关于c++中new和delete[]的一些内容。

Question

我花了2个多小时，光是调试和检查代码的错误就花了2个多小时，但我还是找不到它崩溃的原因。line 91 到 line 93，源码运行在 vs 和 vscode但失败的是 dev-c. 的反馈。dev-c 是 program received signal sigsegv谁能告诉我原因和解决方法，谢谢。

#include <iostream>
using namespace std;
template <class T>
class DynamicVector
{
private:
    T *array;
    unsigned mallocSize, numofItems;
    int virtualZero;

public:
    DynamicVector(int Vindex)
    {
        array = NULL;
        numofItems = 0;
        mallocSize = 0;
        virtualZero = Vindex;
    }
    DynamicVector(const DynamicVector &another)
    {
        if (mallocSize < another.numofItems)
        {
            if (array)
            {
                delete[] array;
                array = NULL;
            }
            array = new T[another.mallocSize];
        }
        virtualZero = another.virtualZero;
        mallocSize = another.mallocSize;
        numofItems = another.numofItems;
        for (int i = 0; i < another.numofItems; i++)
            *(array + i) = *(another.array + i);
    }
    ~DynamicVector()
    {
        if (array)
        {
            delete[] array;
            array = NULL;
        }
    }
    DynamicVector<T> &operator=(const DynamicVector<T> &another)
    {
        if (mallocSize < another.mallocSize)
        {
            delete[] array;
            array = NULL;
            array = new T[another.mallocSize];
        }
        virtualZero = another.virtualZero;
        mallocSize = another.mallocSize;
        numofItems = another.numofItems;
        for (int i = 0; i < another.numofItems; i++)
            *(array + i) = *(another.array + i);
        return *this;
    }
    inline void push_back(const T &n)
    {
        if (numofItems < mallocSize)
        {
            *(array + numofItems) = n;
        }
        else if (numofItems == mallocSize)
        {
            T *num = new T[numofItems + 1];
            for (int i = 0; i < numofItems; i++)
                *(num + i) = *(array + i);
            if (array)
            {
                delete[] array;
                array = NULL;
            }
            array = new T[2 * mallocSize + 1];
            mallocSize = 2 * mallocSize + 1;
            for (int i = 0; i < numofItems; i++)
                *(array + i) = *(num + i);
            *(array + numofItems) = n;
            delete[] num;
            num = NULL;
        }
        numofItems++;
    }
    void push_back(const DynamicVector<T> &another)
    {
        T *num = new T[numofItems + 1];
        for (int i = 0; i < numofItems; i++)
            *(num + i) = *(array + i);
        if (array)
91        {
92            delete[] array;
            array = NULL;
        }
        array = new T[mallocSize + another.mallocSize];
        mallocSize = mallocSize + another.mallocSize;
        for (int i = 0; i < numofItems; i++)
            *(array + i) = *(num + i);
        delete[] num;
        num = NULL;
        for (int i = numofItems, j = 0; i < numofItems + another.numofItems; i++, j++)
            *(array + i) = *(another.array + j);
        numofItems = numofItems + another.numofItems;
    }
    T &operator[](int Vindex)
    {
        int _entry = Vindex - virtualZero;
        if (_entry < 0 || _entry >= numofItems)
        {
            cout << endl
                 << "Out Of Range";
            exit(1);
        }
        return array[_entry];
    }

    bool operator==(const DynamicVector<T> &dv) const
    {
        if (virtualZero == dv.virtualZero)
        {
            for (int i = 0; i < numofItems; i++)
                if (*(array + i) != *(dv.array + i))
                    return false;
            return true;
        }
        else
            return false;
    }
    unsigned length() const
    {
        return numofItems;
    }
    unsigned capacity() const
    {
        return mallocSize;
    }
    int firstIndex() const
    {
        return virtualZero;
    }
};
int main()
{

    DynamicVector<int> ra(-2);
    int i, n;
    cin >> n;
    ra.push_back(-3);
    ra.push_back(-2);
    ra.push_back(-1);
    for (i = 0; i < n; i++)
    {
        ra.push_back(i);
    }
    cout << "\n malloSize is " << ra.capacity();
    cout << "\n numofItems is " << ra.length();
    cout << "\n StartIndex is " << ra.firstIndex() << endl;
    for (i = -2; i < n + 1; i++)
    {
        cout << ra[i] << " ";
    }
    cout << endl;
    DynamicVector<int> raCopy(ra);
    cout << "\n malloSize is " << raCopy.capacity();
    cout << "\n numofItems is " << raCopy.length();
    cout << "\n StartIndex is " << raCopy.firstIndex() << endl;
    cout << endl;
    for (i = -2; i < n + 1; i++)
    {
        cout << ++ra[i] << " ";
    }
    cout << endl;
    for (i = -2; i < n + 1; i++)
    {
        cout << raCopy[i] << " ";
    }

    raCopy = ra;
    if (ra == raCopy)
        cout << "\n ra == raCopy";
    else
        cout << "\n ra != raCopy";

    ra[-2] = 100;

    if (ra == raCopy)
        cout << "\n ra == raCopy";
    else
        cout << "\n ra != raCopy";

    raCopy.push_back(ra);
    cout << endl;
    int firstI = raCopy.firstIndex();
    for (i = 0; i < raCopy.length(); i++)
    {
        cout << raCopy[i + firstI] << " ";
    }
    system("pause");
    return 0;
}

Answer 1

你是在复制构造函数中读取未初始化的成员变量。

    DynamicVector(const DynamicVector &another)
    {
        if (mallocSize < another.numofItems)
        {
            if (array)
            {
                delete[] array;
                array = NULL;
                }
            array = new T[another.mallocSize];
        }
        ...

这里 mallocSize 和 array 成员没有被初始化，并且包含随机垃圾，所以检查有可能做任何事情。你有三种情况。

1. mallocSize 是随机小的，因此。array 没有被初始化，指向随机内存。因此第91行在最终被调用时将导致sigfault。
2. mallocSize 是randoly大，并且 array 不是空的。你把 delete 在随机值上，从而破坏你的内存.要么导致segfault，要么搞乱内存，所以91行是。
3. mallocSize 是随机大的，而且 array 是随机空的。它的工作原理（三分一的机会）。

应该是

    DynamicVector(const DynamicVector &another)
    {
        array = new T[another.mallocSize];
        ...

另外，在代码中的每一个地方你都可以写 array[i] 而不是 *(array + i)例如 array[i] = another.array[i] 而不是 *(array + i) = *(another.array + i).

PS. 一旦你有它的工作得到它的代码审查。

https:/codereview.stackexchange.com）。