Memory C++ 中的分配（抛出异常：读取访问冲突。）

Question

I'm trying to learn a little bit more of C++. After working around with memory allocation for a while I got to a place where I'm struggling to understand it.

I wrote a code that works well (not really sure of that but at least doesn't show any memory violation) for a type of initialization (of an object of some class) but it crashes for a similar initialization.

I would appreciate if someone could me explain what is happening and how can I solve this problem.

My thought: The problem is in the line bellow because I'm trying to delete an array of allocated objects when in the problematic initialization I only have one object allocated and not an array.

delete[] pointer; //PROBLEMATIC LINE

PS.: I'm not looking for alternative solutions (like using smart-pointers or whatever). Sorry for my English!

The code:

class class1
{
private:
    unsigned int    s;
    double* pointer;
public:
/* Constructors */
    class1() { s = 0; pointer = nullptr; }
    class1(unsigned int us, double* uarray)
    {
        pointer = new double[us];
        for (unsigned int i = 0; i < us; i++)
            pointer[i] = uarray[i];
    }
    class1(const class1& other)
    {
        pointer = new double[s];
        for (unsigned int i = 0; i < s; i++)
            pointer[i] = other.pointer[i];
    }
    ~class1() { if (!s && pointer != nullptr) delete[] pointer; }

public:
/* Operators Overloading */
    class1& operator=(const class1& other)
    {
        s = other.s;
        pointer = new double[s];
        for (unsigned int i = 0; i < s; i++)
            pointer[i] = other.pointer[i];
        return *this;
    }
};

class class2
{
private:
    unsigned int    m;
    unsigned int    n;
    class1* pointer;

public:
/* Constructors */
    class2(unsigned int un, double* uarray, bool flag = false) : n(un)
    {
        m = 1;
        pointer = new class1(un, uarray);
        if (flag) { this->function(); }
    }
    ~class2() { if (!m && !n) delete[] pointer; }

public:
/* Public Methods */
    void function()
    {
        class1* newpointer = new class1[n];
        //**... some code (when commented show the same error)**
        delete[] pointer; //**PROBLEMATIC LINE**
        pointer = newpointer;
    }

public:
/*Template Constructor*/
    template<unsigned int m, unsigned int n>
    class2(unsigned int um, unsigned int un, double(&uarray)[m][n], bool flag = false) : m(um), n(un)
    {
        pointer = new class1[um];
        for (unsigned int i = 0; i < um; i++)
        {
            class1 object1(un, uarray[i]);
            pointer[i] = object1;
        }
        if (flag) { this->function(); }
    }
};

int main()
{
    double test3[] = { 1, 2, 3 };
    double test4[][3] = { {3, 2, 1}, {6, 5, 4}, {9, 8, 7} };
    double test5[][3] = { {1, 2, 3}, {4, 5, 6}, {7, 8, 9} };

    class2 m4(3, test3, true);      //**NOT OK - VIOLATION OF MEMORY**
    class2 m5(3, 3, test4, true);   //**OK**
}

Answer 1

Your copy constructor for class1 is not setting the s member, but uses its indeterminate value here:

pointer = new double[s];

causing undefined behavior. Set s from other.s before using it.

---

Your second constructor has the same problem.

---

Your assignment operator of class1 is leaking memory, because it doesn't delete[] the previous array.

---

In class2 you use new in the non-array form, eg here:

pointer = new class1(un, uarray);

but in the destructor you call delete[] to delete pointer . This is also causing undefined behavior. Pointers returned from the non-array version of new need to be deleted by delete , eg delete pointer .

But since you are also using the array version of new for pointer , you cannot use delete pointer either. As using delete instead of delete[] on a pointer returned from a array-new has also undefined behavior.

Be consistent and use always the array-new, eg:

pointer = new class1[1]{{un, uarray}};

---

class2 causes undefined behavior when an object of its type is copied or moved, because you didn't implement a copy constructor and assignment operator although you have defined a destructor. This is a violation of the rule-of-three .

---

There is probably more that I missed. The code is not readable at all. Please use proper variable names next time. (I hope that the real code does not use this naming scheme...) Eg having a non-type template parameter m with the same name as a member of that class and then using m in multiple places of that context is not ok. I had to check the lookup rules to make sure that this actually compiles and does something sensible.