可以以便携式方式使用新的数组放置吗?

The following example shows the problem. Compiled with Visual Studio, this example results in memory corruption:

#include <new>
#include <stdio.h>

class A
{
    public:

    A() : data(0) {}
    virtual ~A() {}
    int data;
};

int main()
{
    const int NUMELEMENTS=20;

    char *pBuffer = new char[NUMELEMENTS*sizeof(A)];
    A *pA = new(pBuffer) A[NUMELEMENTS];

    // With VC++, pA will be four bytes higher than pBuffer
    printf("Buffer address: %x, Array address: %x
", pBuffer, pA);

    // Debug runtime will assert here due to heap corruption
    delete[] pBuffer;

    return 0;
}

查看内存，编译器似乎正在使用缓冲区的前四个字节来存储其中项目数的计数.这意味着因为缓冲区只有 sizeof(A)*NUMELEMENTS 大，所以数组中的最后一个元素被写入未分配的堆中.

int main(int argc, char* argv[])
{
  const int NUMELEMENTS=20;

  char *pBuffer = new char[NUMELEMENTS*sizeof(A)];
  A *pA = (A*)pBuffer;

  for(int i = 0; i < NUMELEMENTS; ++i)
  {
    pA[i] = new (pA + i) A();
  }

  printf("Buffer address: %x, Array address: %x
", pBuffer, pA);

  // dont forget to destroy!
  for(int i = 0; i < NUMELEMENTS; ++i)
  {
    pA[i].~A();
  }    

  delete[] pBuffer;

  return 0;
}