sizeof小览

最新推荐文章于 2025-01-09 15:49:20 发布

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本文通过一系列实例探讨了C++中sizeof运算符的用法及其特点，包括对不同类型、数组及函数的处理方式。

http://blog.youkuaiyun.com/scythe666/article/details/47012347

一、文章来由—一道面试题迁出的探究

我发现我已经形成一种习惯写来由了，以后看博客的时候可以让我回忆起为什么出现这个问题，我用什么方法解决的，既然形成习惯就让这个习惯保持下去吧。今天实验室师姐在看书，一处不解，是关于sizeof的，大家讨论此问题后，我一向信服做了才知道答案，于是有了这篇文章。但是只能叫小览，因为不可能总结完sizeof的用法，欢迎补充和讨论。

二、从这道题目说起

我直接将问题的关键部分提出来：

<code class="language-c++ hljs cpp has-numbering" style="display: block; padding: 0px; background-color: transparent; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background-position: initial initial; background-repeat: initial initial;"><span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">string</span> strArr1[] = { <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"Trend"</span>, <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"Micro"</span>, <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"Soft"</span> };
<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">cout</span> << <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"sizeof(strArr1) == "</span> << <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">sizeof</span>(strArr1) << endl;</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; background-color: rgb(238, 238, 238); top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right;"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li></ul>

请问输出多少，书上的答案这样写道：

而字符串strArr1是由3段构成的，所以sizeof(strArr1)大小是12。
首先要明确sizeof不是函数，也不是一元运算符，它是个类似宏定义的特殊关键字，特别是sizeof(string)=4。

这个不去试是不知道的，因为编译器那么多，编译器做什么事情不去试怎么可能知道

果然，在vs2013 release Win32模式（x64模式还要更大）下结果是

sizeof(string) == 24
sizeof(strArr1) == 72

debug模式

sizeof(string) == 28
sizeof(strArr1) == 84

也是3倍关系，因为这是一个string数组，里面有三个string对象

那么为什么string有不同的结果？

查阅了相关资料得出结论：我们知道char*肯定是4字节，string里面可能不止包含一个char*那么简单，还包含有长度信息等其他信息，string的实现在各库中可能有所不同，但是在同一库中相同一点是，无论你的string里放多长的字符串，它的sizeof()都是固定的，字符串所占的空间是从堆中动态分配的，与sizeof()无关。

  sizeof(string)=4可能是最典型的实现之一，不过也有sizeof()为12、32字节的库实现。 但是VC6.0测试后sizeof(string)=16.还是跟编译器有关

为了完全测试一些东西，我测试越写越多，然后我来了威力加强版，见代码：

<code class="language-c++ hljs cpp has-numbering" style="display: block; padding: 0px; background-color: transparent; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background-position: initial initial; background-repeat: initial initial;"><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">#include<iostream></span>

<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">using</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">namespace</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">std</span>;

<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> main()
{
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">char</span> p[] = { <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'a'</span>, <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'b'</span>, <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'c'</span>, <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'a'</span>, <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'b'</span>, <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'c'</span> };
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">char</span> *p1 = <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"abcabc"</span>;
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">char</span> p2[] = <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"abcabc"</span>;
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">char</span> p3[][<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>] = { { <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'a'</span>, <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'b'</span> }, { <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'c'</span>, <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'a'</span> }, { <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'b'</span>, <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'c'</span> } };
    <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">printf</span>(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"p == %s\n"</span>, p);
    <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">cout</span> << p << endl;
    <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">cout</span> << <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"sizeof(p) == "</span> << <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">sizeof</span>(p) << endl;
    <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">cout</span> << <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"sizeof(p1) == "</span> << <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">sizeof</span>(p1) << endl;
    <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">cout</span> << <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"sizeof(p2) == "</span> << <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">sizeof</span>(p2) << endl;
    <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">cout</span> << <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"sizeof(p3) == "</span> << <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">sizeof</span>(p3) << endl;

    <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">cout</span><<<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"sizeof(string) == "</span> << <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">sizeof</span>(<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">string</span>) << endl;

    <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">string</span> strArr1[] = { <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"Trend"</span>, <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"Micro"</span>, <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"Soft"</span> };
    <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">cout</span> << <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"sizeof(strArr1) == "</span> << <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">sizeof</span>(strArr1) << endl;

    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> a = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>;
    <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">cout</span> <<<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"sizeof(a = 3) == "</span> << <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">sizeof</span>(a = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">3</span>) << endl;
    <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">cout</span> << <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"a == "</span> << a << endl;

    <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">cout</span> << <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"sizeof(999999) == "</span> << <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">sizeof</span>(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">999999</span>) << endl;
    <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">cout</span> << <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"sizeof(9999999999999999999) == "</span> << <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">sizeof</span>(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">9999999999999999999</span>) << endl;
    <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">cout</span> << <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"sizeof(9 / 5) == "</span> << <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">sizeof</span>(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">9</span> / <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">5</span>) << endl;
    <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">cout</span> << <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"sizeof((double)9 / 5) == "</span> << <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">sizeof</span>((<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">double</span>)<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">9</span> / <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">5</span>) << endl;

