Question

我有一个2 * N整数的数据数组，表示对，即使i = 0,2,4，...，2 * N（对[i]，对[i + 1]）是这样一对。数据的格式是这样的，因为我使用的是Matlab的mex库。我这样做：

int N=5;
int data[10] = {1,2,3,4,5,6,7,8,9,10};
struct Pair { int first; int second; };
Pair * pairs = (Pair *)data;

但问题是没有办法保证Pair按顺序排列为第一，第二顺序为两个sizeof（整数）。请参阅：Is the member field order of a class "stable"?

我不想处理并将所有数据复制到新数组中，因为它不是必需的，我需要（据我所知）使用

typedef int Pair[2];

确保它正确对齐（没有尾随垃圾字节等）。如果我想根据第一个元素对对进行排序，我可以这样做：

#include <iostream>
#include <algorithm>

typedef int Pair[2];

int compare(Pair n1, Pair n2) { return n1[0] < n2[0]; }

int main() {
    int N=5;
    int data[10] = {1,2, 7,8, 13,14, 4,5, 10,11};
    Pair *pairs  = (Pair *)((void *)data); 

    std::cout << "unsorted" << std::endl;
    for(int i=0; i<N;++i) std::cout << i << ": (" << pairs[i][0] << ", " << pairs[i][1] << ")" << std::endl;

    std::sort(data, data+N, compare);

    std::cout << "sorted" << std::endl;
    for(int i=0; i<N;++i) std::cout << i << ": (" << pairs[i][0] << ", " << pairs[i][1] << ")" << std::endl;

    return 0;
}

请参阅：http://ideone.com/VyBUvc

我可以将错误消息汇总为错误：数组必须使用大括号括起初始化程序进行初始化，请参阅下面的完整消息。它是由std :: sort调用引起的。

我将Pair typedef包装在一个联合中（http://ideone.com/TVmEeZ），这似乎有效。 为什么c ++（或std :: sort）没有以与union类似的方式看到int [2]？

完成编译器输出：

In file included from /usr/include/c++/4.8/bits/stl_pair.h:59:0,
                                from /usr/include/c++/4.8/bits/stl_algobase.h:64,
                                from /usr/include/c++/4.8/bits/char_traits.h:39,
                                from /usr/include/c++/4.8/ios:40,
                                from /usr/include/c++/4.8/ostream:38,
                                from /usr/include/c++/4.8/iostream:39,
                                from prog.cpp:1:
/usr/include/c++/4.8/bits/stl_algo.h: In instantiation of ‘void std::__insertion_sort(_RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = int (*)[2]; _Compare = bool (*)(int*, int*)]’:
/usr/include/c++/4.8/bits/stl_algo.h:2250:70:   required from ‘void std::__final_insertion_sort(_RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = int (*)[2]; _Compare = bool (*)(int*, int*)]’
/usr/include/c++/4.8/bits/stl_algo.h:5514:55:   required from ‘void std::sort(_RAIter, _RAIter, _Compare) [with _RAIter = int (*)[2]; _Compare = bool (*)(int*, int*)]’
prog.cpp:16:35:   required from here
/usr/include/c++/4.8/bits/stl_algo.h:2186:11: error: array must be initialized with a brace-enclosed initializer
    __val = _GLIBCXX_MOVE(*__i);
                    ^
In file included from /usr/include/c++/4.8/algorithm:62:0,
                                from prog.cpp:2:
/usr/include/c++/4.8/bits/stl_algo.h:2188:17: error: invalid array assignment
                *__first = _GLIBCXX_MOVE(__val);
                                ^
/usr/include/c++/4.8/bits/stl_algo.h: In instantiation of ‘_RandomAccessIterator std::__unguarded_partition(_RandomAccessIterator, _RandomAccessIterator, const _Tp&, _Compare) [with _RandomAccessIterator = int (*)[2]; _Tp = int [2]; _Compare = bool (*)(int*, int*)]’:
/usr/include/c++/4.8/bits/stl_algo.h:2319:78:   required from ‘_RandomAccessIterator std::__unguarded_partition_pivot(_RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = int (*)[2]; _Compare = bool (*)(int*, int*)]’
/usr/include/c++/4.8/bits/stl_algo.h:2360:62:   required from ‘void std::__introsort_loop(_RandomAccessIterator, _RandomAccessIterator, _Size, _Compare) [with _RandomAccessIterator = int (*)[2]; _Size = int; _Compare = bool (*)(int*, int*)]’
/usr/include/c++/4.8/bits/stl_algo.h:5513:44:   required from ‘void std::sort(_RAIter, _RAIter, _Compare) [with _RAIter = int (*)[2]; _Compare = bool (*)(int*, int*)]’
prog.cpp:16:35:   required from here
/usr/include/c++/4.8/bits/stl_algo.h:2287:35: error: invalid conversion from ‘const int*’ to ‘int*’ [-fpermissive]
        while (__comp(*__first, __pivot))
                                                                    ^
/usr/include/c++/4.8/bits/stl_algo.h:2290:34: error: invalid conversion from ‘const int*’ to ‘int*’ [-fpermissive]
        while (__comp(__pivot, *__last))
                                                                    ^
In file included from /usr/include/c++/4.8/bits/stl_pair.h:59:0,
                                from /usr/include/c++/4.8/bits/stl_algobase.h:64,
                                from /usr/include/c++/4.8/bits/char_traits.h:39,
                                from /usr/include/c++/4.8/ios:40,
                                from /usr/include/c++/4.8/ostream:38,
                                from /usr/include/c++/4.8/iostream:39,
                                from prog.cpp:1:
/usr/include/c++/4.8/bits/stl_heap.h: In instantiation of ‘void std::make_heap(_RAIter, _RAIter, _Compare) [with _RAIter = int (*)[2]; _Compare = bool (*)(int*, int*)]’:
/usr/include/c++/4.8/bits/stl_algo.h:1970:47:   required from ‘void std::__heap_select(_RandomAccessIterator, _RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = int (*)[2]; _Compare = bool (*)(int*, int*)]’
/usr/include/c++/4.8/bits/stl_algo.h:5363:59:   required from ‘void std::partial_sort(_RAIter, _RAIter, _RAIter, _Compare) [with _RAIter = int (*)[2]; _Compare = bool (*)(int*, int*)]’
/usr/include/c++/4.8/bits/stl_algo.h:2355:68:   required from ‘void std::__introsort_loop(_RandomAccessIterator, _RandomAccessIterator, _Size, _Compare) [with _RandomAccessIterator = int (*)[2]; _Size = int; _Compare = bool (*)(int*, int*)]’
/usr/include/c++/4.8/bits/stl_algo.h:5513:44:   required from ‘void std::sort(_RAIter, _RAIter, _Compare) [with _RAIter = int (*)[2]; _Compare = bool (*)(int*, int*)]’
prog.cpp:16:35:   required from here
/usr/include/c++/4.8/bits/stl_heap.h:446:25: error: array must be initialized with a brace-enclosed initializer
        _ValueType __value = _GLIBCXX_MOVE(*(__first + __parent));
                                                ^
/usr/include/c++/4.8/bits/stl_heap.h: In instantiation of ‘void std::__pop_heap(_RandomAccessIterator, _RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = int (*)[2]; _Compare = bool (*)(int*, int*)]’:
/usr/include/c++/4.8/bits/stl_algo.h:1973:50:   required from ‘void std::__heap_select(_RandomAccessIterator, _RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = int (*)[2]; _Compare = bool (*)(int*, int*)]’
/usr/include/c++/4.8/bits/stl_algo.h:5363:59:   required from ‘void std::partial_sort(_RAIter, _RAIter, _RAIter, _Compare) [with _RAIter = int (*)[2]; _Compare = bool (*)(int*, int*)]’
/usr/include/c++/4.8/bits/stl_algo.h:2355:68:   required from ‘void std::__introsort_loop(_RandomAccessIterator, _RandomAccessIterator, _Size, _Compare) [with _RandomAccessIterator = int (*)[2]; _Size = int; _Compare = bool (*)(int*, int*)]’
/usr/include/c++/4.8/bits/stl_algo.h:5513:44:   required from ‘void std::sort(_RAIter, _RAIter, _Compare) [with _RAIter = int (*)[2]; _Compare = bool (*)(int*, int*)]’
prog.cpp:16:35:   required from here
/usr/include/c++/4.8/bits/stl_heap.h:339:28: error: array must be initialized with a brace-enclosed initializer
            _ValueType __value = _GLIBCXX_MOVE(*__result);
                                                        ^
In file included from /usr/include/c++/4.8/bits/stl_algo.h:61:0,
                                from /usr/include/c++/4.8/algorithm:62,
                                from prog.cpp:2:
/usr/include/c++/4.8/bits/stl_heap.h:340:17: error: invalid array assignment
            *__result = _GLIBCXX_MOVE(*__first);
                                ^

