = { some_value }; // always illegal
some_type c[z]; // ok in c99 only}

</C>

<...>

>The std::vector is preferable in most respects, anyway. The down-side of
vector is that it uses dynamic storage, which may incur more performance
overhead than automatic (stack-based) storage.

Ah ha yes, but does the dynamic array actually just look like a
std::vector underneath (use heap allocation) or is there some other
magic.

Its a sort of holy grail isnt it, that you can create a dynamically
sized array without the heap, so if C99 dynamic array does use another
method to allocate, would be interesting.

(my guess is that it woud have same performance as std::vector. There
is no "magic")

I believe VLAs use the stack, just like traditional arrays. The "magic"
is that the compiler doesn't know a priori how big the function's stack
frame will be; in fact, multiple invocations of the same function might
need frames of different sizes. It would be interesting to know what
kinds of complications this creates for compiler implementers, and how
those problems have been worked around.

[Pete Becker]

On 2008-02-13 15:50:05 -0500, Jeff Schwab <je**@schwabcen ter.comsaid:

>
I believe VLAs use the stack, just like traditional arrays.

They're not required to.

--
Pete
Roundhouse Consulting, Ltd. (www.versatilecoding.com) Author of "The
Standard C++ Library Extensions: a Tutorial and Reference
(www.petebecker.com/tr1book)

[=?UTF-8?B?RXJpayBXaWtzdHLDtm0=?=]

On 2008-02-13 21:50, Jeff Schwab wrote:

kwikius wrote:

>On Feb 13, 4:52 am, Jeff Schwab <j...@schwabcen ter.comwrote:

>>kwikius wrote:
Does anyone know what a C99 dynamic array is
A variable-length array, i.e. an array whose size can be set by a
run-time variable.

<C>
#include <stdlib.h>

void f(size_t z) {

typedef enum some_t { some_value } some_type;

some_type a[5] = { some_value }; // always ok
// some_type b[z] = { some_value }; // always illegal
some_type c[z]; // ok in c99 only}

</C>

<...>

>>The std::vector is preferable in most respects, anyway. The down-side of
vector is that it uses dynamic storage, which may incur more performance
overhead than automatic (stack-based) storage.

Ah ha yes, but does the dynamic array actually just look like a
std::vector underneath (use heap allocation) or is there some other
magic.

Its a sort of holy grail isnt it, that you can create a dynamically
sized array without the heap, so if C99 dynamic array does use another
method to allocate, would be interesting.

(my guess is that it woud have same performance as std::vector. There
is no "magic")

I believe VLAs use the stack, just like traditional arrays. The "magic"
is that the compiler doesn't know a priori how big the function's stack
frame will be; in fact, multiple invocations of the same function might
need frames of different sizes. It would be interesting to know what
kinds of complications this creates for compiler implementers, and how
those problems have been worked around.

There should be little trouble to create the actual frame, nor to use
VLAs, you just have to remember the size of each VLA and use it to
calculate the offsets. Which means that some operations (probably all
that accesses elements in an VLA) will require one or more additional
computations to calculate the offset from the stack-pointer.

--
Erik Wikström

[Jeff Schwab]

Erik Wikström wrote:

On 2008-02-13 21:50, Jeff Schwab wrote:

>kwikius wrote:

>>On Feb 13, 4:52 am, Jeff Schwab <j...@schwabcen ter.comwrote:
kwikius wrote:
Does anyone know what a C99 dynamic array is
A variable-length array, i.e. an array whose size can be set by a
run-time variable.

<C>
#include <stdlib.h>

void f(size_t z) {

typedef enum some_t { some_value } some_type;

some_type a[5] = { some_value }; // always ok
// some_type b[z] = { some_value }; // always illegal
some_type c[z]; // ok in c99 only}

</C>
<...>

The std::vector is preferable in most respects, anyway. The down-side of
vector is that it uses dynamic storage, which may incur more performance
overhead than automatic (stack-based) storage.
Ah ha yes, but does the dynamic array actually just look like a
std::vector underneath (use heap allocation) or is there some other
magic.

Its a sort of holy grail isnt it, that you can create a dynamically
sized array without the heap, so if C99 dynamic array does use another
method to allocate, would be interesting.

(my guess is that it woud have same performance as std::vector. There
is no "magic")

I believe VLAs use the stack, just like traditional arrays. The "magic"
is that the compiler doesn't know a priori how big the function's stack
frame will be; in fact, multiple invocations of the same function might
need frames of different sizes. It would be interesting to know what
kinds of complications this creates for compiler implementers, and how
those problems have been worked around.

There should be little trouble to create the actual frame, nor to use
VLAs, you just have to remember the size of each VLA and use it to
calculate the offsets. Which means that some operations (probably all
that accesses elements in an VLA) will require one or more additional
computations to calculate the offset from the stack-pointer.

That's per-invocation overhead, right? So recursive functions with VLAs
could get very messy...

[Jeff Schwab]

Pete Becker wrote:

On 2008-02-13 15:50:05 -0500, Jeff Schwab <je**@schwabcen ter.comsaid:

>>
I believe VLAs use the stack, just like traditional arrays.

They're not required to.

Are there any implementations that use free store? If the compiler has
to ensure that all possible return paths free the memory, it seems like
it's half way to C++-style destructors.

[kwikius]

On Feb 13, 4:52*am, Jeff Schwab <j...@schwabcen ter.comwrote:

<C>
#include <stdlib.h>

void f(size_t z) {

<..>

* * * * some_type c[z]; // ok in c99 only}

Ah I think I get it. As far as the array is concerned z is a constant,
so it cant resize after its created. Thats quite neat, if you can grab
an arbitrary amount of space on the stack in a function.

As observed later, it then raises questions about how this affects
optimisations, as you have a runtime constant.

So it looks like you arent getting something for nothing, but maybe
you are getting it quite cheap :-)

regards
Andy Little

[Ian Collins]

kwikius wrote:

On Feb 13, 4:52 am, Jeff Schwab <j...@schwabcen ter.comwrote:

><C>
#include <stdlib.h>

void f(size_t z) {

<..>

> some_type c[z]; // ok in c99 only}

Ah I think I get it. As far as the array is concerned z is a constant,
so it cant resize after its created. Thats quite neat, if you can grab
an arbitrary amount of space on the stack in a function.

As observed later, it then raises questions about how this affects
optimisations, as you have a runtime constant.

There is also one potentially crippling problem with VLAs: no indication
of failure. You can bugger your stack without realising.

Another complication is the incompatibility with C caused by the
different interpretation of const,

void f() {
const size_t s = 42;

int bla[s];
}

requires a VLA in C, but not in C++.

--
Ian Collins.