blocks|key|1000672|text|根据#if文档，明确禁止使用第一种情况：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1000673|表达式不能使用|1000674|或类型转换操作符.|blockquote|1000675|1000676|1000677|至于警告，您可以忽略它(因为您知道您的代码是正常的)，使用#pragma禁用它，或者直接从if中去掉条件。|1000678|bool+sizeMatch+=+(sizeof(a)+==+sizeof(b));
if+(!sizeMatch){
++++printf("sizes+don't+match\n");
++++return+-1;
}|code-block|syntax|javascript|1000679|1000680|编辑:由于禁用错误似乎引起了一些注意，下面是使用#pragma+warning实现这一目标的几种方法|1000681|#pragma+warning+(push)+
#pragma+warning+(disable:+4127)
++++if(sizeof(a)+!=+sizeof(b)){
#pragma+warning+(pop)+
++++++++++//+...|1000682|很明显，pop可以在代码的更远的地方完成。另一种选择可以是：|1000683|#pragma+warning+(disable:+4127)
++++if(sizeof(a)+!=+sizeof(b)){
#pragma+warning+(default:+4127)+|1000684|这会使警告恢复正常而不会被推和弹出。|1000685|不管怎样，这段代码看起来确实很难看。仅使用bool来获得sizeof比较的结果(正如我的第一个片段显示的那样)，将是最干净的解决方案。|1000686|entityMap|0|LINK|mutability|MUTABLE|url|http://msdn.microsoft.com/en-us/library/2c8f766e%2528v=vs.80%2529.aspx^0|2|3|0|0|0|0|0|T|7|19|2|0|0|0|O|F|O|F|0|0|0|0|0|0|L|4|S|6|0^^$0|@$1|2|3|4|5|6|7|1E|8|@$9|1F|A|1G|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|1H|8|@]|D|@]|E|$]]|$1|H|3|I|5|J|7|1I|8|@]|D|@]|E|$]]|$1|K|3|-4|5|6|7|1J|8|@]|D|@]|E|$]]|$1|L|3|-4|5|6|7|1K|8|@]|D|@]|E|$]]|$1|M|3|N|5|6|7|1L|8|@$9|1M|A|1N|B|C]|$9|1O|A|1P|B|C]]|D|@]|E|$]]|$1|O|3|P|5|Q|7|1Q|8|@]|D|@]|E|$R|S]]|$1|T|3|-4|5|6|7|1R|8|@]|D|@]|E|$]]|$1|U|3|V|5|6|7|1S|8|@$9|1T|A|1U|B|C]]|D|@$9|1V|A|1W|1|1X]]|E|$]]|$1|W|3|X|5|Q|7|1Y|8|@]|D|@]|E|$R|S]]|$1|Y|3|Z|5|6|7|1Z|8|@]|D|@]|E|$]]|$1|10|3|11|5|Q|7|20|8|@]|D|@]|E|$R|S]]|$1|12|3|13|5|6|7|21|8|@]|D|@]|E|$]]|$1|14|3|15|5|6|7|22|8|@$9|23|A|24|B|C]|$9|25|A|26|B|C]]|D|@]|E|$]]|$1|16|3|-4|5|6|7|27|8|@]|D|@]|E|$]]]|17|$18|$5|19|1A|1B|E|$1C|1D]]]]

The first case is explicitly forbidden according to <code>#if</code> documentation: 

<blockquote>
 The expression cannot use sizeof or a type-cast operator.
</blockquote>

As for the warning, you can either ignore it (because you know your code is ok), disable it using a <code>#pragma</code>, or just take the condition out of the <code>if</code>:

<pre><code>bool sizeMatch = (sizeof(a) == sizeof(b));
if (!sizeMatch){
 printf("sizes don't match\n");
 return -1;
}
</code></pre>

<hr>

Edit: since disabling the error seems to have drawn some attention, here are a couple of ways to achieve that using <a href="http://msdn.microsoft.com/en-us/library/2c8f766e%28v=vs.80%29.aspx" rel="nofollow"><code>#pragma warning</code></a>:

<pre><code>#pragma warning (push) 
#pragma warning (disable: 4127)
 if(sizeof(a) != sizeof(b)){
#pragma warning (pop) 
 // ...
</code></pre>

The pop could obviously be done further down the code. Another option could be:

<pre><code>#pragma warning (disable: 4127)
 if(sizeof(a) != sizeof(b)){
#pragma warning (default: 4127) 
</code></pre>

Which will turn the warning back on without pushing and popping.