Answer 1

std::sort(data, data+N, compare);

您正在排序data，而不是pairs。也就是说，您的新方法仍然是未定义的行为，因此无法保证工作¹。你基本上是想把一个方形钉固定在一个圆孔里。如果要使用std::sort，则显示有效数据 - 这意味着在您的情况下进行复制，或者编写自定义迭代器，将数组视为连续对的集合。

¹这是一种轻率的轻描淡写。 - 不要这样做。

Answer 2

为int交换两个std::pair<int,int>的数组为我做了伎俩（live at ideone）：

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

typedef std::pair<int,int> Pair;

bool compare(const Pair& i, const Pair& j) { return i.first < j.first; }

int main() {
    const int N=5;
    Pair pairs[N] = {{1,2}, {7,8}, {13,14}, {4,5}, {10,11}};

    std::cout << "unsorted" << std::endl;
    for(int i=0; i<N;++i) std::cout << i << ": (" << pairs[i].first << ", " << pairs[i].second << ")" << std::endl;

    std::sort(pairs, pairs+N, compare);

    std::cout << "sorted" << std::endl;
    for(int i=0; i<N;++i) std::cout << i << ": (" << pairs[i].first << ", " << pairs[i].second << ")" << std::endl;
}

另一种方法是在结构中封装两个int的数组。您的代码中的问题是std::sort需要一个可比较的数组（您使用compare函数修复它）和 copy-or-move-assignable 项（数组都不是）

使用std::array：

可能更好（代码更改更少）

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

typedef std::array<int, 2> Pair;

bool compare(const Pair& i, const Pair& j) { return i[0] < j[0]; }

int main() {
    const int N=5;
    Pair pairs[N] = {1,2, 7,8, 13,14, 4,5, 10,11};

    std::cout << "unsorted" << std::endl;
    for(int i=0; i<N;++i) std::cout << i << ": (" << pairs[i][0] << ", " << pairs[i][1] << ")" << std::endl;

    std::sort(pairs, pairs+N, compare);

    std::cout << "sorted" << std::endl;
    for(int i=0; i<N;++i) std::cout << i << ": (" << pairs[i][0] << ", " << pairs[i][1] << ")" << std::endl;
}

排序int [2]数组不会编译

2 个答案: