C++之Lambda研究

客官°小女子只卖身不卖艺 2022-04-25 06:42 232阅读 0赞

# **目录** #

目录 1

1. 前言 1

2. 示例1 1

3. 示例2 2

4. 示例3 3

5. 示例4 3

6. 示例5 6

7. 匿名类规则 6

8. 参考资料 7

# **1. 前言** #

本文代码测试环境为“GCC-9.1.0”，有关编译器的安装请参考《[安装GCC-8.3.0及其依赖][GCC-8.3.0]》，适用于“GCC-9.1.0”。

本文试图揭露Lambda背后一面，以方便更好的理解和掌握Lambda。Lambda代码段实际为一个编译器生成的类的“**operator ()**”函数，编译器会为每一个Lambda函数生成一个匿名的类（在C++中，类和结构体实际一样，无本质区别，除了默认的访问控制）。

对Lambda的最简单理解，是将它看作一个匿名类（或结构体），实际上也确实如此，编译器把Lambda编译成了匿名类。

# **2. 示例1** #

先看一段几乎最简单的Lambda代码：

如果Lambda表达式（或函数）没有以“**;**”结尾，则编译时将报如下错误：

Lambda之所以神奇，这得益于C++编译器的工作，上述“f”实际长这样：

<table style="width:426.1pt;"> 
 <tbody> 
  <tr> 
   <td style="vertical-align:top;width:426.1pt;"> <p style="margin-left:0pt;">type&nbsp;=&nbsp;<strong>struct&nbsp;</strong>&lt;lambda()&gt;&nbsp;{ </p> <p style="margin-left:0pt;">}</p> </td> 
  </tr> 
 </tbody> 
</table>

一个匿名的类（或结构体），实际上还有一个成员函数“operator () const”。注意这里成员函数是”const”类型，这是默认的。如果需非”const”成员函数，需要加”mutable”修饰，如下所示：

<table style="width:426.1pt;"> 
 <tbody> 
  <tr> 
   <td style="vertical-align:top;width:426.1pt;"> <p style="margin-left:0pt;">auto&nbsp;f&nbsp;=&nbsp;[n]()&nbsp;mutable&nbsp;{&nbsp;printf("%d\n",&nbsp;n);&nbsp;};</p> </td> 
  </tr> 
 </tbody> 
</table>

上面例子对应的匿名类没有任何类数据成员，现在来个有类数据成员的代码：

<table style="width:426.1pt;"> 
 <tbody> 
  <tr> 
   <td style="vertical-align:top;width:426.1pt;"> <p style="margin-left:0pt;">//&nbsp;g++&nbsp;-g&nbsp;-o&nbsp;a1&nbsp;a1.cpp&nbsp;-std=c++11</p> <p style="margin-left:0pt;">#include&nbsp;&lt;stdio.h&gt;</p> <p style="margin-left:0pt;">int&nbsp;main()&nbsp;{ </p> <p style="margin-left:0pt;">&nbsp;&nbsp;int&nbsp;n&nbsp;=&nbsp;3;</p> <p style="margin-left:0pt;">&nbsp;&nbsp;<strong>auto&nbsp;f&nbsp;=&nbsp;[n]&nbsp;{&nbsp;printf("%d\n",&nbsp;n);&nbsp;};</strong></p> <p style="margin-left:0pt;">&nbsp;&nbsp;f();&nbsp;<span style="color:#ff0000;"><strong>//&nbsp;这里实际调用的是匿名类的成员函数“</strong></span><span style="color:#0000ff;"><strong>operator&nbsp;()</strong></span><span style="color:#ff0000;"><strong>”</strong></span></p> <p style="margin-left:0pt;">&nbsp;&nbsp;return&nbsp;0;</p> <p style="margin-left:0pt;">}</p> </td> 
  </tr> 
 </tbody> 
</table>

