C++中一个类对象占用的内存空间大小

一个class对象需要占用的内存空间大小，最权威的结论是：

非静态成员变量总合
加上编译器为了CPU计算，作出的数据对齐处理
加上为了支持虚函数，产生的额外负担

可以分为基类和派生类两种情况来查看一个类占用的内存空间大小。

基类

(一)

<code class="hljs ruby has-numbering" style="display: block; padding: 0px; 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: transparent;"><span class="hljs-class" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">class</span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">CBase</span> </span>
{ 
}; </code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; 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; background-color: rgb(238, 238, 238);"><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>

<code class="hljs fix has-numbering" style="display: block; padding: 0px; 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: transparent;"><span class="hljs-attribute" style="box-sizing: border-box;">sizeof(CBase)</span>=<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">1；</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; 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; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li></ul>

c++要求每个实例在内存中都有独一无二的地址，空类也会被实例化，所以编译器会给空类隐含的添加一个字节，这样空类实例化之后就有了独一无二的地址了。所以空类的sizeof为1。

(二)

<code class="hljs vala has-numbering" style="display: block; padding: 0px; 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: transparent;"><span class="hljs-class" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">class</span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">CBase</span> 
{</span> 
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">private</span>:
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> a; 
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">char</span> p; 
}; </code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; 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; background-color: rgb(238, 238, 238);"><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>

<code class="hljs fix has-numbering" style="display: block; padding: 0px; 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: transparent;"><span class="hljs-attribute" style="box-sizing: border-box;">sizeof(CBase)</span>=<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">8;</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; 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; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li></ul>

记得对齐的问题。int 占4字节，char占一字节，补齐3字节。

(三)

<code class="hljs vala has-numbering" style="display: block; padding: 0px; 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: transparent;"><span class="hljs-class" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">class</span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">CBase</span> 
{</span> 
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">private</span>:
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> a; 
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">char</span> p; 
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">static</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> b;
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; 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; background-color: rgb(238, 238, 238);"><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>

<code class="hljs fix has-numbering" style="display: block; padding: 0px; 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: transparent;"><span class="hljs-attribute" style="box-sizing: border-box;">sizeof(CBase)</span>=<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">8;</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; 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; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li></ul>

静态成员变量属于整个类，不单独属于某个对象。

(四)

<code class="hljs cs has-numbering" style="display: block; padding: 0px; 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: transparent;">class CBase 
{ 
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span>:
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">func</span>();
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">private</span>:
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> a; 
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">char</span> p; 
}; </code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; 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; background-color: rgb(238, 238, 238);"><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></ul>

<code class="hljs fix has-numbering" style="display: block; padding: 0px; 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: transparent;"><span class="hljs-attribute" style="box-sizing: border-box;">sizeof(CBase)</span>=<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">8;</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; 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; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li></ul>

成员函数只是在名义上是类里的。其实成员函数的大小不在类的对象里面，同一个类的多个对象共享函数代码。而我们访问类的成员函数是通过类里面的一个指针实现，而这个指针指向的是一个table，table里面记录的各个成员函数的地址.

(五)

<code class="hljs cs has-numbering" style="display: block; padding: 0px; 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: transparent;">class CBase 
{ 
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span>: 
    <span class="hljs-title" style="box-sizing: border-box;">CBase</span>(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span>); 
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">virtual</span> ~CBase(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span>); 
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">private</span>: 
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> a; 
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">char</span> *p; 
}; </code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; 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; background-color: rgb(238, 238, 238);"><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>

<code class="hljs fix has-numbering" style="display: block; padding: 0px; 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: transparent;"><span class="hljs-attribute" style="box-sizing: border-box;">sizeof(CBase)</span>=<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">12</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; 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; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li></ul>

C++类中有虚函数的时候有一个指向虚函数表的指针（vptr），在32位系统分配指针大小为4字节。一个基类对象无论有多少个虚函数，只有这一个指针，4字节。在内存布局中，C++的编译器应该是保证虚函数表的指针存在于对象实例中最前面的位置（这是为了保证取到虚函数表的有最高的性能——如果有多层继承或是多重继承的情况下）。

派生类

派生类的大小是本身成员变量的大小加上基类的大小。如果派生类继承了多个基类的虚函数，那个这个派生类的所有虚函数所占用的内存空间等于4*（基类个数），即为每个拥有虚函数的基类维护一张虚函数表。

<code class="hljs cpp has-numbering" style="display: block; padding: 0px; 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: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">class</span> B1
{
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span>:
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> x;
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">virtual</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> v1(){ <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;">"B1::v1"</span> << endl; }
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> f1(){<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;">"B1::f1"</span> << endl; }
};

<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">class</span> B2
{
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span>:
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> y;
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">virtual</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> v2(){ <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;">"B2::v2"</span> << endl; }
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> f2(){ <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;">"B2::f2"</span> << endl; }
};

<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">class</span> B3
{
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span>:
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> z;
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">virtual</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> v3(){ <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;">"B3::v3"</span> << endl; }
    <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-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"B3::f3"</span> << endl; }
};

<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">class</span> D : <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> B1, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> B2, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> B3
{
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span>:
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> a;
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> v3(){ <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;">"D::v3"</span> << endl; }
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">virtual</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> vD(){ <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;">"D::vD"</span> << endl; }
};</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; 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; background-color: rgb(238, 238, 238);"><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></ul>

内存布局为：

C++中一个类对象占用的内存空间大小

与单继承相同的是所有的虚函数都包含在虚函数表中，所不同的是多重继承有多个虚函数表，当子类对父类的虚函数有重写时，子类的函数覆盖父类的函数在对应的虚函数位置，当子类有新的虚函数时，这些虚函数被加在第一个虚函数表的后面。

原文地址：
http://blog.sina.com.cn/s/blog_69c189bf0100mkeu.html
http://www.cnblogs.com/itech/archive/2009/02/28/1399995.html

秒客网

C++中一个类对象占用的内存空间大小

基类

派生类

相关文章