    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>;
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; background-color: rgb(238, 238, 238); top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right;"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li><li style="box-sizing: border-box; padding: 0px 5px;">30</li><li style="box-sizing: border-box; padding: 0px 5px;">31</li><li style="box-sizing: border-box; padding: 0px 5px;">32</li><li style="box-sizing: border-box; padding: 0px 5px;">33</li></ul>

运行结果如图所示：
这里写图片描述

我来一一解释我的这些测试在做什么：
（1）首先p的这种初始化方式，在末尾不会加’\0’，所以 sizeof(p) == 6；而且一个有趣的问题是printf和cout直接输出p是不同的，printf是要碰到’\0’结束，我也看了printf和cout的汇编代码，但是没有细究，之后又空详细研究一下
（2）p1和p2的sizeof不同，因为一个是指针，一个是字符数组，指针在Win32编译环境下的sizeof都是4，因为是4字节的地址，32bits可寻址空间
（3）p3是另一个有趣的问题，p3不能写成p3[3][]来初始化，因为这样初始化要保证二维数组每一行的个数相同，也就是不能出现“参差不齐”的情况，那种情况要动态分配
（4）这是一个陷阱

<code class="language-c++ hljs cpp has-numbering" style="display: block; padding: 0px; background-color: transparent; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background-position: initial initial; background-repeat: initial initial;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> a = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>;
<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">cout</span><<<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">sizeof</span>(a=<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">3</span>)<<endl;
<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">cout</span><<a<<endl;</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; background-color: rgb(238, 238, 238); top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right;"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li></ul>

输出为什么是4，0 而不是期望中的4，3？？？就在于sizeof在编译阶段处理的特性。由于sizeof不能被编译成机器码，所以sizeof作用范围内，也就是()里面的内容不能被编译，而是被替换成类型。=操作符返回左操作数的类型，所以a=3相当于int，而代码也被替换为：

<code class="language-c++ hljs cpp has-numbering" style="display: block; padding: 0px; background-color: transparent; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background-position: initial initial; background-repeat: initial initial;"><span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">cout</span><<<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">4</span><<endl;
<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">cout</span><<a<<endl;</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; background-color: rgb(238, 238, 238); top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right;"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li></ul>

所以，sizeof是不可能支持链式表达式的，这也是和一元操作符不一样的地方。

不要把sizeof当成函数，也不要看作一元操作符，把他当成一个特殊的编译预处理。

（5）这样的原因是999999是一个编译器int型可以搞定的数，所以按int来处理，太大的数不能用int搞定，但是8个字节一定可以搞定的

sizeof(999999) == 4
sizeof(9999999999999999999) == 8

（6）最后是看了（9 / 5）编译器是作为int看待的，强制转换后才是double

三、sizeof 能总结多少是多少

sizeof博大精深，即使看了很多资料，一口气总结完也是不可能了，总结常用的就好。

1、什么是sizeof

首先看一下sizeof在msdn上的定义：

The sizeof keyword gives the amount of storage, in bytes, associated with a variable or a type (including aggregate types). This keyword returns a value of type size_t.

看到return这个字眼，是不是想到了函数？错了，sizeof不是一个函数，你见过给一个函数传参数，而不加括号的吗？sizeof可以，所以sizeof不是函数。网上有人说sizeof是一元操作符，但是我并不这么认为，因为sizeof更像一个特殊的宏，它是在编译阶段求值的。举个例子：

<code class="language-c++ hljs cpp has-numbering" style="display: block; padding: 0px; background-color: transparent; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background-position: initial initial; background-repeat: initial initial;"><span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">cout</span><<<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">sizeof</span>(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span>)<<endl; <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 32位机上int长度为4</span>
<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">cout</span><<<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">sizeof</span>(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>==<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>)<<endl; <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// == 操作符返回bool类型，相当于 cout<<sizeof(bool)<<endl;</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; background-color: rgb(238, 238, 238); top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right;"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li></ul>

在编译阶段已经被翻译为：

<code class="language-c++ hljs cpp has-numbering" style="display: block; padding: 0px; background-color: transparent; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background-position: initial initial; background-repeat: initial initial;"><span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">cout</span><<<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">4</span><<endl;
<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">cout</span><<<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span><<endl;</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; background-color: rgb(238, 238, 238); top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right;"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li></ul>

2、语法

sizeof有三种语法形式，如下：

1) sizeof( object ); // sizeof( 对象 );
2) sizeof( type_name ); // sizeof( 类型 );
3) sizeof object; // sizeof 对象;
所以，

int i;
sizeof( i ); // ok
sizeof i; // ok
sizeof( int ); // ok
sizeof int; // error
既然写法1可以完全代替写法3，为求形式统一以及减少我们大脑的负担，第3种写法，忘掉它吧！