这时，“f”实际长这样，它是一个含有类数据成员的匿名类，而不再是空无一特的类：

<table style="width:426.1pt;"> 
 <tbody> 
  <tr> 
   <td style="vertical-align:top;width:426.1pt;"> <p style="margin-left:0pt;">type&nbsp;=&nbsp;struct&nbsp;&lt;lambda()&gt;&nbsp;{ </p> <p style="margin-left:0pt;">&nbsp;&nbsp;&nbsp;&nbsp;<strong>int&nbsp;__n;</strong></p> <p style="margin-left:0pt;">}</p> </td> 
  </tr> 
 </tbody> 
</table>

# **3. 示例2** #

继续来个变种：

这时，“f”实际长这样，一个包含了引用类型的匿名类：

<table style="width:426.1pt;"> 
 <tbody> 
  <tr> 
   <td style="vertical-align:top;width:426.1pt;"> <p style="margin-left:0pt;">type&nbsp;=&nbsp;struct&nbsp;&lt;lambda()&gt;&nbsp;{ </p> <p style="margin-left:0pt;">&nbsp;&nbsp;&nbsp;&nbsp;int&nbsp;&amp;__n;</p> <p style="margin-left:0pt;">}</p> </td> 
  </tr> 
 </tbody> 
</table>

# **4. 示例3** #

继续变种，“**&**”的作用让Lambda函数可使用Lambda所在作用域内所有可见的局部变量（包括Lambda所在类的this），并且是以引用传递方式：

“f”实际长这样：

<table style="width:426.1pt;"> 
 <tbody> 
  <tr> 
   <td style="vertical-align:top;width:426.1pt;"> <p style="margin-left:0pt;">type&nbsp;=&nbsp;struct&nbsp;&lt;lambda()&gt;&nbsp;{ </p> <p style="margin-left:0pt;">&nbsp;&nbsp;&nbsp;&nbsp;int&nbsp;&amp;__n;</p> <p style="margin-left:0pt;">}</p> </td> 
  </tr> 
 </tbody> 
</table>

变稍复杂一点：

<table style="width:426.1pt;"> 
 <tbody> 
  <tr> 
   <td style="vertical-align:top;width:426.1pt;"> <p style="margin-left:0pt;">//&nbsp;g++&nbsp;-g&nbsp;-o&nbsp;a1&nbsp;a1.cpp&nbsp;-std=c++11</p> <p style="margin-left:0pt;">#include&nbsp;&lt;stdio.h&gt;</p> <p style="margin-left:0pt;">int&nbsp;main()&nbsp;{ </p> <p style="margin-left:0pt;">&nbsp;&nbsp;int&nbsp;n&nbsp;=&nbsp;3;</p> <p style="margin-left:0pt;">&nbsp;&nbsp;int&nbsp;m&nbsp;=&nbsp;5;</p> <p style="margin-left:0pt;">&nbsp;&nbsp;<strong>auto&nbsp;f&nbsp;=&nbsp;[</strong><span style="color:#0000ff;"><strong>&amp;</strong></span><strong>]()&nbsp;mutable&nbsp;{&nbsp;printf("%d\n",&nbsp;n);&nbsp;};</strong></p> <p style="margin-left:0pt;">&nbsp;&nbsp;f();</p> <p style="margin-left:0pt;">&nbsp;&nbsp;return&nbsp;0;</p> <p style="margin-left:0pt;">}</p> </td> 
  </tr> 
 </tbody> 
</table>

可以看到，“f”并没有发生变化：

<table style="width:426.1pt;"> 
 <tbody> 
  <tr> 
   <td style="vertical-align:top;width:426.1pt;"> <p style="margin-left:0pt;">type&nbsp;=&nbsp;struct&nbsp;&lt;lambda()&gt;&nbsp;{ </p> <p style="margin-left:0pt;">&nbsp;&nbsp;&nbsp;&nbsp;int&nbsp;&amp;__n;</p> <p style="margin-left:0pt;">}</p> </td> 
  </tr> 
 </tbody> 
</table>

