C++中涉及new和delete []函数的奇怪错误

Question

我正在编写关于Coursera的算法:设计和分析课程(https://www.coursera.org/course/algo)的第一个编程作业.它涉及使用合并排序来计算反转(http://en.wikipedia.org/wiki/Inversion_(discrete_mathematics)).我认为这将是一个相对明智的因为我之前(在学校)遇到过合并排序.

#include <iostream>
#include <fstream>

using namespace std;

int *half(int *array, int n, int start, int end)
{
    /*
     * Creates a new array which contains elements from ''array'' starting with ''start''
     * and ending with ''end - 1''.
     */

     int *new_array = new int[end-start];

     for(int i = start; i < end; i++)
     {
        new_array[i-start] = array[i];
     }

     return new_array;
}

int merge(int *array1, int n1, int *array2, int n2, int *new_array)
{
    /*
     *  Merges arrays 1 and 2 (with lengths n1 and n2) into a new_array, counting
     *  ''split inversions'' by the way.
     */
    int i = 0, j = 0, count = 0;

    for(int k = 0; k < n1 + n2; k++)
    {

         if(i >= n1)
        {
            new_array[k] = array2[j];
            j++;
            continue;
        }

         if(j >= n2)
        {

            new_array[k] = array1[i];
            i++;
            continue;
        }

        if( array1[i] <= array2[j] )
        {
            new_array[k] = array1[i];
            i++;
        }
        else
        {
            new_array[k] = array2[j];
            j++;
            count++;
        }
    }

    return count;
}

int mergesort(int *array, int n)
{

    if(n == 1) return 0; //base case

    int x, y, z;
    int odd;

    if(n%2 == 0) odd = 0;
    else odd = 1;

    int *half1 = new int [n/2];
    int *half2 = new int [n/2 + odd];

    half1 = half(array, n, 0, n/2);
    half2 = half(array, n, n/2, n);

    x = mergesort(half1, n/2);
    y = mergesort(half2, n/2 + odd);  //if n is odd, we add one
    z = merge(half1, n/2, half2, n/2 + odd, array); //we write a sorted array back in our starting array

    delete [] half1;
    delete [] half2;

    return x + y + z;
}

int main()
{
    int n;
    int *array = new int[n];

    cin >> n;

    for(int i = 0; i < n; i++)
    {
        int x;
        cin >> x;
        array[i] = x;
    }

    for(int i = 0; i < n; i++)
        cout << array[i] << " ";

    cout << endl;
    cout << "Number of inversions: " << mergesort(array, n) << endl;

    for(int i = 0; i < n; i++)
    cout << array[i] << " ";

    cout << endl;
    delete[] array;

    return 0;
}

那么,这里有什么奇怪的？第一件事:对我来说,它适用于某些数组,但其他数组崩溃(后面的例子).第二件事:我发送代码给我的朋友,他说一切对他来说都很好,甚至是那些让我崩溃的例子.

所以,例子:

对于数组[1 2 3 4 5 6 7],g ++产生这样的:

malloc.c:2451: sYSMALLOc: Assertion `(old_top == (((mbinptr) (((char *) &((av)->bins[((1) - 1) * 2])) - __builtin_offsetof (struct malloc_chunk, fd)))) && old_size == 0) || ((unsigned long) (old_size) >= (unsigned long)((((__builtin_offsetof (struct malloc_chunk, fd_nextsize))+((2 * (sizeof(size_t))) - 1)) & ~((2 * (sizeof(size_t))) - 1))) && ((old_top)->size & 0x1) && ((unsigned long)old_end & pagemask) == 0)' failed.
Aborted (core dumped)

当我使用gdb"回溯"它时:

#0  0x00007ffff7753445 in __GI_raise (sig=<optimized out>) at ../nptl/sysdeps/unix/sysv/linux/raise.c:64
#1  0x00007ffff7756bab in __GI_abort () at abort.c:91
#2  0x00007ffff779abed in __malloc_assert (assertion=<optimized out>, file=<optimized out>, line=<optimized out>, function=<optimized out>) at malloc.c:300
#3  0x00007ffff779e0f4 in sYSMALLOc (av=0x7ffff7ad3720, nb=32) at malloc.c:2448
#4  _int_malloc (av=0x7ffff7ad3720, bytes=12) at malloc.c:3892
#5  0x00007ffff779fa45 in __GI___libc_malloc (bytes=12) at malloc.c:2924
#6  0x00007ffff7b8fded in operator new(unsigned long) () from /usr/lib/x86_64-linux-gnu/libstdc++.so.6
#7  0x00007ffff7b8ff09 in operator new[](unsigned long) () from /usr/lib/x86_64-linux-gnu/libstdc++.so.6
#8  0x0000000000400b12 in mergesort (array=0x603010, n=7) at jedan.cpp:81
#9  0x0000000000400cfe in main () at jedan.cpp:120

它为数组[1 2 3 4 5 6 7 8 9 10] 做了类似的事情(但不一样!),再次连接到new和delete []函数.如果有人认为这会有所帮助,我可以稍后在这里发布,但我不想过多地夸大这篇文章.它适用于我尝试过的大多数数组(我对大小<= 6的数组没有问题,对于相当多的大数组也没有问题).

我正在使用昨天安装的Ubuntu 12.04 ......非常干净清新.救命？

PS如果你发现变量名有点奇怪,抱歉......我用我的母语翻译它们,这样代码就更具可读性了.

Answer 1

int n;
int *array = new int[n]; // Undefined behavior

n 在这里使用未初始化,因此您将获得"随机"长度分配.

如果您运气不好并且n拥有"大"垃圾值,那么您的代码似乎可以正常工作.如果它保持较小的值,那么在填充初始数组时可能会损坏堆 - 这将产生您所看到的错误类型.

cin >> n;在array分配之前移动该行.

旁注:我认为你正在进行的两个分配mergesort是泄露的(你只是delete在分配的内存中half,mergesort如果我正确读取你的代码,你实际上并不需要分配).