Either way, this code does look ugly. IMO, just using a <code>bool</code> to get the result of the <code>sizeof</code> comparison (as my first snippet shows) would be the cleanest solution.

blocks|key|3008346|text|#include+<stdio.h>

struct+tag+{
int+a;
};
main()+{
++++struct+tag+a,b;
++++++++if(sizeof(a)+!=+sizeof(b))
++++++++printf("not");
++++else
++++++++printf("same");

}|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|3008347|这个程序运行良好，没有任何警告.！|unstyled|3008348|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$B|C]]|$1|D|3|E|5|F|7|J|8|@]|9|@]|A|$]]|$1|G|3|-4|5|F|7|K|8|@]|9|@]|A|$]]]|H|$]]

<pre><code>#include &lt;stdio.h&gt;

struct tag {
int a;
};
main() {
 struct tag a,b;
 if(sizeof(a) != sizeof(b))
 printf("not");
 else
 printf("same");

}
</code></pre>

this programs works fine without any warning...!!!

blocks|key|3024427|text|虽然可以与编译器玩捉迷藏，在编译器看不到的地方隐藏常量条件，但这并不是抑制警告的最佳方法。对于下一个要维护代码的人来说，这只会使代码显得不必要的模糊。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3024428|如果需要禁止编译器警告，请明确说明。在Visual+2005中，最好使用语用：|3024429|http://msdn.microsoft.com/en-us/library/2c8f766e(v=vs.80).aspx|offset|length|3024430|entityMap|0|LINK|mutability|MUTABLE|url^0|0|0|0|1Q|0|0^^$0|@$1|2|3|4|5|6|7|O|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|P|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|Q|8|@]|9|@$F|R|G|S|1|T]]|A|$]]|$1|H|3|-4|5|6|7|U|8|@]|9|@]|A|$]]]|I|$J|$5|K|L|M|A|$N|E]]]]

While it is possible to play hide and seek with the compiler and hide the constant conditional in places the compiler cannot see it, it's not the best way to suppress warnings. It will only make the code seem unnecessarily obscure for the next person who is going to maintain it.

If you need to suppress compiler warnings, please be explicit about it. In Visual Studio 2005, you are best off using pragmas:

<a href="http://msdn.microsoft.com/en-us/library/2c8f766e(v=vs.80).aspx" rel="nofollow">http://msdn.microsoft.com/en-us/library/2c8f766e(v=vs.80).aspx</a>

In my code, I want to ensure that <code>sizeof(a) == sizeof(b)</code>. 

First approach was to let the preprocessor do the checking:

<pre><code>#if (sizeof(a) != sizeof(b))
# error sizes don't match
#endif
</code></pre>

which doesn't compile because of <code>fatal error C1017: invalid integer constant expression</code>. Okay. Understand.

Next try:

<pre><code>if(sizeof(a) != sizeof(b)){
 printf("sizes don't match\n");
 return -1;
}
</code></pre>

Which results in a warning: <code>warning C4127: conditional expression is constant</code>.

Now, I'm stuck. Is there a warning-and-error-free way to make sure that the two structs <code>a</code> and <code>b</code> have the same size?

<hr>

Edit: 
Compiler is Visual Studio 2005, Warning level is set to 4.

How do I ensure that two types have the same size?

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

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在我的代码中，我想确保sizeof(a) == sizeof(b)。第一种方法是让预处理器进行检查：#if (sizeof(a) != sizeof(b))#  error sizes don't match#endif因为fatal error C1017: invalid integer constant expr...

问如何确保两种类型的大小相同？
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问如何确保两种类型的大小相同？EN