Re: std^shared_ptr compile error for 2D array type (G++7.2)

[goldenhawking <goldenhawking at 163 dot com>]

OK! This is a convenient way to learn new standard changesets. By the 
way, our engineers usually take several different ways to maintain 
dynamic m-D array,  but none is perfect.

(1) C-style multi-dimension array pointer: "int (*p)[16][32];"

     Advantage:  continuous memory address for low-level conversion and 
operation.

     Weakness:   only 1 dimension can be dynamically assigned. The size 
'16' and '32' must be constant.

(2) STL container vector: vector< vector< vector<int> > > p;

     Advantage: fully dynamic size assignment for each dimension.

     Weakness:  discontinuous memory address.

(3) Sparse matrix using std::unordered_map: unordered_map<int, 
unordered_map<int, int> > p

     Advantage: memory-saving for sparse matrix .

     Weakness:  discontinuous memory address, low performance.

(4) STL array: array< array< int, MAX1> , MAX2> Darray = {0};

     Advantage: Seems to be continuous memory address?  Mordern STL 
features enabled.

     Weakness:  Dimension size is static. We must predict the max size 
for each dimension.

We choose carefully, and consider that C-style multi-dimension array 
pointer with shared_ptr is the best way for dynamic m-D array.  If 
continuous memory is not required, STL container vector should be the best.

2017-10-06 19:52, Jonathan Wakely:
> You're welcome.
>
> Instead of reading the code to understand what changed it might be
> simpler to see the changes made by the proposal to change the C++
> standard: https://wg21.link/p0414
>
>
>
>
>
> On 6 October 2017 at 12:49, goldenhawking@163.com <goldenhawking@163.com> wrote:
> > Oh, amazing ! I read about the new code of template class in <memory>,
> > really complex , indeed a great job. There seems to be a lot of new
> > knowledge for me to learn, tks!
> >
> > ---Original---
> > From: "Jonathan Wakely"<jwakely.gcc@gmail.com>
> > Date: 2017/10/6 19:25:02
> > To: "goldenhawking"<goldenhawking@163.com>;
> > Cc: "libstdc++"<libstdc++@gcc.gnu.org>;
> > Subject: Re: std::shared_ptr compile error for 2D array type (G++7.2)
> >
> > On 6 October 2017 at 12:02, Jonathan Wakely wrote: > On 6 October 2017 at
> > 11:48, Jonathan Wakely wrote: >> On 6 October 2017 at 09:47, goldenhawking
> > wrote: >>> We use std::shared_ptr to hold 2D array like this: >>> >>>
> > std::shared_ptr< int[24] > bar(new int[N][24], [=](int(*p)[24])->void { >>>
> > delete[] p; } ); >>> >>> GNU C++ 4/5/6 is ok . GNU C++ 7.2 gives an error
> > message: >>> >>> error: no matching function for call to 'std::shared_ptr>>
> > [24]>::shared_ptr(int (*)[24])' shared_ptr< int [24] > pt (new int >>>
> > [4096][24]); >>> >>> This may be a problem caused by default construct
> > mechanism of a 2D array. >> >> No, it's because in C++11 and C++14
> > shared_ptr was not designed to be >> used with arrays. In C++17 arrays are
> > fully supported, but in a way >> that is incompatible with your code. >> >>
> > It will work if you use shared_ptr because that's actually a >> shared_ptr
> > that owns a 2D array. > > Oops, sorry for the type. I meant shared_ptr not
> > "int[]24]" > which isn't a valid type :-) Also you don't need to use the
> > custom deleter, because shared_ptr will do the right thing, and use delete[]
> > to release the memory. So simply: std::shared_ptr< int[][24] > bar(new
> > int[N][24]);