blocks|key|398803|text|[]比sizeof具有更高的优先级。所以sizeof(my_arr)[0]和sizeof((my_arr)[0])是一样的。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|398804|Here是指向优先表的链接。|398805|entityMap|0|LINK|mutability|MUTABLE|url|http://en.cppreference.com/w/c/language/operator_precedence^0|0|2|3|6|K|H|12|J|0|0|4|0|0^^$0|@$1|2|3|4|5|6|7|P|8|@$9|Q|A|R|B|C]|$9|S|A|T|B|C]|$9|U|A|V|B|C]|$9|W|A|X|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|Y|8|@]|D|@$9|Z|A|10|1|11]]|E|$]]|$1|H|3|-4|5|6|7|12|8|@]|D|@]|E|$]]]|I|$J|$5|K|L|M|E|$N|O]]]]

<code>[]</code> have a higher precendence than <code>sizeof</code>. So <code>sizeof(my_arr)[0]</code> is the same as <code>sizeof((my_arr)[0])</code>.

<a href="http://en.cppreference.com/w/c/language/operator_precedence" rel="noreferrer">Here</a> is a link to a precedence table.

blocks|key|3439350|text|sizeof不是一个函数。它是一个一元运算符，类似于!或~。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|3439351|sizeof(my_arr)[0]解析为sizeof+(my_arr)[0]，它只是带有冗余括号的sizeof+my_arr[0]。|3439352|这就像!(my_arr)[0]解析为!(my_arr[0])一样。|3439353|通常，后缀运算符的优先级高于C中的前缀运算符。sizeof+*a[i]%2B%2B解析为sizeof+(*((a[i])%2B%2B))+(后缀运算符[]和%2B%2B首先应用于a，然后是前缀运算符*和sizeof)。|3439354|(这是sizeof的表达式版本。还有一个类型版本，它接受一个带括号的类型名称：sizeof+(TYPE)。在这种情况下，需要使用括号，并将其作为sizeof语法的一部分。)|3439355|entityMap^0|0|6|Q|1|S|1|0|0|H|K|I|1D|G|0|3|C|I|C|0|N|E|14|K|1V|2|1Y|2|25|1|2F|1|2H|6|0|3|6|13|D|20|6|0^^$0|@$1|2|3|4|5|6|7|P|8|@$9|Q|A|R|B|C]|$9|S|A|T|B|C]|$9|U|A|V|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|W|8|@$9|X|A|Y|B|C]|$9|Z|A|10|B|C]|$9|11|A|12|B|C]]|D|@]|E|$]]|$1|H|3|I|5|6|7|13|8|@$9|14|A|15|B|C]|$9|16|A|17|B|C]]|D|@]|E|$]]|$1|J|3|K|5|6|7|18|8|@$9|19|A|1A|B|C]|$9|1B|A|1C|B|C]|$9|1D|A|1E|B|C]|$9|1F|A|1G|B|C]|$9|1H|A|1I|B|C]|$9|1J|A|1K|B|C]|$9|1L|A|1M|B|C]]|D|@]|E|$]]|$1|L|3|M|5|6|7|1N|8|@$9|1O|A|1P|B|C]|$9|1Q|A|1R|B|C]|$9|1S|A|1T|B|C]]|D|@]|E|$]]|$1|N|3|-4|5|6|7|1U|8|@]|D|@]|E|$]]]|O|$]]

<code>sizeof</code> is not a function. It's a unary operator like <code>!</code> or <code>~</code>.

<code>sizeof(my_arr)[0]</code> parses as <code>sizeof (my_arr)[0]</code>, which is just <code>sizeof my_arr[0]</code> with redundant parentheses.

This is just like <code>!(my_arr)[0]</code> parses as <code>!(my_arr[0])</code>.

In general, postfix operators have higher precedence than prefix operators in C. <code>sizeof *a[i]++</code> parses as <code>sizeof (*((a[i])++))</code> (the postfix operators <code>[]</code> and <code>++</code> are applied to <code>a</code> first, then the prefix operators <code>*</code> and <code>sizeof</code>).

(This is the expression version of <code>sizeof</code>. There's also a type version, which takes a parenthesized type name: <code>sizeof (TYPE)</code>. In that case the parens would be required and part of the <code>sizeof</code> syntax.)

blocks|key|396625|text|您使用的是接受表达式作为参数的sizeof操作符的版本。与接受类型的方法不同，它不需要括号。因此，操作数是简单的(my_arr)[0]，括号是多余的。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|396626|entityMap^0|F|6|1K|B|0^^$0|@$1|2|3|4|5|6|7|H|8|@$9|I|A|J|B|C]|$9|K|A|L|B|C]]|D|@]|E|$]]|$1|F|3|-4|5|6|7|M|8|@]|D|@]|E|$]]]|G|$]]

You're using the version of the <code>sizeof</code> operator that takes an expression as parameter. Unlike the one that takes a type, it doesn't require parentheses. Hence, the operand is simply <code>(my_arr)[0]</code>, with the parentheses being redundant.

