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digitalmars.D - Length of fixed size arrays

reply "Benjamin Thaut" <code benjamin-thaut.de> writes:
Do we already have something like this in phobos? If not should 
we add it? I'm not quite sure if the .length property of a static 
array is always a compile time constant.

I use something similar in C++ quite often.

template staticLength(alias symbol)
{
   enum size_t staticLength = staticLengthImpl!(typeof(symbol));
}

template staticLengthImpl(T : U[N], U, size_t N)
{
   enum size_t staticLengthImpl = N;
}

template staticLengthImpl(T)
{
   static assert(0, T.stringof ~ " is not an static array");
}

Kind Regards
Benjamin Thaut
Nov 17 2012
next sibling parent reply Andrej Mitrovic <andrej.mitrovich gmail.com> writes:
On 11/17/12, Benjamin Thaut <code benjamin-thaut.de> wrote:

 I'm not quite sure if the .length property of a static
 array is always a compile time constant.


When is it not a constant?
Nov 17 2012
parent "Benjamin Thaut" <code benjamin-thaut.de> writes:
On Saturday, 17 November 2012 at 16:55:46 UTC, Andrej Mitrovic 
wrote:

 On 11/17/12, Benjamin Thaut <code benjamin-thaut.de> wrote:

 I'm not quite sure if the .length property of a static
 array is always a compile time constant.


 When is it not a constant?


I had a problem in the past where it complained that accessing 
the length is not possible without a instance. But that could 
have been a bug.

Kind Regards
Benjamin Thaut
Nov 17 2012
prev sibling next sibling parent reply "bearophile" <bearophileHUGS lycos.com> writes:
Benjamin Thaut:


 I'm not quite sure if the .length property of a static array is 
 always a compile time constant.


Currently it's always a compile-time constant.

But I have asked for some kind of Variable Length arrays 
allocated on the stack, as in C99, because using alloca() to 
create a 2D array on the stack is not good, it's noisy, unsafe 
and bug-prone. In this case their length is contained in some 
scoped variable or in an argument, at run-time.

They are discussing VLAs again for C++14:
http://root.cern.ch/drupal/content/c14

Bye,
bearophile
Nov 17 2012
parent reply "Benjamin Thaut" <code benjamin-thaut.de> writes:
On Saturday, 17 November 2012 at 17:10:40 UTC, bearophile wrote:

 Benjamin Thaut:


 I'm not quite sure if the .length property of a static array 
 is always a compile time constant.


 Currently it's always a compile-time constant.

 But I have asked for some kind of Variable Length arrays 
 allocated on the stack, as in C99, because using alloca() to 
 create a 2D array on the stack is not good, it's noisy, unsafe 
 and bug-prone. In this case their length is contained in some 
 scoped variable or in an argument, at run-time.

 They are discussing VLAs again for C++14:
 http://root.cern.ch/drupal/content/c14

 Bye,
 bearophile


That would be nice, but a bigger problem I'm currently having, is 
that you can not initialize fixed size arrays like this:

int[4] bla = [1,2,3,4];

Without causing a allocation. The literal will be allocated on 
the heap, then copied onto the stack, and then it will become 
garbage.

Kind Regards
Benjamin Thaut
Nov 17 2012
parent Jonathan M Davis <jmdavisProg gmx.com> writes:
On Saturday, November 17, 2012 18:19:50 Benjamin Thaut wrote:

 That would be nice, but a bigger problem I'm currently having, is
 that you can not initialize fixed size arrays like this:
 
 int[4] bla = [1,2,3,4];
 
 Without causing a allocation. The literal will be allocated on
 the heap, then copied onto the stack, and then it will become
 garbage.


It's a well-known issue and will be fixed at some point. I suspect that it's 
simply a matter of one of the compiler devs making it a priority, but I don't 
know what all is involved in making the change. Certainly, it's an 
implementation issue rather than a language issue. My guess though is that it 
hasn't been tackled primarily because it's a performance issue, whereas so 
many other bugs actually result in incorrect programs.

- Jonathan M Davis
Nov 17 2012
prev sibling parent Jonathan M Davis <jmdavisProg gmx.com> writes:
On Saturday, November 17, 2012 17:55:37 Andrej Mitrovic wrote:

 On 11/17/12, Benjamin Thaut <code benjamin-thaut.de> wrote:

 I'm not quite sure if the .length property of a static
 array is always a compile time constant.


 
 When is it not a constant?


It's always a constant. The length of static arrays must always be known at 
compile time.

- Jonathan M Davis
Nov 17 2012