C++将 new 放置到 NULL 中的不寻常用法
在 以下 C++98 语句中：
multiThreadService[nextBuffer] = new (NULL) MultiThreadService(binder);
这样说是否正确：
- 这是“placement new”，
- 该对象将被创建（以某种方式为 NULL？）并被丢弃，并且
multiThreadService[nextBuffer]现在将为 NULL？
我还被告知这可能是 UB - 是这样吗？
In the following C++98 statement:
multiThreadService[nextBuffer] = new (NULL) MultiThreadService(binder);
Would it be correct to say:
- this is "placement new",
- the object will be created (at NULL, somehow?) and thrown away, and
multiThreadService[nextBuffer]will now be NULL?
I was also told this could be UB - is that the case?
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首先，它不一定是调用全局非分配placement-new
operator new重载void*operator new(std::size_t size, void* ptr)（什么是通常表示“placement new”。）因为 new 表达式未限定为
::new，所以它可能更喜欢operator new 的类内重载是否存在合适的以及类型NULL不是void*，而是std::nullptr_t或整型（前者在 C++98 中不可能） 。因此，即使在全局情况下，重载解析也存在一些潜在的替代结果。在项目中搜索
operator new重载，可以在 https://github.com/aneto0/MARTe2/blob/master/Source/Core/BareMetal/L2Objects/CLASSREGISTER.h#L85，即使放置参数（第二个函数参数）是一个空指针。如果所选重载确实是全局非分配放置新
operator new，那么它基本上是 C++98 中的 noop，导致空指针。如果operator new返回空指针，则new表达式需要不执行任何初始化。但是，CWG 1748 对此进行了更改，现在如果全局非分配placement-new运算符new返回空指针值。这也可能被认为是针对 C++98 的缺陷（我不知道），在这种情况下
-std=c++98不会阻止当前编译器上的未定义行为。First, it is not necessarily calling the global non-allocating placement-new
operator newoverloadvoid* operator new(std::size_t size, void* ptr)(What is commonly meant by "placement new".)Because the new-expression is not qualified as
::new, it may prefer an in-class overload ofoperator newif a suitable one exists and furthermore the type ofNULLis notvoid*, but eitherstd::nullptr_tor an integral type (the former not being possible in C++98). Therefore there are some potential alternative results of overload resolution even in the global case.Searching the project for
operator newoverloads one can find at least one possible candidate at https://github.com/aneto0/MARTe2/blob/master/Source/Core/BareMetal/L2Objects/CLASSREGISTER.h#L85, which seems to return a valid pointer even if the placement argument (second function argument) is a null pointer.If the selected overload is really the global non-allocating placement-new
operator new, then it was basically a noop in C++98 resulting in a null pointer.newexpressions were required to not do any initialization ifoperator newreturns a null pointer.However this was changed with CWG 1748 and now the behavior is undefined if the global non-allocating placement-new
operator newreturns a null pointer value. This may have been considered a defect against C++98 as well (I don't know), in which case-std=c++98will not prevent the undefined behavior on a current compiler.