问题描述
c ++中的new运算符有很多面孔，但是我对new放置感兴趣.
there are quite a few faces for the new operator in c++, but I'm interested in placement new.
假设您在特定的内存位置分配内存
Suppose you allocate memory at a specific memory location
int memoryPool[poolSize*sizeof(int)];
int* p = new (mem) int; //allocates memory inside the memoryPool buffer
delete p; //segmentation fault
在这种情况下如何正确分配内存?如果我不使用内置类型int而是使用名为myClass的类怎么办?
How can I correctly deallocate memory in this case?What if instead of built-in type int I would use some class called myClass?
myClass memoryPool[poolSize*sizeof(myClass )];
myClass * p = new (mem) myClass ; //allocates memory inside the memoryPool buffer
delete p; //segmentation fault
感谢您的帮助.
推荐答案
在第一种情况下，由于int没有构造函数，因此没有必要使用new放置.
In the first case, there's no point in using placement new, since int doesn't have a constructor.
在第二种情况下，它是没有意义的(如果myClass是微不足道的)或错误的，因为数组中已经有对象.
In the second case, it's either pointless (if myClass is trivial) or wrong, since there are already objects in the array.
您可以使用new放置来初始化内存块中的对象，该对象必须适当对齐，并且不能已经包含(非平凡的)对象.
You use placement new to initialise an object in a block of memory, which must be suitably aligned, and mustn't already contain a (non-trivial) object.
char memory[enough_bytes]; // WARNING: may not be properly aligned.
myClass * c = new (memory) myClass;
完成后，您需要通过调用其析构函数来销毁该对象:
Once you've finished with it, you need to destroy the object by calling its destructor:
c->~myClass();
这将对象的生存期与其内存的生存期分开.您可能还必须在某个时候释放内存，具体取决于分配方式.在这种情况下，它是一个自动数组，因此当它超出范围时会自动释放.
This separates the object's lifetime from that of its memory. You might also have to release the memory at some point, depending on how you allocated it; in this case, it's an automatic array, so it's automatically released when it goes out of scope.
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