ISO C++ forbids initialization in array new?

ISO C++ forbids initialization in array new?

[WU Yongwei]

Well, I see this in the gcc error message.  Can someone here kindly
point to me which part of the Standard specified this behaviour?  I
thought it should be in 5.3.4, but was not able to find the words
there.

By the way, anyone knows the rationale of this behaviour?

Thanks in advance.

Best regards,

Yongwei

Re: ISO C++ forbids initialization in array new?

[Jonathan Wakely]

WU Yongwei wrote:

> Well, I see this in the gcc error message.  Can someone here kindly
> point to me which part of the Standard specified this behaviour?  I
> thought it should be in 5.3.4, but was not able to find the words
> there.

It might be better if the error message said "non-default
initialization" since default initialization is allowed (and required).

I assume you are trying something like this:

    int* i = new int[5](23);


A new initializer must have the form (...) (see 5.3.4/1) and the only
way that can be used to initialize an array is for default construction.


This is OK, and default initialises the array, which default initialises
each element (8.5/5)

    int* i = new int[5]();

but this is not OK:

    int* i = new int[5](23);

because it is not valid to initialise an array like this:

    typedef int (five_ints)[5];
    five_ints i(23);

this gives:

array_init.cc:8: error: cannot initialize arrays using this syntax

HTH

jon

-- 
sigfault

Re: ISO C++ forbids initialization in array new?

[Jonathan Wakely]

Jonathan Wakely wrote:

> WU Yongwei wrote:
> 
> > Well, I see this in the gcc error message.  Can someone here kindly
> > point to me which part of the Standard specified this behaviour?  I
> > thought it should be in 5.3.4, but was not able to find the words
> > there.
> 
> It might be better if the error message said "non-default
> initialization" since default initialization is allowed (and required).

Oops! that was meant to say "required for some types" (e.g.
const-qualified types, non-POD types)

Too early in the morning for me to be answering questions like this!

jon

Re: ISO C++ forbids initialization in array new?

[Alisdair Meredith]

WU Yongwei wrote:

> Well, I see this in the gcc error message.  Can someone here kindly
> point to me which part of the Standard specified this behaviour?  I
> thought it should be in 5.3.4, but was not able to find the words
> there.
> 
> By the way, anyone knows the rationale of this behaviour?

It is not explicitly forbidden. Rather, there is no syntax defined that
would enable it (so it is implicitly forbidden)

you might be interested in the following paper from the last ISO
mailing ;¬)

http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2005/n1824.htm


AlisdairM

Re: ISO C++ forbids initialization in array new?

[Joe Buck]

On Fri, Aug 19, 2005 at 09:19:55AM +0100, Jonathan Wakely wrote:
> WU Yongwei wrote:
> 
> > Well, I see this in the gcc error message.  Can someone here kindly
> > point to me which part of the Standard specified this behaviour?  I
> > thought it should be in 5.3.4, but was not able to find the words
> > there.
> 
> It might be better if the error message said "non-default
> initialization" since default initialization is allowed (and required).
> 
> I assume you are trying something like this:
> 
>     int* i = new int[5](23);

While this is not gcc-help, maybe we could make a FAQ item for this.
For people who want to do something like this, I suggest

    std::vector<int> i(5, 23);

If the reason vector wasn't used in the first place was because some
API needs an array, then use

    std::vector<int> vector_i(5, 23);
    int* i = &*vector.begin();

The reason for the &* is to convert the vector iterator into a
pointer to the first element.

Re: ISO C++ forbids initialization in array new?

[Andreas Schwab]

Joe Buck <Joe.Buck@synopsys.COM> writes:

> On Fri, Aug 19, 2005 at 09:19:55AM +0100, Jonathan Wakely wrote:
>> I assume you are trying something like this:
>> 
>>     int* i = new int[5](23);
>
> While this is not gcc-help, maybe we could make a FAQ item for this.
> For people who want to do something like this, I suggest
>
>     std::vector<int> i(5, 23);
>
> If the reason vector wasn't used in the first place was because some
> API needs an array, then use
>
>     std::vector<int> vector_i(5, 23);
>     int* i = &*vector.begin();

This is not quite the same, because when vector_i goes out of scope it
will be destructed, so you can't use the pointer beyond that point.

Andreas.

-- 
Andreas Schwab, SuSE Labs, schwab@suse.de
SuSE Linux Products GmbH, Maxfeldstraße 5, 90409 Nürnberg, Germany
Key fingerprint = 58CA 54C7 6D53 942B 1756  01D3 44D5 214B 8276 4ED5
"And now for something completely different."

Re: ISO C++ forbids initialization in array new?

[WU Yongwei]

On 8/19/05, Jonathan Wakely <cow@compsoc.man.ac.uk> wrote:
> WU Yongwei wrote:
> > Well, I see this in the gcc error message.  Can someone here kindly
> > point to me which part of the Standard specified this behaviour?  I
> > thought it should be in 5.3.4, but was not able to find the words
> > there.
[snipped]
> This is OK, and default initialises the array, which default initialises
> each element (8.5/5)
> 
>    int* i = new int[5]();
> 
> but this is not OK:
> 
>    int* i = new int[5](23);
> 
> because it is not valid to initialise an array like this:
> 
>    typedef int (five_ints)[5];
>    five_ints i(23);
> 
> this gives:
> 
> array_init.cc:8: error: cannot initialize arrays using this syntax

This sounds reasonable.  The only problem is that it does not
constitute proof.  It is complete OK if the standard disallowed `int
a[5](23)' while allowing `new int[5](23)', just as older GCC did.

On 8/19/05, Alisdair Meredith <alisdair.meredith@uk.renaultf1.com> wrote:
> WU Yongwei wrote:
> 
> > Well, I see this in the gcc error message.  Can someone here kindly
> > point to me which part of the Standard specified this behaviour?  I
> > thought it should be in 5.3.4, but was not able to find the words
> > there.
> >
> > By the way, anyone knows the rationale of this behaviour?
> 
> It is not explicitly forbidden. Rather, there is no syntax defined that
> would enable it (so it is implicitly forbidden)

My observations.  According to 5.3.4, an expression like `new
int[5](23)' is well-formed.  Just that no semantics are formally
defined for it.

The behaviour might be OK, but the message seems a little misleading. 
Would something like `cannot specify initializer for arrays' be better
since it really has little to do with `array new'?

Also, the trends of gcc removing existing `extensions' are a little
worrisome.  IMHO, the extension should be allowed and a warning in the
case of `-W' or `-ansi' should be issued.

Best regards,

Yongwei