mem访问冲突

Question

我想知道这段代码是如何导致内存访问冲突的？

{
   Vector3f *a = new Vector3f [10];
   Vector3f *b = a;
   b[9] = Vector3f (2,3,4);
   delete[] a;
   a = new Vector3f [10];
   b[4] = Vector3f (1,2,3);
   delete[] a;
}

Answer 1

当您致电b时，a仍指向与delete[] a相同的数组，然后您尝试将该内存与b[4]一起使用。

Vector3f *a = new Vector3f [10]; // a initialised to a memory block x
Vector3f *b = a;                 // b initialised to point to x also
b[9] = Vector3f (2,3,4);         // x[9] is assigned a new vector
delete[] a;                      // x is deallocated
a = new Vector3f [10];           // a is assigned a new memory block y
b[4] = Vector3f (1,2,3);         // x is used (b still points to x)
                                 // x was deallocated and this causes segfault
delete[] a;                      // y is deallocated

Answer 2

这一行：

b[4] = Vector3f (1,2,3);

b仍然指向旧的a。

Answer 3

b指向首先删除的a（即b未指向新分配的 a），所以当你删除它指向的内存后再次尝试使用b，你正在调用未定义的行为，在这种情况下，它会给你一个内存访问冲突。