在以下代码中，当调用X的ctor时，将首先调用A或B的ctor吗？ 它们在类主体中的放置顺序是否对此有所控制？ 如果有人可以提供有关此问题的C ++标准的摘要，那将是完美的。

该顺序是它们在类定义中出现的顺序-这来自C ++标准的12.6.2节：

5 Initialization shall proceed in the
following order:

— First, and only for
the constructor of the most derived
class as described below, virtual base
classes shall be initialized in the
order they appear on a depth-first
left-to-right traversal of the
directed acyclic graph of base
classes, where"left-to-right" is the
order of appearance of the base class
names in the derived class
base-specifier-list.

— Then, direct
base classes shall be initialized in
declaration order as they appear in
the base-specifier-list (regardless of
the order of the mem-initializers).

— Then, nonstatic data members shall be
initialized in the order they were
declared in the class definition
(again regardless of the order of the
mem-initializers).

— Finally, the body
of the constructor is executed. [Note:
the declaration order is mandated to
ensure that base and member subobjects
are destroyed in the reverse order of
initialization. ]

初始化总是按照类成员出现在类定义中的顺序进行，因此在您的示例中，依次为a和b。

每个成员的初始化之间都有一个序列点，您可以将对尚未初始化的成员的引用传递到类成员的构造函数中，但您只能以有限的方式使用它(例如，使用它的 地址以形成一个指针)，其他用途很可能导致不确定的行为。

破坏班级成员总是以相反的顺序进行。

基数和成员的初始化顺序在12.6.2 [class.base.init] / 5中定义。