blocks|key|396656|text|sizeof有两个“版本”：sizeof(type+name)和sizeof+expression。前者需要一对()围绕其论点。但后者-使用表达式作为参数-在其参数周围没有()。在参数中使用的任何()都被视为参数表达式的一部分，而不是sizeof语法本身的一部分。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|396657|因为编译器知道my_arr是一个对象名，而不是类型名，所以您的sizeof(my_arr)[0]实际上被编译器视为应用于表达式的sizeof：sizeof+(my_arr)[0]，其中(my_arr)[0]是参数表达式。数组名称周围的()纯粹是多余的。整个表达式被解释为sizeof+my_arr[0]。这等同于您之前的sizeof(my_arr[0])。|396658|(顺便说一句，您之前的sizeof(my_arr[0])还包含一对多余的()。)|396659|一种相当普遍的误解是，sizeof的语法需要在其参数周围加上一对()，这种误解是人们在将这类表达式解释为sizeof(my_arr)[0]时误导直觉的原因。|396660|entityMap^0|0|6|E|H|W|H|1K|2|2E|2|2Q|2|39|6|0|7|6|V|H|1S|6|1Z|I|2K|B|39|2|3R|G|4G|H|0|B|H|10|2|0|B|6|W|2|1G|H|0^^$0|@$1|2|3|4|5|6|7|N|8|@$9|O|A|P|B|C]|$9|Q|A|R|B|C]|$9|S|A|T|B|C]|$9|U|A|V|B|C]|$9|W|A|X|B|C]|$9|Y|A|Z|B|C]|$9|10|A|11|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|12|8|@$9|13|A|14|B|C]|$9|15|A|16|B|C]|$9|17|A|18|B|C]|$9|19|A|1A|B|C]|$9|1B|A|1C|B|C]|$9|1D|A|1E|B|C]|$9|1F|A|1G|B|C]|$9|1H|A|1I|B|C]]|D|@]|E|$]]|$1|H|3|I|5|6|7|1J|8|@$9|1K|A|1L|B|C]|$9|1M|A|1N|B|C]]|D|@]|E|$]]|$1|J|3|K|5|6|7|1O|8|@$9|1P|A|1Q|B|C]|$9|1R|A|1S|B|C]|$9|1T|A|1U|B|C]]|D|@]|E|$]]|$1|L|3|-4|5|6|7|1V|8|@]|D|@]|E|$]]]|M|$]]

<code>sizeof</code> has two "versions": <code>sizeof(type name)</code> and <code>sizeof expression</code>. The former requires a pair of <code>()</code> around its argument. But the latter - the one with an expression as an argument - does not have <code>()</code> around its argument. Whatever <code>()</code> you use in the argument is seen as part of the argument expression, not part of <code>sizeof</code> syntax itself. 

Since <code>my_arr</code> is known to the compiler as an object name, not a type name, your <code>sizeof(my_arr)[0]</code> is actually seen by the compiler as <code>sizeof</code> applied to an expression: <code>sizeof (my_arr)[0]</code>, where <code>(my_arr)[0]</code> is the argument expression. The <code>()</code> surrounding the array name is purely superfluous. The whole expression is interpreted as <code>sizeof my_arr[0]</code>. This is equivalent to your previous <code>sizeof(my_arr[0])</code>.

(This means, BTW, that your previous <code>sizeof(my_arr[0])</code> also contains a pair of superfluous <code>()</code>.) 

It is a rather widespread misconception that <code>sizeof</code>'s syntax somehow requires a pair of <code>()</code> around its argument. This misconception is what misleads people's intuition when interpreting such expressions as <code>sizeof(my_arr)[0]</code>.

Why does this code compile?

<pre><code>_Static uint32_t my_arr[2];
_Static_assert(sizeof(my_arr) == 8, "");
_Static_assert(sizeof(my_arr[0]) == 4, "");
_Static_assert(sizeof(my_arr)[0] == 4, "");
</code></pre>

The first 2 asserts are obviously correct, but I would have expected the last line to fail, as my understanding is that <code>sizeof()</code> should evaluate to an integer literal, which can't be treated as an array. In other words, it would fail in the same way that the following line fails:

<pre><code>_Static_assert(4[0] == 4, "");
</code></pre>

Interestingly, the following does indeed fail to compile (which should be doing the same thing, no?):

<pre><code>_Static_assert(*sizeof(my_arr) == 4, "");
</code></pre>

<blockquote>
 error: invalid type argument of unary '*' (have 'long unsigned int') 
 _Static_assert(*sizeof(my_arr) == 4, "");
</blockquote>

If it matters, I'm using gcc 5.3.0

Why does sizeof(my_arr)[0] compile and equal sizeof(my_arr[0])?

编译器

为什么要编译这段代码？_Static uint32_t my_arr[2];_Static_assert(sizeof(my_arr) == 8, "");_Static_assert(sizeof(my_arr[0]) == 4, "");_Static_assert(sizeof(my_arr)[0] == 4, ...

问为什么sizeof(my_arr)[0]编译和sizeof(my_arr[0])相等？
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