用非默认可构造类型填充std :: array(没有可变参数模板) [英] Populate std::array with non-default-constructible type (no variadic templates)

问题描述

假设我有一个A类型，没有默认构造函数:

Suppose I have a type A with no default constructor:

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

我想将std::array设置为A.我可以使用初始化列表轻松实现它:

I want to make an std::array of A. I can easily make it with initializer list:

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

您可以在此处看到一个模式.是的，我可以使用一个生成器函数来计算数组的每个值:

You can see a pattern here. Yes, I can have a generator function to compute each value of the array:

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5个以上的元素(即64个).因此，最好不要重复makeElement 64次.我想出的唯一解决方案是使用可变参数模板将参数包解包到初始化程序列表中: https://ideone.com/yEWZVq (它还会检查所有副本是否被正确删除).此解决方案的灵感来自此问题.

And yes, in fact I have much more than 5 elements (64, namely). So it would be nice not to repeat makeElement 64 times. The only solution I came up with is to use variadic templates to unpack parameter pack into the initializer list: https://ideone.com/yEWZVq (it also checks that all copies are properly elided). This solution was inspired by this question.

它可以工作，但是我不想滥用可变参数模板来完成这样一个简单的任务.您知道，膨胀可执行文件的大小，减慢了编译速度，所有这些工作.我想做这样的事情:

It works, but I would like not to abuse variadic templates for such a simple task. You know, bloating executable size, slowing down the compilation, all that stuff. I would like to do something like this:

1. 创建一些适当大小的未初始化存储
2. 使用放置位置new
初始化循环中的所有元素
3. 神奇地将存储转换为std::array并返回

1. Create some uninitialized storage with a proper size
2. Initialize all elements in the loop with placement new
3. Magically convert the storage to std::array and return it

我可以做一些肮脏的技巧以在动态内存中实现此目标: https://ideone.com/tbw5lm但这并不比std::vector好，我根本没有这样的问题.

I can do some dirty hacks to implement this in dynamic memory: https://ideone.com/tbw5lm But this is not better than a std::vector, where I do not have such problems at all.

我不知道如何在自动记忆中做到这一点. IE.具有相同的便捷功能，可以按值返回std::array，而所有这些东西都在后台.有什么想法吗?

And I have no idea how I can do it in automatic memory. I.e. to have the same convenient function returning std::array by value with all this stuff behind the hood. Any ideas?

我想，boost::container::static_vector对我来说可能是一个很好的解决方案.不幸的是，我不能使用boost来完成特定任务.

I suppose, that boost::container::static_vector might be good solution for me. Unfortunately, I cannot use boost for that particular task.

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缺少什么.

PS. Note please that this question is more like of research interest. In a real world both variadic templates and std::vector would work just fine. I just want to know maybe I am missing something.

推荐答案

这是允许向生成器输入任意范围的另一种方法:

Here's another way which allows an arbitrary range of inputs to the generator:

这是用例:

/// 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;
}

这里是样板允许它工作

#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;
}

这是汇编之前的代码膨胀:

here's the code bloat as viewed prior to assembly:

.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

即什么都没有.

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