可能的重复:
C++的“placement new”
什么是 C++ 中的就地构造函数?
What is an in-place constructor in C++?
例如Datatype *x = new(y) Datatype();
这称为放置新操作符.它允许您提供将分配数据的内存，而无需 new 运算符分配它.例如:
This is called the placement new operator. It allows you to supply the memory the data will be allocated in without having the new operator allocate it. For example:
Foo * f = new Foo();
上面会为你分配内存.
void * fm = malloc(sizeof(Foo));
Foo *f = new (fm) Foo();
以上将使用调用malloc分配的内存.new 不会再分配了.但是，您不仅限于课程.您可以对通过调用 new 分配的任何类型使用放置 new 运算符.
The above will use the memory allocated by the call to malloc. new will not allocate any more. You are not, however, limited to classes. You can use a placement new operator for any type you would allocate with a call to new.
placement new 的一个问题"是，您不应该释放通过使用delete 关键字调用placement new 运算符所分配的内存.您将通过直接调用析构函数来销毁对象.
A 'gotcha' for placement new is that you should not release the memory allocated by a call to the placement new operator using the delete keyword. You will destroy the object by calling the destructor directly.
f->~Foo();
手动调用析构函数后，内存可以按预期释放.
After the destructor is manually called, the memory can then be freed as expected.
free(fm);
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