好吧，这基本上是直接从标准复制粘贴

3.4.1 1 implementation-defined behavior unspecified behavior where each implementation documents how the choice is made 2 EXAMPLE An example of implementation-defined behavior is the propagation of the high-order bit when a signed integer is shifted right. 3.4.3 1 undefined behavior behavior, upon use of a nonportable or erroneous program construct or of erroneous data, for which this International Standard imposes no requirements 2 NOTE Possible undefined behavior ranges from ignoring the situation completely with unpredictable results, to behaving during translation or program execution in a documented manner characteristic of the environment (with or without the issuance of a diagnostic message), to terminating a translation or execution (with the issuance of a diagnostic message). 3 EXAMPLE An example of undefined behavior is the behavior on integer overflow. 3.4.4 1 unspecified behavior use of an unspecified value, or other behavior where this International Standard provides two or more possibilities and imposes no further requirements on which is chosen in any instance 2 EXAMPLE An example of unspecified behavior is the order in which the arguments to a function are evaluated.

实现定义的,

实现者希望，应该有良好的文档，标准给出了选择，但一定要编译

未指定的,

与实现定义的相同，但没有文档化

未定义的,

不管发生什么事，都要小心。

好吧，这基本上是直接从标准复制粘贴

3.4.1 1 implementation-defined behavior unspecified behavior where each implementation documents how the choice is made 2 EXAMPLE An example of implementation-defined behavior is the propagation of the high-order bit when a signed integer is shifted right. 3.4.3 1 undefined behavior behavior, upon use of a nonportable or erroneous program construct or of erroneous data, for which this International Standard imposes no requirements 2 NOTE Possible undefined behavior ranges from ignoring the situation completely with unpredictable results, to behaving during translation or program execution in a documented manner characteristic of the environment (with or without the issuance of a diagnostic message), to terminating a translation or execution (with the issuance of a diagnostic message). 3 EXAMPLE An example of undefined behavior is the behavior on integer overflow. 3.4.4 1 unspecified behavior use of an unspecified value, or other behavior where this International Standard provides two or more possibilities and imposes no further requirements on which is chosen in any instance 2 EXAMPLE An example of unspecified behavior is the order in which the arguments to a function are evaluated.

Historically, both Implementation-Defined Behavior and Undefined Behavior represented situations in which the authors of the Standard expected that people writing quality implementations would use judgment to decide what behavioral guarantees, if any, would be useful for programs in the intended application field running on the intended targets. The needs of high-end number-crunching code are quite different from those of low-level systems code, and both UB and IDB give compiler writers flexibility to meet those different needs. Neither category mandates that implementations behave in a way that's useful for any particular purpose, or even for any purpose whatsoever. Quality implementations that claim to be suitable for a particular purpose, however, should behave in a manner befitting such purpose whether the Standard requires it or not.

The only difference between Implementation-Defined Behavior and Undefined Behavior is that the former requires that implementations define and document a consistent behavior even in cases where nothing the implementation could possibly do would be useful. The dividing line between them is not whether it would generally be useful for implementations to define behaviors (compiler writers should define useful behaviors when practical whether the Standard requires them to or not) but whether there might be implementations where defining a behavior would be simultaneously costly and useless. A judgment that such implementations might exist does not in any way, shape, or form, imply any judgment about the usefulness of supporting a defined behavior on other platforms.

不幸的是，自20世纪90年代中期以来，编译器作者开始将缺乏行为授权解释为一种判断，即行为保证不值得付出代价，即使在它们至关重要的应用领域，甚至在它们几乎不花费任何成本的系统上。编译器编写者不再将UB视为进行合理判断的邀请，而是开始将其视为不这样做的借口。

例如，给定以下代码:

int scaled_velocity(int v, unsigned char pow)
{
  if (v > 250)
    v = 250;
  if (v < -250)
    v = -250;
  return v << pow;
}

两个互补的实现不需要花费任何精力 将表达式v << pow视为二补移位 不管v是正还是负。

The preferred philosophy among some of today's compiler writers, however, would suggest that because v can only be negative if the program is going to engage in Undefined Behavior, there's no reason to have the program clip the negative range of v. Even though left-shifting of negative values used to be supported on every single compiler of significance, and a large amount of existing code relies upon that behavior, modern philosophy would interpret the fact that the Standard says that left-shifting negative values is UB as implying that compiler writers should feel free to ignore that.

未定义行为vs.未指定行为有一个简短的描述。

他们最后的总结是:

总而言之，未指定的行为通常是不应该发生的 担心，除非你的软件被要求是可移植的。 相反，未定义的行为总是不受欢迎的，也不应该这样做 发生。

也许简单的措辞比严格的标准定义更容易理解。

实现定义的行为: 语言说我们有数据类型。编译器供应商指定了他们应该使用的大小，并提供了他们所做的工作的文档。

未定义的行为: 你做错了什么。例如，int型中有一个非常大的值，但不适合char型。如何将该值放入char?其实没有办法!任何事情都可能发生，但最明智的做法是将int的第一个字节放入char类型。这样做分配第一个字节是错误的，但这是在引擎盖下发生的事情。

未指明的行为: 这两个函数哪个先执行?

void fun(int n, int m);

int fun1() {
    std::cout << "fun1";
    return 1;
}
int fun2() {
    std::cout << "fun2";
    return 2;
}

//...

fun(fun1(), fun2()); // which one is executed first?

语言没有指定计算值，从左到右还是从右到左!因此，未指定的行为可能会导致未定义的行为，也可能不会导致未定义的行为，但可以肯定的是，您的程序不应该产生未指定的行为。

@eSKay我认为你的问题值得编辑答案以澄清更多:)

For fun(fun1()， fun2());行为不是“实现定义的”吗?编译器必须选择一个或另一个过程，毕竟?

The difference between implementation-defined and unspecified, is that the compiler is supposed to pick a behavior in the first case but it doesn't have to in the second case. For example, an implementation must have one and only one definition of sizeof(int). So, it can't say that sizeof(int) is 4 for some portion of the program and 8 for others. Unlike unspecified behavior, where the compiler can say: "OK I am gonna evaluate these arguments left-to-right and the next function's arguments are evaluated right-to-left." It can happen in the same program, that's why it is called unspecified. In fact, C++ could have been made easier if some of the unspecified behaviors were specified. Take a look here at Dr. Stroustrup's answer for that:

It is claimed that the difference between what can be produced giving the compiler this freedom and requiring "ordinary left-to-right evaluation" can be significant. I'm unconvinced, but with innumerable compilers "out there" taking advantage of the freedom and some people passionately defending that freedom, a change would be difficult and could take decades to penetrate to the distant corners of the C and C++ worlds. I am disappointed that not all compilers warn against code such as ++i+i++. Similarly, the order of evaluation of arguments is unspecified. IMO far too many "things" are left undefined, unspecified, that's easy to say and even to give examples of, but hard to fix. It should also be noted that it is not all that difficult to avoid most of the problems and produce portable code.