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.

它可能在使用共享内存时很方便，在其他用途中…例如:http://www.boost.org/doc/libs/1_51_0/doc/html/interprocess/synchronization_mechanisms.html#interprocess.synchronization_mechanisms.conditions.conditions_anonymous_example

如果你正在构建一个内核，它很有用——你把从磁盘或页表读取的内核代码放在哪里?你得知道该往哪里跳。

或者在其他非常罕见的情况下，比如当你有大量分配的空间，想要把几个结构放在彼此后面。它们可以这样打包，而不需要使用offset()操作符。不过，还有其他的技巧。

我也相信一些STL实现使用了新位置，比如std::vector。它们以这种方式为2^n个元素分配空间，而不需要总是realloc。

The one place I've run across it is in containers which allocate a contiguous buffer and then fill it with objects as required. As mentioned, std::vector might do this, and I know some versions of MFC CArray and/or CList did this (because that's where I first ran across it). The buffer over-allocation method is a very useful optimization, and placement new is pretty much the only way to construct objects in that scenario. It is also used sometimes to construct objects in memory blocks allocated outside of your direct code.

我在类似的情况下使用过它，尽管它不经常出现。不过，它是c++工具箱中的一个有用工具。

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。

我也有个主意。 c++确实有零开销原则。 但是异常不遵循这个原则，所以有时它们会被编译器开关关闭。

让我们来看看这个例子:

#include <new>
#include <cstdio>
#include <cstdlib>

int main() {
    struct A {
        A() {
            printf("A()\n");
        }
        ~A() {
            printf("~A()\n");
        }
        char data[1000000000000000000] = {}; // some very big number
    };

    try {
        A *result = new A();
        printf("new passed: %p\n", result);
        delete result;
    } catch (std::bad_alloc) {
        printf("new failed\n");
    }
}

我们在这里分配一个大的结构体，检查分配是否成功，然后删除它。

但是如果我们关闭了异常，我们就不能使用try block，并且无法处理new[]失败。

我们怎么做呢?以下是如何做到的:

#include <new>
#include <cstdio>
#include <cstdlib>

int main() {
    struct A {
        A() {
            printf("A()\n");
        }
        ~A() {
            printf("~A()\n");
        }
        char data[1000000000000000000] = {}; // some very big number
    };

    void *buf = malloc(sizeof(A));
    if (buf != nullptr) {
        A *result = new(buf) A();
        printf("new passed: %p\n", result);
        result->~A();
        free(result);
    } else {
        printf("new failed\n");
    }
}

使用简单的malloc 检查是否是C方式失败 如果成功了，我们就使用新位置 手动调用析构函数(不能直接调用delete) 电话免费，由于我们叫malloc

UPD @Useless写了一个注释，它向我的视图打开了new(nothrow)的存在，在这种情况下应该使用它，但不是我之前写的方法。请不要使用我之前写的代码。对不起。