关于动态数组的c++素数加示例的混淆

I was currently reading C++ prime plus in order to learn dynamic memory allocate. I tried a piece of code on the book, but after I added one line after it I was confused by what I got: I asked the computer to print out p3[2] after I freed the memory for p3 and the computer actually printed out the correct value for p3. But shouldn't it be NOT possible for it to print after you free the memory already? I was currently reading C++ prime plus in order