# **5. 示例4** #

继续增加复杂度：

<table style="width:426.1pt;"> 
 <tbody> 
  <tr> 
   <td style="vertical-align:top;width:426.1pt;"> <p style="margin-left:0pt;">//&nbsp;g++&nbsp;-g&nbsp;-o&nbsp;a1&nbsp;a1.cpp&nbsp;-std=c++11</p> <p style="margin-left:0pt;">#include&nbsp;&lt;stdio.h&gt;</p> <p style="margin-left:0pt;">int&nbsp;main()&nbsp;{ </p> <p style="margin-left:0pt;">&nbsp;&nbsp;int&nbsp;n&nbsp;=&nbsp;3;</p> <p style="margin-left:0pt;">&nbsp;&nbsp;int&nbsp;m&nbsp;=&nbsp;5;</p> <p style="margin-left:0pt;">&nbsp;&nbsp;<strong>auto&nbsp;f&nbsp;=&nbsp;[</strong><span style="color:#0000ff;"><strong>&amp;</strong></span><strong>]()&nbsp;mutable&nbsp;{&nbsp;printf("%d,%d\n",&nbsp;n,&nbsp;m);&nbsp;};</strong></p> <p style="margin-left:0pt;">&nbsp;&nbsp;f();</p> <p style="margin-left:0pt;">&nbsp;&nbsp;return&nbsp;0;</p> <p style="margin-left:0pt;">}</p> </td> 
  </tr> 
 </tbody> 
</table>

可以看到“f”变了：

<table style="width:426.1pt;"> 
 <tbody> 
  <tr> 
   <td style="vertical-align:top;width:426.1pt;"> <p style="margin-left:0pt;">type&nbsp;=&nbsp;struct&nbsp;&lt;lambda()&gt;&nbsp;{ </p> <p style="margin-left:0pt;">&nbsp;&nbsp;&nbsp;&nbsp;int&nbsp;&amp;__n;</p> <p style="margin-left:0pt;">&nbsp;&nbsp;&nbsp;&nbsp;int&nbsp;&amp;__m;</p> <p style="margin-left:0pt;">}</p> </td> 
  </tr> 
 </tbody> 
</table>

从上面不难看出，编译器只会把Lambda函数用到的变量打包进对应的匿名类。继续一个稍复杂点的：

<table style="width:426.1pt;"> 
 <tbody> 
  <tr> 
   <td style="vertical-align:top;width:426.1pt;"> <p style="margin-left:0pt;">//&nbsp;g++&nbsp;-g&nbsp;-o&nbsp;a1&nbsp;a1.cpp&nbsp;-std=c++11</p> <p style="margin-left:0pt;">#include&nbsp;&lt;stdio.h&gt;</p> <p style="margin-left:0pt;">struct&nbsp;X&nbsp;{ </p> <p style="margin-left:0pt;">&nbsp;&nbsp;void&nbsp;foo()&nbsp;{&nbsp;printf("foo\n");&nbsp;}</p> <p style="margin-left:0pt;">&nbsp;&nbsp;void&nbsp;xoo()&nbsp;{ </p> <p style="margin-left:0pt;">&nbsp;&nbsp;&nbsp;&nbsp;<strong>auto&nbsp;f&nbsp;=&nbsp;[</strong><span style="color:#0000ff;"><strong>&amp;</strong></span><strong>]&nbsp;{&nbsp;foo();&nbsp;};</strong></p> <p style="margin-left:0pt;">&nbsp;&nbsp;&nbsp;&nbsp;f();</p> <p style="margin-left:0pt;">&nbsp;&nbsp;}</p> <p style="margin-left:0pt;">};</p> <p style="margin-left:0pt;">int&nbsp;main()&nbsp;{ </p> <p style="margin-left:0pt;">&nbsp;&nbsp;X().xoo();</p> <p style="margin-left:0pt;">&nbsp;&nbsp;return&nbsp;0;</p> <p style="margin-left:0pt;">}</p> </td> 
  </tr> 
 </tbody> 
</table>

这时，“f”实际长这样：

<table style="width:426.1pt;"> 
 <tbody> 
  <tr> 
   <td style="vertical-align:top;width:426.1pt;"> <p style="margin-left:0pt;">type&nbsp;=&nbsp;struct&nbsp;X::&lt;lambda()&gt;&nbsp;{ </p> <p style="margin-left:0pt;">&nbsp;&nbsp;&nbsp;&nbsp;X&nbsp;*&nbsp;const&nbsp;__this;&nbsp;<span style="color:#ff0000;"><strong>//&nbsp;X类型的指针（非对象）</strong></span></p> <p style="margin-left:0pt;">}</p> </td> 
  </tr> 
 </tbody> 
</table>

