C++ array of char using unique_ptr

Question

First of all, I know this is not the best way to do this, I'm just looking how it should be done. I created a class called bord,which holds a member

        std::unique_ptr<std::unique_ptr<char>[] > char_bord;

Which should be the correct syntax, then I try to initialize this in the constructor:

bord::bord():char_bord(new std::unique_ptr<char>[10])
{
    //char_bord=new std::unique_ptr<char>[10]; //This did not seem to work aswell.
    for(int i=0;i<10;i++)
      char_bord[i]=new std::unique_ptr<char>[](new char[10]); 
    return;
}

This results in the following heap of errors, which I did not manage to decipher.

jelmer@jelmer-N56JN:~/Git/Board/lib$ g++ -std=c++0x bord.c
In file included from bord.c:1:0:
bord.h:20:1: error: new types may not be defined in a return type
 class bord
 ^
bord.h:20:1: note: (perhaps a semicolon is missing after the definition of ‘bord’)
bord.c:3:12: error: return type specification for constructor invalid
 bord::bord():char_bord(new std::unique_ptr<char>[10])
            ^
bord.c: In constructor ‘bord::bord()’:
bord.c:7:46: error: expected primary-expression before ‘]’ token
       char_bord[i]=new std::unique_ptr<char>[](new char[10]); 
                                              ^
bord.c:7:60: error: parenthesized initializer in array new [-fpermissive]
       char_bord[i]=new std::unique_ptr<char>[](new char[10]); 
                                                            ^
bord.c:7:19: error: no match for ‘operator=’ (operand types are ‘std::unique_ptr<char>’ and ‘std::unique_ptr<char>*’)
       char_bord[i]=new std::unique_ptr<char>[](new char[10]); 
                   ^
bord.c:7:19: note: candidates are:
In file included from /usr/include/c++/4.9/memory:81:0,
                 from bord.h:19,
                 from bord.c:1:
/usr/include/c++/4.9/bits/unique_ptr.h:249:7: note: std::unique_ptr<_Tp, _Dp>& std::unique_ptr<_Tp, _Dp>::operator=(std::unique_ptr<_Tp, _Dp>&&) [with _Tp = char; _Dp = std::default_delete<char>]
       operator=(unique_ptr&& __u) noexcept
       ^
/usr/include/c++/4.9/bits/unique_ptr.h:249:7: note:   no known conversion for argument 1 from ‘std::unique_ptr<char>*’ to ‘std::unique_ptr<char>&&’
/usr/include/c++/4.9/bits/unique_ptr.h:269:2: note: template<class _Up, class _Ep> typename std::enable_if<std::__and_<std::is_convertible<typename std::unique_ptr<_Up, _Ep>::pointer, typename std::unique_ptr<_Tp, _Dp>::_Pointer::type>, std::__not_<std::is_array<_Up> > >::value, std::unique_ptr<_Tp, _Dp>&>::type std::unique_ptr<_Tp, _Dp>::operator=(std::unique_ptr<_Up, _Ep>&&) [with _Up = _Up; _Ep = _Ep; _Tp = char; _Dp = std::default_delete<char>]
  operator=(unique_ptr<_Up, _Ep>&& __u) noexcept
  ^
/usr/include/c++/4.9/bits/unique_ptr.h:269:2: note:   template argument deduction/substitution failed:
bord.c:7:19: note:   mismatched types ‘std::unique_ptr<_Tp, _Dp>’ and ‘std::unique_ptr<char>*’
       char_bord[i]=new std::unique_ptr<char>[](new char[10]); 
                   ^
In file included from /usr/include/c++/4.9/memory:81:0,
                 from bord.h:19,
                 from bord.c:1:
/usr/include/c++/4.9/bits/unique_ptr.h:278:7: note: std::unique_ptr<_Tp, _Dp>& std::unique_ptr<_Tp, _Dp>::operator=(std::nullptr_t) [with _Tp = char; _Dp = std::default_delete<char>; std::nullptr_t = std::nullptr_t]
       operator=(nullptr_t) noexcept
       ^
/usr/include/c++/4.9/bits/unique_ptr.h:278:7: note:   no known conversion for argument 1 from ‘std::unique_ptr<char>*’ to ‘std::nullptr_t’

What am I doing wrong, assuming I am doing something wrong.

Solution 1

Here is some code that demonstrates what i think you want:

#include <iostream>
#include <algorithm>
#include <iterator>
#include <memory>

using namespace std;

using array_ptr_type = std::unique_ptr<char[]>;
using array_of_arrays_type = std::unique_ptr<array_ptr_type[]>;

auto main() -> int
{
    auto x = array_ptr_type(new char[10]);
    auto y = array_ptr_type(new char[10]);
    for (int i = 0 ; i < 10 ; ++i)
    {
        x[i] = char('0' + i);
        y[i] = char('0' + 9 - i);
    }

    auto pxy = array_of_arrays_type(new array_ptr_type[2]);
    pxy[0] = move(x);
    pxy[1] = move(y);

    for (int i = 0 ; i < 2 ; ++i) {
        copy(&pxy[i][0], &pxy[i][10], ostream_iterator<char>(cout, ", "));
        cout << endl;
    }
    return 0;
}

expected output:

0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 
9, 8, 7, 6, 5, 4, 3, 2, 1, 0, 

Of course as you know, none of this is recommended - vector<vector<char>> would be a lot cleaner and more maintainable.

Solution 2

You have to use specialization:

std::unique_ptr<char[]> chars(new char[1024]);

That is because std::unique_ptr does not support custom deleter as std::shared_ptr does (in this style of writing).

std::unique_ptr uses std::default_delete as deleter. Shortly, if you specify argument type as class T it will use default delete but if you write class T[] (in this specialization) std::unique_ptr will use delete[].

But it is better to use some container and not c-style arrays.

Comments

[jelmew]

I kkow I should use a container. Could you elaborate on your answer? It it not totally clear to me. Is it just a smart pointer to a list of dumb pointers now?

[Victor Polevoy]

What is use of std::unique_ptr and other smart pointers? Just to delete the pointer properly when it goes out of scope or it's reference count is zero. What your std::unique_ptr<std::unique_ptr<char>[] > is for? Do you want delete unique_ptr when it ... I can't even describe that meaningless code. If you want to make a smart pointer to array of chars - use one smart pointer for an array. Otherwise I don't understand what are you trying to do.

[Puppy]

This... is technically correct but does not explain all of the compiler errors the OP got in his question.

[Victor Polevoy]

@Puppy I think if the author would rewrite his code to normal one (yes, my humble opinion) he will escape of all these errors because there are all about meaningless array's thing.

[jelmew]

Hi, Thanks, that was what I wanted to know, how it would be done. Indeed, back to the more sensible vector<vector<char> >. Thanks for answering my curiosity.