Placement new 来初始化就地对象
我最近遇到了一种相当有趣的序列化方法，该方法利用未初始化变量的透明度（编译器中常见的未定义行为？）来实现“高效”反序列化。
内存被分配并分配有预定值。然后使用 Placement new 来实例化一个结构（例如复杂的就地数据结构），将未初始化的变量“初始化”为底层内存的值。 （参见下面的代码）
除了相当有风险并且可能不是一个非常令人愉快的编码约定之外......我只是想知道是否有人遇到过这种方法或更重要的是 - 它叫什么？
class SomeClass {
public:
SomeClass() {}
int someInt;
};
int main(...) {
int dummy = 42;
int *pSomeClass = new (&dummy) SomeClass();
cout << pSomeClass->someInt << endl;
}
这将打印出数字 42...neato！
I recently came across a rather interesting serialization approach that utilized the transparency (common undefined behavior among compilers?) of uninitialized variables for "efficient" deserialization.
Memory is allocated and assigned a predetermined value. Placement new is then used to instantiate a struct (for example a complex inplace data structure) "initializing" the uninitialized variables to the value of the underlying memory. (see code below)
Besides from being rather risky and possibly not a very agreeable coding convention... I was just wondering whether anyone had come across this method or more importantly -- what is it called?
class SomeClass {
public:
SomeClass() {}
int someInt;
};
int main(...) {
int dummy = 42;
int *pSomeClass = new (&dummy) SomeClass();
cout << pSomeClass->someInt << endl;
}
This will print out the number 42... neato!
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这就是所谓的“靠UB”，或者通俗地说，就是“愚”。
It's called "relying on UB" or, in laymen's terms, "foolishness".
我在 eCos（一种 RTOS）中看到过这样做，以初始化一些内核对象。
正如托马拉克所指出的，缺点之一是不允许使用虚拟函数。
他们试图通过测试大小相等来确保
sizeof(kernel object) == sizeof(variable used for初始化)。尽管他们的代码更复杂，但使用 C 结构来模拟 c 接口的 C++ 类成员变量，而不是使用 C 函数来获取/设置 C++ 类中的变量。
尽管他们的行为完全相反，他们使用在构造函数中设置的 C++ 类中的值来填充
placement new中的内存。我不建议这样做。
I have seen this done in eCos, an RTOS, to initialize some of their kernel objects.
As pointed out by Tomalak one of the draw-backs is no virtual functions allowed.
They try to ensure that by testing for equal size
sizeof(kernel object) == sizeof(variable used for initialization).Their code although was way more complex using a C-struct to mimic the C++ class member variables for the c interface instead of using C functions to get/set the variables in the C++ class
Although the behaviour they went for there was the exact opposite, they used values from the C++ class, set in the constructor, to fill the memory from
placement new.I do not advise on doing this tho.