如果将“auto f = \[**&**\] \{ foo(); \};”中的“**&**”去掉，则会遇到编译错误，提示“this”没有被Lambda函数捕获：

<table style="width:426.1pt;"> 
 <tbody> 
  <tr> 
   <td style="vertical-align:top;width:426.1pt;"> <p style="margin-left:0pt;">a2.cpp:&nbsp;In&nbsp;lambda&nbsp;function:</p> <p style="margin-left:0pt;">a2.cpp:5:23:&nbsp;error:&nbsp;<span style="color:#0000ff;"><strong>'this'&nbsp;</strong></span><span style="color:#ff0000;"><strong>was&nbsp;not&nbsp;captured&nbsp;for&nbsp;this&nbsp;lambda&nbsp;function</strong></span></p> <p style="margin-left:0pt;">&nbsp;&nbsp;&nbsp;&nbsp;5&nbsp;|&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;auto&nbsp;f&nbsp;=&nbsp;[]&nbsp;{&nbsp;foo();&nbsp;};</p> <p style="margin-left:0pt;">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;|&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;^</p> <p style="margin-left:0pt;">a2.cpp:5:23:&nbsp;error:&nbsp;cannot&nbsp;call&nbsp;member&nbsp;function&nbsp;'void&nbsp;X::foo()'&nbsp;without&nbsp;object</p> </td> 
  </tr> 
 </tbody> 
</table>

改成下列方式捕获也是可以的：

<table style="width:426.1pt;"> 
 <tbody> 
  <tr> 
   <td style="vertical-align:top;width:426.1pt;"> <p style="margin-left:0pt;">//&nbsp;g++&nbsp;-g&nbsp;-o&nbsp;a1&nbsp;a1.cpp&nbsp;-std=c++11</p> <p style="margin-left:0pt;">#include&nbsp;&lt;stdio.h&gt;</p> <p style="margin-left:0pt;">struct&nbsp;X&nbsp;{ </p> <p style="margin-left:0pt;">&nbsp;&nbsp;void&nbsp;foo()&nbsp;{&nbsp;printf("foo\n");&nbsp;}</p> <p style="margin-left:0pt;">&nbsp;&nbsp;void&nbsp;xoo()&nbsp;{ </p> <p style="margin-left:0pt;">&nbsp;&nbsp;&nbsp;&nbsp;<strong>auto&nbsp;f&nbsp;=&nbsp;[</strong><span style="color:#0000ff;"><strong>this</strong></span><strong>]&nbsp;{&nbsp;foo();&nbsp;};</strong></p> <p style="margin-left:0pt;">&nbsp;&nbsp;&nbsp;&nbsp;f();</p> <p style="margin-left:0pt;">&nbsp;&nbsp;}</p> <p style="margin-left:0pt;">};</p> <p style="margin-left:0pt;">int&nbsp;main()&nbsp;{ </p> <p style="margin-left:0pt;">&nbsp;&nbsp;X().xoo();</p> <p style="margin-left:0pt;">&nbsp;&nbsp;return&nbsp;0;</p> <p style="margin-left:0pt;">}</p> </td> 
  </tr> 
 </tbody> 
</table>

