c++ - 运算符 new(n) 与 new unsigned char[n] 用于放置 new

Question

我正在分配内存，稍后将用于构造带有放置的对象new。我应该使用operator new(n)，还是应该使用new unsigned char[n]？为什么？

Answer 1

Factors:

• new[] must be matched with delete[] / new() with delete
• They communicate different things. operator new(n) is a request for memory for unspecified purposes, whereas new unsigned char[n] loosely implies intent to store characters there.

The array form may be slightly worse performance / efficiency wise - exact details depending on your implementation:

5.3.4/12 new T[5] results in a call of operator new where x is a non-neagtive unspecified value representing array allocation overhead: the result of the new-expression will be offset by this amount from the value returned by operator new[]....

BTW - neither are initialised:

• operator new() returns a void* to uninitialised memory: see 3.7.4.1/2 "There are no constraints on the contents of the allocated storage on return from the allocation function", whereas 5.3.4/15 says "a new-expression that creates an object of type T initializes that object as follows: if the new-initializer is ommitted, the object is default-initialized (8.5)"; 8.5/6 says only class types default constructors provide initialisation.

Answer 2

前者返回一个指向某个存储区域的指针。后者返回一个指向包含一些对象的数组的第一个元素的指针。如果您需要存储，请使用提供存储而不是对象的存储。