问题描述
什么是 C 和 C++ 中的未定义行为 (UB)?未指定的行为和实现定义的行为呢?它们有什么区别?
What is undefined behavior (UB) in C and C++? What about unspecified behavior and implementation-defined behavior? What is the difference between them?
推荐答案
未定义行为 是 C 和 C++ 语言中令程序员感到惊讶的方面之一来自其他语言(其他语言试图更好地隐藏它).基本上，即使许多 C++ 编译器不会报告程序中的任何错误，也可以编写不以可预测方式运行的 C++ 程序！
Undefined behavior is one of those aspects of the C and C++ language that can be surprising to programmers coming from other languages (other languages try to hide it better). Basically, it is possible to write C++ programs that do not behave in a predictable way, even though many C++ compilers will not report any errors in the program!
我们来看一个经典的例子:
Let's look at a classic example:
#include <iostream>
int main()
{
char* p = "hello!
"; // yes I know, deprecated conversion
p[0] = 'y';
p[5] = 'w';
std::cout << p;
}
变量 p 指向字符串文字 "hello!"，下面的两个赋值尝试修改该字符串文字.这个程序有什么作用?根据 C++ 标准的第 2.14.5 节第 11 段，它调用了未定义行为:
The variable p points to the string literal "hello!", and the two assignments below try to modify that string literal. What does this program do? According to section 2.14.5 paragraph 11 of the C++ standard, it invokes undefined behavior:
尝试修改字符串文字的效果未定义.
我可以听到人们在尖叫但是等等，我可以毫无问题地编译它并获得输出yellow"；或您是什么意思未定义，字符串文字存储在只读内存中，因此第一次分配尝试会导致核心转储".这正是未定义行为的问题.基本上，一旦您调用未定义的行为(甚至是鼻恶魔)，该标准就会允许任何事情发生.如果有一个正确"根据您对语言的心理模型的行为，该模型完全是错误的；C++ 标准拥有唯一的投票权，句号.
I can hear people screaming "But wait, I can compile this no problem and get the output yellow" or "What do you mean undefined, string literals are stored in read-only memory, so the first assignment attempt results in a core dump". This is exactly the problem with undefined behavior. Basically, the standard allows anything to happen once you invoke undefined behavior (even nasal demons). If there is a "correct" behavior according to your mental model of the language, that model is simply wrong; The C++ standard has the only vote, period.
未定义行为的其他示例包括访问超出其边界的数组、取消引用空指针、在对象生命周期结束后访问对象 或编写据称是聪明的表达式，例如i++ + ++i.
Other examples of undefined behavior include accessing an array beyond its bounds, dereferencing the null pointer, accessing objects after their lifetime ended or writing allegedly clever expressions like i++ + ++i.
C++ 标准的第 1.9 节还提到了未定义行为的两个不太危险的兄弟，未指定行为和实现定义的行为:
Section 1.9 of the C++ standard also mentions undefined behavior's two less dangerous brothers, unspecified behavior and implementation-defined behavior:
本国际标准中的语义描述定义了一个参数化的非确定性抽象机器.
抽象机的某些方面和操作在本国际标准中被描述为实现定义(例如，sizeof(int)).这些构成了抽象机的参数.每个实现都应包含描述其在这些方面的特征和行为的文档.
Certain aspects and operations of the abstract machine are described in this International Standard as implementation-defined (for example, sizeof(int)). These constitute the parameters of the abstract machine. Each implementation shall include documentation describing its characteristics and behavior in these respects.
抽象机的某些其他方面和操作在本国际标准中被描述为未指定(例如，函数参数的求值顺序).在可能的情况下，本国际标准定义了一组允许的行为.这些定义了抽象机器的不确定性方面.
Certain other aspects and operations of the abstract machine are described in this International Standard as unspecified (for example, order of evaluation of arguments to a function). Where possible, this International Standard defines a set of allowable behaviors. These define the nondeterministic aspects of the abstract machine.
本国际标准中将某些其他操作描述为未定义(例如，取消引用空指针的效果).[ 注意:本国际标准对包含未定义行为的程序的行为没有要求. —尾注 ]
Certain other operations are described in this International Standard as undefined (for example, the effect of dereferencing the null pointer). [ Note: this International Standard imposes no requirements on the behavior of programs that contain undefined behavior. —end note ]
具体而言，第 1.3.24 节规定:
Specifically, section 1.3.24 states:
允许的未定义行为范围从完全无视情况并产生不可预测的结果，在翻译或程序执行期间以环境特征的记录方式行为(有或没有发布诊断消息)，终止翻译或执行(发出诊断消息).
你能做些什么来避免遇到未定义的行为?基本上，您必须阅读了解自己在说什么的作者的优秀的 C++ 书籍.避免网络教程.避免废话.
What can you do to avoid running into undefined behavior? Basically, you have to read good C++ books by authors who know what they're talking about. Avoid internet tutorials. Avoid bullschildt.
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