这里有人用过c++的“placement new”吗?如果有,为什么?在我看来,它只在内存映射硬件上有用。
当前回答
Head Geek: BINGO! You got it totally - that's exactly what it's perfect for. In many embedded environments, external constraints and/or the overall use scenario forces the programmer to separate the allocation of an object from its initialization. Lumped together, C++ calls this "instantiation"; but whenever the constructor's action must be explicitly invoked WITHOUT dynamic or automatic allocation, placement new is the way to do it. It's also the perfect way to locate a global C++ object that is pinned to the address of a hardware component (memory-mapped I/O), or for any static object that, for whatever reason, must reside at a fixed address.
其他回答
在http://xll.codeplex.com上查看xll项目中的fp.h文件,它解决了喜欢随身携带维度的数组“与编译器的不必要的亲密关系”问题。
typedef struct _FP
{
unsigned short int rows;
unsigned short int columns;
double array[1]; /* Actually, array[rows][columns] */
} FP;
它被std::vector<>使用,因为std::vector<>通常分配比vector<>中的对象更多的内存。
我使用它来创建基于内存的对象,其中包含从网络接收到的消息。
Head Geek: BINGO! You got it totally - that's exactly what it's perfect for. In many embedded environments, external constraints and/or the overall use scenario forces the programmer to separate the allocation of an object from its initialization. Lumped together, C++ calls this "instantiation"; but whenever the constructor's action must be explicitly invoked WITHOUT dynamic or automatic allocation, placement new is the way to do it. It's also the perfect way to locate a global C++ object that is pinned to the address of a hardware component (memory-mapped I/O), or for any static object that, for whatever reason, must reside at a fixed address.
我认为任何答案都没有强调这一点,但新位置的另一个好例子和用法是减少内存碎片(通过使用内存池)。这在嵌入式和高可用性系统中特别有用。在最后一种情况下,这一点特别重要,因为对于一个必须运行24/365天的系统来说,没有碎片是非常重要的。此问题与内存泄漏无关。
Even when a very good malloc implementation is used (or similar memory management function) it's very difficult to deal with fragmentation for a long time. At some point if you don't manage cleverly the memory reservation/release calls you could end up with a lot of small gaps that are difficult to reuse (assign to new reservations). So, one of the solutions that are used in this case is to use a memory pool to allocate before hand the memory for the application objects. After-wards each time you need memory for some object you just use the new placement to create a new object on the already reserved memory.
这样,一旦应用程序启动,就已经预留了所需的所有内存。所有新的内存预留/释放都将分配到已分配的池中(您可能有几个池,每个池对应一个不同的对象类)。在这种情况下不会发生内存碎片,因为没有间隙,您的系统可以运行很长时间(数年)而不会出现内存碎片。
我在实践中看到过这种情况,特别是在VxWorks RTOS中,因为它的默认内存分配系统受到了很多碎片的影响。因此,通过标准的new/malloc方法分配内存在项目中基本上是被禁止的。所有的内存预留都应该到一个专用的内存池中。
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