c++中的运算符new和delete

我需要一些关于操作员new和delete 的帮助

我试图创建一个名为big的类来处理巨大的数字

#include <iostream>
using namespace std;
class big
{
protected:
    char *a;
    long lenght,maxlenght;
public:
    big()
    {
        maxlenght=256;
        a=new char[maxlenght];
        memset(a,'',maxlenght);
    }
    void read();
    void set_maxlenght(long x);
    long msize();
    long size();
    char* return_big();
    long at(long poz);
    char* operator[](long poz);
    void operator=(big x);
    void modify (long poz,long val);
    void dec (long poz);
    void inc (long poz);
    void operator-(big x);
    int compare(big x);
};


//functions
void write(big x)
{
    char *ptr;
    ptr=x.return_big();
    cout<<ptr;
}

//class big
    void big::read()
    {
        char *buffer;
        buffer=new char[maxlenght];
        memset(buffer,'',maxlenght);
        cin>>buffer;
        delete[] a;
        a=new char[strlen(buffer)];
        memset(a,'',maxlenght);
        for(long i=0;i<(long)strlen(buffer);i++)
            a[i]=buffer[i];
        lenght=strlen(buffer);
        delete[] buffer;
    }
    void big::set_maxlenght(long x)
    {
        maxlenght=x;
    }
    long big::msize()
    {
        return maxlenght;
    }
    long big::size()
    {
        return lenght;
    }
    char* big::return_big()
    {
        char *x;
        x=a;
        return x;
    }
    long big::at(long poz)
    {
        if(poz>=lenght)
            return -1;
        else
            return this->a[poz];
    }
    char* big::operator[](long poz)
    {
        if(poz>=lenght)
            return NULL;
        else
            return a+poz;
    }
    void big::operator=(big x)
    {
        a=new char[x.size()];
        for(long i=0;i<(long)strlen(x.a);i++)
            this->modify(i,x.at(i));
        this->lenght=strlen(x.a);
    }
    void big::modify (long poz,long val)
    {
        a[poz]=(char)val;
    }
    void big::dec (long poz)
    {
        if(a[poz])
            a[poz]--;
    }
    void big::inc (long poz)
    {
        if(a[poz]<255)
            a[poz]++;
    }
    void big::operator-(big z)
    {
        long n,m,i,j,aux;
        big rez,x,y;
        if(compare(z))
        {
            x=*this;
            y=z;
        }
        else
        {
            y=*this;
            x=z;
        }
        n=x.size();
        m=y.size();
        i=n;
        j=m;
        while(j>=0)
        {
            if(x.at(i)<0)
            {
                x.modify(i-1,x.at(i-1)+x.at(i));
                x.modify(i,0);
            }
            aux=x.at(i)-y.at(j);
            if(aux<0)
            {
                x.dec(i-1);
                rez.modify(i,aux+10);
            }
            else
                rez.modify(i,aux);
        }
        while(i)
        {
            i--;
            if(x.at(i)<0)
            {
                x.modify(i-1,x.at(i-1)+x.at(i));
                x.modify(i,0);
            }
            rez.modify(i,x.at(i));
        }
    }
    int big::compare(big x)
    {
        return strcmp(this->a,x.return_big());
    }
    int main()
    {
        big a,b;
        a.read();
        b.read();
        write(a);
        endl(cout);
        write(b);
        return 0;
    }

第一次读取是可以的，但第二次读取在想要执行a=new char[strlen(buffer)]:时出现此错误

---Windows has triggered an error in MyProject.exe
This may be due to the corruption of the heap, which indicates a bug in MyProject.exe or any DLLs it has loaded.
This may also be due to the user pressing F12 while MyProject.exe has focus.
The output window may have more diagnostic information.---

还有两个按钮"断开"answers"继续"
如果我按下continue，它会显示以下内容：

Unhandled exception at 0x77b8380e in Olimpiada a IX-a.exe: 0xC0000374: A heap has been corrupted.

我对指针不是很有经验（6个月）。如果有任何答案，我将不胜感激。