是的，这行为是不确定的，但出于不同的原因比大多数人都知道的。

Yes this behavior is undefined but for different reasons than most people are aware of.

首先，使用未初始化值本身不是不确定的行为，但价值简直是不确定的。访问这便是UB如果值正好是该类型陷阱重新presentation。无符号类型很少有重陷presentations，所以你会在那边比较安全的。

First, using an unitialized value is by itself not undefined behavior, but the value is simply indeterminate. Accessing this then is UB if the value happens to be a trap representation for the type. Unsigned types have rarely trap representations, so you would be relatively safe on that side.

是什么让未定义行为是您的变量的一个附加属性，即它可能已被宣布与注册这是它的地址从来没有采取。这样的变量被特殊处理，因为有有有几分额外的状态是未初始化，并且不符合在类型域的值真正的CPU寄存器架构。

What makes the behavior undefined is an additional property of your variable, namely that it "could have been declared with register" that is its address is never taken. Such variables are treated specially because there are architectures that have real CPU registers that have a sort of extra state that is "uninitialized" and that doesn't correspond to a value in the type domain.

的编辑：的标准的相关语句是6.3.2.1p2：

The relevant phrase of the standard is 6.3.2.1p2:

如果左值指定自动存储持续时间的对象，
  可能已宣布与寄存器存储类（从未有过
  其地址获取），并且该对象是未初始化（未声明
  有一个初始化并没有分配给它之前已经被执行
  使用），则该行为是不确定的。

If the lvalue designates an object of automatic storage duration that could have been declared with the register storage class (never had its address taken), and that object is uninitialized (not declared with an initializer and no assignment to it has been performed prior to use), the behavior is undefined.

和以使其更清晰，下面code的是的情况下都合法的：

And to make it clearer, the following code is legal under all circumstances:

unsigned char a, b;
memcpy(&a, &b, 1);
a -= a;

• 这里的地址 A 和 B 已被占用，所以他们的价值仅仅是
  不确定的。


• 由于 unsigned char型从来没有陷阱再presentations
  这不确定的值就是不确定的，的任何值unsigned char型能
  发生。


• 在结束 A 的必须的保存值 0 。


• Here the addresses of a and b are taken, so their value is just indeterminate.
• Since unsigned char never has trap representations that indeterminate value is just unspecified, any value of unsigned char could happen.
• At the end a must hold the value 0.

的 EDIT2：的 A 和 B 有未指定的值：

3.19.3 未指定值

    相关类型，其中本国际标准中并没有规定要求的有效价值上的价值
  选择在任何情况

3.19.3 unspecified value
valid value of the relevant type where this International Standard imposes no requirements on which value is chosen in any instance