如果是C++17，还可以这样：

<table style="width:426.1pt;"> 
 <tbody> 
  <tr> 
   <td style="vertical-align:top;width:426.1pt;"> <p style="margin-left:0pt;">//&nbsp;g++&nbsp;-g&nbsp;-o&nbsp;a1&nbsp;a1.cpp&nbsp;-std=c++17</p> <p style="margin-left:0pt;">#include&nbsp;&lt;stdio.h&gt;</p> <p style="margin-left:0pt;">struct&nbsp;X&nbsp;{ </p> <p style="margin-left:0pt;">&nbsp;&nbsp;void&nbsp;foo()&nbsp;{&nbsp;printf("foo\n");&nbsp;}</p> <p style="margin-left:0pt;">&nbsp;&nbsp;void&nbsp;xoo()&nbsp;{ </p> <p style="margin-left:0pt;">&nbsp;&nbsp;&nbsp;&nbsp;<strong>auto&nbsp;f&nbsp;=&nbsp;[</strong><span style="color:#0000ff;"><strong>*this</strong></span><strong>]()&nbsp;mutable&nbsp;{&nbsp;foo();&nbsp;};</strong></p> <p style="margin-left:0pt;">&nbsp;&nbsp;&nbsp;&nbsp;f();</p> <p style="margin-left:0pt;">&nbsp;&nbsp;}</p> <p style="margin-left:0pt;">};</p> <p style="margin-left:0pt;">int&nbsp;main()&nbsp;{ </p> <p style="margin-left:0pt;">&nbsp;&nbsp;X().xoo();</p> <p style="margin-left:0pt;">&nbsp;&nbsp;return&nbsp;0;</p> <p style="margin-left:0pt;">}</p> </td> 
  </tr> 
 </tbody> 
</table>

注意得有“**mutable**”修饰，不然报如下编译错误：

<table style="width:426.1pt;"> 
 <tbody> 
  <tr> 
   <td style="vertical-align:top;width:426.1pt;"> <p style="margin-left:0pt;">a2.cpp:&nbsp;In&nbsp;lambda&nbsp;function:</p> <p style="margin-left:0pt;">a2.cpp:5:30:&nbsp;error:&nbsp;passing&nbsp;'<strong>const&nbsp;X</strong>'&nbsp;as&nbsp;'<strong>this</strong>'&nbsp;argument&nbsp;discards&nbsp;qualifiers&nbsp;[-fpermissive]</p> <p style="margin-left:0pt;">&nbsp;&nbsp;&nbsp;&nbsp;5&nbsp;|&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;auto&nbsp;f&nbsp;=&nbsp;[*this]()&nbsp;{&nbsp;foo();&nbsp;};</p> <p style="margin-left:0pt;">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;|&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;^</p> <p style="margin-left:0pt;">a2.cpp:3:8:&nbsp;note:&nbsp;&nbsp;&nbsp;in&nbsp;call&nbsp;to&nbsp;'void&nbsp;X::foo()'</p> <p style="margin-left:0pt;">&nbsp;&nbsp;&nbsp;&nbsp;3&nbsp;|&nbsp;&nbsp;&nbsp;void&nbsp;foo()&nbsp;{&nbsp;printf("foo\n");&nbsp;}</p> <p style="margin-left:0pt;">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;|&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;^~~</p> </td> 
  </tr> 
 </tbody> 
</table>

也可以这样：

<table style="width:426.1pt;"> 
 <tbody> 
  <tr> 
   <td style="vertical-align:top;width:426.1pt;"> <p style="margin-left:0pt;">//&nbsp;g++&nbsp;-g&nbsp;-o&nbsp;a1&nbsp;a1.cpp&nbsp;-std=c++17</p> <p style="margin-left:0pt;">#include&nbsp;&lt;stdio.h&gt;</p> <p style="margin-left:0pt;">struct&nbsp;X&nbsp;{ </p> <p style="margin-left:0pt;">&nbsp;&nbsp;void&nbsp;foo()&nbsp;{&nbsp;printf("foo\n");&nbsp;}</p> <p style="margin-left:0pt;">&nbsp;&nbsp;void&nbsp;xoo()&nbsp;{ </p> <p style="margin-left:0pt;">&nbsp;&nbsp;&nbsp;&nbsp;<strong>auto&nbsp;f&nbsp;=&nbsp;[</strong><span style="color:#0000ff;"><strong>&amp;</strong></span><strong>,</strong><span style="color:#0000ff;"><strong>*this</strong></span><strong>]()&nbsp;mutable&nbsp;{&nbsp;foo();&nbsp;};</strong></p> <p style="margin-left:0pt;">&nbsp;&nbsp;&nbsp;&nbsp;f();</p> <p style="margin-left:0pt;">&nbsp;&nbsp;}</p> <p style="margin-left:0pt;">};</p> <p style="margin-left:0pt;">int&nbsp;main()&nbsp;{ </p> <p style="margin-left:0pt;">&nbsp;&nbsp;X().xoo();</p> <p style="margin-left:0pt;">&nbsp;&nbsp;return&nbsp;0;</p> <p style="margin-left:0pt;">}</p> </td> 
  </tr> 
 </tbody> 
</table>

