用非default-constructible类型填充std :: array(没有可变参数模板)

时间:2016-06-18 12:04:40

标签: c++ initialization c++14 stdarray

假设我的类型A没有默认构造函数:

struct A
{
  int x;
  A(int x) : x(x) {}
};

我想制作一个std::array A。我可以使用初始化列表轻松地创建它:

std::array<A, 5> arr = { 0, 1, 4, 9, 16 };

你可以在这里看到一个模式。是的,我可以使用生成器函数来计算数组的每个值:

int makeElement(size_t i)
{
  return i * i;
}

std::array<A, 5> arr = { 
  makeElement(0), 
  makeElement(1),
  makeElement(2),
  makeElement(3),
  makeElement(4)
};

是的,实际上我有超过5个元素(64,即)。所以不重复makeElement 64次会很好。我想出的唯一解决方案是使用可变参数模板将参数包解压缩到初始化列表中:https://ideone.com/yEWZVq(它还检查所有副本是否被正确删除)。此解决方案的灵感来自this question

它有效,但我不想滥用可变参数模板来完成这么简单的任务。你知道,膨胀的可执行文件大小,减慢编译速度,所有这些。我想做这样的事情:

  1. 创建一些大小合适的未初始化存储
  2. 使用展示位置new
  3. 初始化循环中的所有元素
  4. 将存储空间神奇地转换为std::array并将其返回
  5. 我可以做一些肮脏的黑客在动态内存中实现它:https://ideone.com/tbw5lm但这并不比std::vector更好,我根本没有这样的问题。

    我不知道如何在自动记忆中做到这一点。即具有相同的方便功能返回std::array值与所有这些东西后面的引擎盖。有什么想法吗?

    我想,boost::container::static_vector对我来说可能是个好方法。不幸的是,我无法将boost用于该特定任务。

    PS。请注意,这个问题更像是研究兴趣。在现实世界中,可变参数模板和std::vector都可以正常工作。我只是想知道也许我错过了什么。

2 个答案:

答案 0 :(得分:3)

我认为,你对代码臃肿的担忧被误解了。这是一个示例:

loadData: function (item) {
                return $.ajax({
                    type: "POST",
                    url: "url.html?cmd=fill",
                    data: null,
                    dataType: "json",
                    success: function (data) {
                          var list=eval(data);
                           //this loop is for example
                           for(var i=0;i<list.length;i++)
                            { 
                               list[i].id=i+1;
                                //how to bind this list to Grid Fields
                            }
                    }
                });
            }

它产生非常整洁的装配:

#include <utility>
#include <array>

template<std::size_t... ix>
constexpr auto generate(std::index_sequence<ix...> ) {
    return std::array<int, sizeof...(ix)>{(ix * ix)...};
}

std::array<int, 3> check() {
 return generate(std::make_index_sequence<3>());
}

std::array<int, 5> glob = generate(std::make_index_sequence<5>());

如你所见,看不到任何代码膨胀。静态数组是静态初始化的,对于自动数组,它只是一堆移动。如果你认为一堆动作是一个可怕的代码臃肿,请考虑循环展开 - 每个人都喜欢!

顺便说一下,没有其他符合要求的解决方案。在构造时使用聚合初始化初始化数组,因此元素应该是默认可构造的或者是初始化的。

答案 1 :(得分:2)

这是另一种允许发生器输入任意范围的方法:

以下是用例:

/// generate an integer by multiplying the input by 2
/// this could just as easily be a lambda or function object
constexpr int my_generator(int x) {
    return 2 * x;
}

int main()
{
    // generate a std::array<int, 64> containing the values
    // 0 - 126 inclusive (the 64 acts like an end() iterator)
    static constexpr auto arr = generate_array(range<int, 0, 64>(),
                                               my_generator);

    std::copy(arr.begin(), arr.end(), std::ostream_iterator<int>(std::cout, ", "));
    std::cout << std::endl;
}

这是允许其工作的样板

#include <utility>
#include <array>
#include <iostream>
#include <algorithm>
#include <iterator>