实际上，sizeof计算对象的大小也是转换成对对象类型的计算，也就是说，同种类型的不同对象其sizeof值都是一致的。这里，对象可以进一步延伸至表达式，即sizeof可以对一个表达式求值，编译器根据表达式的最终结果类型来确定大小，一般不会对表达式进行计算。如：

sizeof( 2 ); // 2的类型为int，所以等价于 sizeof( int );
sizeof( 2 + 3.14 ); // 3.14的类型为double，2也会被提升成double类型，所以等价于 sizeof( double );

3、函数的sizeof

函数类型

考虑下面的问题：

<code class="language-c++ hljs cpp has-numbering" style="display: block; padding: 0px; background-color: transparent; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background-position: initial initial; background-repeat: initial initial;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> f1(){<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>;};
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">double</span> f2(){<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0.0</span>;}
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> f3(){}

<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">cout</span><<<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">sizeof</span>(f1())<<endl; <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// f1()返回值为int，因此被认为是int</span>
<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">cout</span><<<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">sizeof</span>(f2())<<endl; <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// f2()返回值为double，因此被认为是double</span>
<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">cout</span><<<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">sizeof</span>(f3())<<endl; <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 错误！无法对void类型使用sizeof</span>
<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">cout</span><<<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">sizeof</span>(f1)<<endl;   <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 错误！无法对函数指针使用sizeof   </span>
<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">cout</span><<<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">sizeof</span>*f2<<endl;   <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// *f2，和f2()等价，因为可以看作object，所以括号不是必要的。被认为是double</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; background-color: rgb(238, 238, 238); top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right;"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li></ul>

结论：对函数使用sizeof，在编译阶段会被函数返回值的类型取代

4、数组的sizeof

<code class="language-c++ hljs cpp has-numbering" style="display: block; padding: 0px; background-color: transparent; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background-position: initial initial; background-repeat: initial initial;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">char</span> a[] = <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"abcdef"</span>;
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> b[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">20</span>] = {<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">3</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">4</span>};
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">char</span> c[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>][<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">3</span>] = {<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"aa"</span>, <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"bb"</span>};

<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">cout</span><<<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">sizeof</span>(a)<<endl; <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 7</span>
<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">cout</span><<<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">sizeof</span>(b)<<endl; <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 20*4=80</span>
<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">cout</span><<<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">sizeof</span>(c)<<endl; <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 6</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; background-color: rgb(238, 238, 238); top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right;"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li></ul>

数组a的大小在定义时未指定，编译时给它分配的空间是按照初始化的值确定的，也就是7。c是多维数组，占用的空间大小是各维数的乘积，也就是6。可以看出，数组的大小就是他在编译时被分配的空间，也就是各维数的乘积*数组元素的大小。

结论：数组的大小是各维数的乘积*数组元素的大小。

这里有一个陷阱：

<code class="language-c++ hljs cpp has-numbering" style="display: block; padding: 0px; background-color: transparent; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background-position: initial initial; background-repeat: initial initial;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> *d = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span>[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">10</span>];

<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">cout</span><<<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">sizeof</span>(d)<<endl; <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 4</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; background-color: rgb(238, 238, 238); top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right;"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li></ul>

d是我们常说的动态数组，但是他实质上还是一个指针，所以sizeof(d)的值是4。
再考虑下面的问题：

<code class="language-c++ hljs cpp has-numbering" style="display: block; padding: 0px; background-color: transparent; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background-position: initial initial; background-repeat: initial initial;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">double</span>* (*a)[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">3</span>][<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">6</span>];
<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">cout</span><<<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">sizeof</span>(a)<<endl;   <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 4</span>
<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">cout</span><<<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">sizeof</span>(*a)<<endl;   <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 72</span>
<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">cout</span><<<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">sizeof</span>(**a)<<endl; <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 24</span>
<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">cout</span><<<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">sizeof</span>(***a)<<endl; <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 4</span>
<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">cout</span><<<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">sizeof</span>(****a)<<endl; <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 8</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; background-color: rgb(238, 238, 238); top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right;"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li></ul>

a是一个很奇怪的定义，他表示一个指向 double*[3][6]类型数组的指针。既然是指针，所以sizeof(a)就是4。

既然a是执行double*[3][6]类型的指针，a就表示一个double[3][6]的多维数组类型，因此sizeof(a)=3*6*sizeof(double)=72。同样的，*a表示一个double[6]类型的数组，所以sizeof(*a)=6*sizeof(double)=24。**a就表示其中的一个元素，也就是double了，所以sizeof(***a)=4。至于****a，就是一个double了，所以sizeof(****a)=sizeof(double)=8。

差不多也要结束了，如果更进一步了解，需要查阅更多的资料~~~

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