使用“**\*this**”时的“f”样子如下：

<table style="width:426.1pt;"> 
 <tbody> 
  <tr> 
   <td style="vertical-align:top;width:426.1pt;"> <p style="margin-left:0pt;">type&nbsp;=&nbsp;struct&nbsp;X::&lt;lambda()&gt;&nbsp;{ </p> <p style="margin-left:0pt;">&nbsp;&nbsp;&nbsp;&nbsp;X&nbsp;__this;&nbsp;<span style="color:#ff0000;"><strong>//&nbsp;X类型的对象（非指针）</strong></span></p> <p style="margin-left:0pt;">}</p> </td> 
  </tr> 
 </tbody> 
</table>

# **6. 示例5** #

继续研究，使用C++ RTTI（Run-Time Type Identification，运行时类型识别）设施“**typeid**”查看Lambda函数：

<table style="width:426.1pt;"> 
 <tbody> 
  <tr> 
   <td style="vertical-align:top;width:426.1pt;"> <p style="margin-left:0pt;">//&nbsp;g++&nbsp;-g&nbsp;-o&nbsp;a1&nbsp;a1.cpp&nbsp;-std=c++11</p> <p style="margin-left:0pt;">#include&nbsp;&lt;stdio.h&gt;</p> <p style="margin-left:0pt;">#include&nbsp;&lt;<strong>typeinfo</strong>&gt;</p> <p style="margin-left:0pt;">struct&nbsp;X&nbsp;{ </p> <p style="margin-left:0pt;">&nbsp;&nbsp;void&nbsp;<span style="color:#0000ff;"><strong>xoo</strong></span>()&nbsp;{ </p> <p style="margin-left:0pt;">&nbsp;&nbsp;&nbsp;&nbsp;auto&nbsp;f&nbsp;=&nbsp;[]&nbsp;{&nbsp;printf("f\n");&nbsp;};</p> <p style="margin-left:0pt;">&nbsp;&nbsp;&nbsp;&nbsp;printf("%s\n",&nbsp;<span style="color:#0000ff;"><strong>typeid</strong></span>(f).name());</p> <p style="margin-left:0pt;">&nbsp;&nbsp;&nbsp;&nbsp;//&nbsp;注：typeid返回值类型为“std::type_info”</p> <p style="margin-left:0pt;">&nbsp;&nbsp;}</p> <p style="margin-left:0pt;">};</p> <p style="margin-left:0pt;">int&nbsp;main()&nbsp;{ </p> <p style="margin-left:0pt;">&nbsp;&nbsp;X().xoo();</p> <p style="margin-left:0pt;">&nbsp;&nbsp;return&nbsp;0;</p> <p style="margin-left:0pt;">}</p> </td> 
  </tr> 
 </tbody> 
</table>

运行输出：

<table style="width:426.1pt;"> 
 <tbody> 
  <tr> 
   <td style="vertical-align:top;width:426.1pt;"> <p style="margin-left:0pt;">ZN1X3<span style="color:#0000ff;"><strong>xoo</strong></span>EvEUlvE_</p> </td> 
  </tr> 
 </tbody> 
</table>