/// the concept of a class that holds a range of something
/// @requires T + 1 results in the next T
/// @requires Begin + 1 + 1 + 1.... eventually results in Tn == End
template<class T, T Begin, T End>
struct range
{
    constexpr T begin() const { return Begin; }
    constexpr T end() const { return End; }

    constexpr T size() const { return end() - begin(); }
    using type = T;
};

/// offset every integer in an integer sequence by a value
/// e.g offset(2, <1, 2, 3>) -> <3, 4, 5>
template<int Offset, int...Is>
constexpr auto offset(std::integer_sequence<int, Is...>)
{
    return std::integer_sequence<int, (Is + Offset)...>();
}

/// generate a std::array by calling Gen(I) for every I in Is
template<class T, class I, I...Is, class Gen>
constexpr auto generate_array(std::integer_sequence<I, Is...>, Gen gen)
{
    return std::array<T, sizeof...(Is)> {
        gen(Is)...
    };
}

/// generate a std::array by calling Gen (x) for every x in Range
template<class Range, class Gen>
constexpr auto generate_array(Range range, Gen&& gen)
{
    using T = decltype(gen(range.begin()));
    auto from_zero = std::make_integer_sequence<typename Range::type, range.size()>();
    auto indexes = offset<range.begin()>(from_zero);
    return generate_array<T>(indexes, std::forward<Gen>(gen));
}

/// generate an integer by multiplying the input by 2
constexpr int my_generator(int x) {
    return 2 * x;
}

int main()
{
    static constexpr auto arr = generate_array(range<int, 0, 64>(),
                                               my_generator);

    std::copy(arr.begin(), arr.end(), std::ostream_iterator<int>(std::cout, ", "));
    std::cout << std::endl;
}

这里是在汇编之前查看的代码膨胀:

.LC0:
    .string ", "
main:
;; this is the start of the code that deals with the array
    pushq   %rbx
    movl    main::arr, %ebx
.L2:
    movl    (%rbx), %esi
    movl    std::cout, %edi
    addq    $4, %rbx
;; this is the end of it

;; all the rest of this stuff is to do with streaming values to cout
    call    std::basic_ostream<char, std::char_traits<char> >::operator<<(int)
    movl    $2, %edx
    movl    $.LC0, %esi
    movl    std::cout, %edi
    call    std::basic_ostream<char, std::char_traits<char> >& std::__ostream_insert<char, std::char_traits<char> >(std::basic_ostream<char, std::char_traits<char> >&, char const*, long)
    cmpq    main::arr+256, %rbx
    jne     .L2
    movl    std::cout, %edi
    call    std::basic_ostream<char, std::char_traits<char> >& std::endl<char, std::char_traits<char> >(std::basic_ostream<char, std::char_traits<char> >&)
    xorl    %eax, %eax
    popq    %rbx
    ret
    subq    $8, %rsp
    movl    std::__ioinit, %edi
    call    std::ios_base::Init::Init()
    movl    $__dso_handle, %edx
    movl    std::__ioinit, %esi
    movl    std::ios_base::Init::~Init(), %edi
    addq    $8, %rsp
    jmp     __cxa_atexit

main::arr:
    .long   0
    .long   2
    .long   4
    .long   6
    .long   8
    .long   10
    .long   12
    .long   14
    .long   16
    .long   18
    .long   20
    .long   22
    .long   24
    .long   26
    .long   28
    .long   30
    .long   32
    .long   34
    .long   36
    .long   38
    .long   40
    .long   42
    .long   44
    .long   46
    .long   48
    .long   50
    .long   52
    .long   54
    .long   56
    .long   58
    .long   60
    .long   62
    .long   64
    .long   66
    .long   68
    .long   70
    .long   72
    .long   74
    .long   76
    .long   78
    .long   80
    .long   82
    .long   84
    .long   86
    .long   88
    .long   90
    .long   92
    .long   94
    .long   96
    .long   98
    .long   100
    .long   102
    .long   104
    .long   106
    .long   108
    .long   110
    .long   112
    .long   114
    .long   116
    .long   118
    .long   120
    .long   122
    .long   124
    .long   126

即。没什么。