blocks|key|271903|text|abort+(留下核心转储)或throw+(允许备用数据捕获)能满足您的需求吗？|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|271904|您真的希望switch语句不包含完整的枚举吗？我几乎总是尝试在没有默认情况的情况下列出所有可能的情况(+no-op)，这样如果添加了新的枚举，gcc就会警告我，因为它可能需要处理它们，而不是让它静默地(在编译期间)落入默认情况。|271905|entityMap^0|0|5|F|5|0|0^^$0|@$1|2|3|4|5|6|7|J|8|@$9|K|A|L|B|C]|$9|M|A|N|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|O|8|@]|D|@]|E|$]]|$1|H|3|-4|5|6|7|P|8|@]|D|@]|E|$]]]|I|$]]

Would <code>abort</code> (leaving a core dump) or <code>throw</code> (allowing for alternate data capture) accommodate your needs?

Do you really want to have switch statements that don't cover the full enumeration? I nearly always try to list all the possible cases (to no-op) with no default case so that gcc will warn me if new enumerations are added, as it may be required to handle them rather than letting it silently (during compile) fall into the default.

blocks|key|2965997|text|template<unsigned+int+LINE>+class+Unreachable_At_Line+{};+
#define+__builtin_unreachable()+throw+Unreachable_At_Line<__LINE__>()|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|2965998|编辑|unstyled|offset|length|style|BOLD|2965999|由于您希望编译器省略无法访问的代码，下面是最简单的方法。|2966000|#define+__builtin_unreachable()+{+struct+X+{X&+operator=(const+X&);+}+x;+x=x;+}|2966001|编译器会优化x+=+x;指令，特别是当它不可用的时候。其用法如下：|CODE|2966002|int+foo+(int+i)
{
++switch(i)
++{
++case+0:++return+0;
++case+1:++return+1;
++default:+return+-1;
++}
++__builtin_unreachable();++//+never+executed;+so+compiler+optimizes+away
}|2966003|如果您将__builtin_unreachable()放在foo()的开头，则编译器会为未实现的operator+=生成链接器错误。我在gcc+3.4.6+(64位)中运行了这些测试。|2966004|entityMap^0|0|0|2|0|0|0|6|6|0|0|4|N|T|5|1C|A|1O|5|0^^$0|@$1|2|3|4|5|6|7|X|8|@]|9|@]|A|$B|C]]|$1|D|3|E|5|F|7|Y|8|@$G|Z|H|10|I|J]]|9|@]|A|$]]|$1|K|3|L|5|F|7|11|8|@]|9|@]|A|$]]|$1|M|3|N|5|6|7|12|8|@]|9|@]|A|$B|C]]|$1|O|3|P|5|F|7|13|8|@$G|14|H|15|I|Q]]|9|@]|A|$]]|$1|R|3|S|5|6|7|16|8|@]|9|@]|A|$B|C]]|$1|T|3|U|5|F|7|17|8|@$G|18|H|19|I|Q]|$G|1A|H|1B|I|Q]|$G|1C|H|1D|I|Q]|$G|1E|H|1F|I|J]]|9|@]|A|$]]|$1|V|3|-4|5|F|7|1G|8|@]|9|@]|A|$]]]|W|$]]

<pre><code>template&lt;unsigned int LINE&gt; class Unreachable_At_Line {}; 
#define __builtin_unreachable() throw Unreachable_At_Line&lt;__LINE__&gt;()
</code></pre>

<h2>Edit:</h2>

Since you want to have unreachable code to be omitted by compiler, below is the simplest way.

<pre><code>#define __builtin_unreachable() { struct X {X&amp; operator=(const X&amp;); } x; x=x; }
</code></pre>

Compiler optimizes away <code>x = x;</code> instruction especially when it's unreachable. Here is the usage:

<pre><code>int foo (int i)
{
 switch(i)
 {
 case 0: return 0;
 case 1: return 1;
 default: return -1;
 }
 __builtin_unreachable(); // never executed; so compiler optimizes away
}
</code></pre>

If you put <code>__builtin_unreachable()</code> in the beginning of <code>foo()</code> then compiler generates a linker error for unimplemented <code>operator =</code>. I ran these tests in gcc 3.4.6 (64-bit).

blocks|key|3179854|text|保持简单：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3179855|assert(false);|code-block|syntax|javascript|3179856|或者，更好的是：|3179857|#define+UNREACHABLE+(!"Unreachable+code+executed!")

assert(UNREACHABLE);|3179858|entityMap^0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|N|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|O|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|P|8|@]|9|@]|A|$E|F]]|$1|K|3|-4|5|6|7|Q|8|@]|9|@]|A|$]]]|L|$]]

keep it simple:

<pre><code>assert(false);
</code></pre>

or, better yet:

<pre><code>#define UNREACHABLE (!"Unreachable code executed!")

assert(UNREACHABLE);
</code></pre>

blocks|key|269967|text|您可以调用一个声明为_Noreturn的内联函数，将该调用之后的任何内容标记为不可达。编译器被允许在这样的函数之后抛出任何代码。如果函数本身是static+(并且确实返回)，编译器通常也会内联该函数。下面是一个示例：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|269968|static+_Noreturn+void+unreachable()+{
++++return;+/*+intentional+*/
}

/*+...+*/

foo();
bar();+/*+should+better+not+return+*/
unreachable();
baz();+/*+compiler+will+know+this+is+not+reachable+*/|code-block|syntax|javascript|269969|请注意，如果标记为_Noreturn的函数确实返回，则会调用未定义的行为。确保所述函数永远不会被调用。|269970|entityMap^0|A|9|1Z|6|0|0|9|9|0^^$0|@$1|2|3|4|5|6|7|O|8|@$9|P|A|Q|B|C]|$9|R|A|S|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|T|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|U|8|@$9|V|A|W|B|C]]|D|@]|E|$]]|$1|M|3|-4|5|6|7|X|8|@]|D|@]|E|$]]]|N|$]]

You can call an inline function declared <code>_Noreturn</code> to mark anything after that call as unreachable. The compiler is allowed to throw out any code after such a function. If the function itself is <code>static</code> (and does return), the compiler will usually also inline the function. Here is an example:

<pre><code>static _Noreturn void unreachable() {
 return; /* intentional */
}

/* ... */

foo();
bar(); /* should better not return */
unreachable();
baz(); /* compiler will know this is not reachable */
</code></pre>

Notice that you invoke undefined behavior if a function marked <code>_Noreturn</code> indeed returns. Be sure that said function will never be called.

I get a whole lot of warnings about switches that only partially covers the range of an enumeration switched over. Therefor, I would like to have a "default" for all those switches and put <code>__builtin_unreachable</code> (GCC builtin) in that case, so that the compiler know that case is not reachable. 

However, I came to know that GCC4.3 does not support that builtin yet. Is there any good way to emulate that functionality? I thought about dereferencing a null pointer instead, but that may have other undesirable effects/warnings and such. Do you have any better idea?

Emulating GCC's __builtin_unreachable?

编译器

我收到了一大堆关于开关的警告，这些开关只覆盖了所切换的枚举的部分范围。因此，我希望所有这些开关都有一个“默认值”，并在这种情况下放入__builtin_unreachable (GCC内置)，这样编译器就知道这种情况是不可访问的。然而，我开始了解到GCC4.3还不支持该内置。有没有什么好的方法来模拟这个功能呢？我想过取...

问模仿GCC的__builtin_unreachable？
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