# **7. 匿名类规则** #

编译器为Lambda生成的匿名类规则（不同标准有区别）：

<table style="width:426.1pt;"> 
 <tbody> 
  <tr> 
   <td style="width:69.35pt;"> <p style="margin-left:0pt;">构造函数</p> <p style="margin-left:0pt;">拷贝构造函数</p> </td> 
   <td style="vertical-align:top;width:250.8pt;"> <p style="margin-left:0pt;">ClosureType()&nbsp;=&nbsp;<strong>delete</strong>;</p> </td> 
   <td style="vertical-align:top;width:105.95pt;"> <p style="margin-left:0pt;">C++14前</p> </td> 
  </tr> 
  <tr> 
   <td style="vertical-align:top;width:250.8pt;"> <p style="margin-left:0pt;">ClosureType()&nbsp;=&nbsp;default;</p> </td> 
   <td style="vertical-align:top;width:105.95pt;"> <p style="margin-left:0pt;">C++20起，</p> <p style="margin-left:0pt;">仅当未指定任何俘获时</p> </td> 
  </tr> 
  <tr> 
   <td style="vertical-align:top;width:250.8pt;"> <p style="margin-left:0pt;">ClosureType(const&nbsp;ClosureType&amp;&nbsp;)&nbsp;=&nbsp;default;</p> </td> 
   <td style="vertical-align:top;width:105.95pt;"> <p style="margin-left:0pt;">C++14起</p> </td> 
  </tr> 
  <tr> 
   <td style="vertical-align:top;width:250.8pt;"> <p style="margin-left:0pt;">ClosureType(ClosureType&amp;&amp;&nbsp;)&nbsp;=&nbsp;default;</p> </td> 
   <td style="vertical-align:top;width:105.95pt;"> <p style="margin-left:0pt;">C++14起</p> </td> 
  </tr> 
  <tr> 
   <td style="width:69.35pt;"> <p style="margin-left:0pt;">拷贝复制函数</p> </td> 
   <td style="vertical-align:top;width:250.8pt;"> <p style="margin-left:0pt;">ClosureType&amp;&nbsp;operator=(const&nbsp;ClosureType&amp;)&nbsp;=&nbsp;<strong>delete</strong>;</p> </td> 
   <td style="vertical-align:top;width:105.95pt;"> <p style="margin-left:0pt;"><span style="color:#0000ff;"><strong>C++20前</strong></span></p> </td> 
  </tr> 
  <tr> 
   <td style="vertical-align:top;width:250.8pt;"> <p style="margin-left:0pt;">ClosureType&amp;&nbsp;operator=(const&nbsp;ClosureType&amp;)&nbsp;=&nbsp;default;</p> <p style="margin-left:0pt;">ClosureType&amp;&nbsp;operator=(ClosureType&amp;&amp;)&nbsp;=&nbsp;default;</p> </td> 
   <td style="vertical-align:top;width:105.95pt;"> <p style="margin-left:0pt;">C++20起，</p> <p style="margin-left:0pt;">仅当未指定任何俘获时</p> </td> 
  </tr> 
  <tr> 
   <td style="vertical-align:top;width:250.8pt;"> <p style="margin-left:0pt;">ClosureType&amp;&nbsp;operator=(const&nbsp;ClosureType&amp;)&nbsp;=&nbsp;<strong>delete</strong>;</p> </td> 
   <td style="vertical-align:top;width:105.95pt;"> <p style="margin-left:0pt;">C++20起，其他情况</p> </td> 
  </tr> 
  <tr> 
   <td style="vertical-align:top;width:69.35pt;"> <p style="margin-left:0pt;">析构函数</p> </td> 
   <td style="vertical-align:top;width:250.8pt;"> <p style="margin-left:0pt;">~ClosureType()&nbsp;=&nbsp;default;</p> </td> 
   <td style="vertical-align:top;width:105.95pt;"> <p style="margin-left:0pt;">析构函数是隐式声明的</p> </td> 
  </tr> 
 </tbody> 
</table>

对于标记为“**delete**”的函数是不能调用的，如下列代码中的“**f2 = f1;**”将触发编译错误：

<table style="width:426.1pt;"> 
 <tbody> 
  <tr> 
   <td style="vertical-align:top;width:426.1pt;"> <p style="margin-left:0pt;">int&nbsp;main()&nbsp;{ </p> <p style="margin-left:0pt;">&nbsp;&nbsp;auto&nbsp;f1&nbsp;=&nbsp;[]{};</p> <p style="margin-left:0pt;">&nbsp;&nbsp;auto&nbsp;f2&nbsp;=&nbsp;f1;</p> <p style="margin-left:0pt;">&nbsp;&nbsp;<strong>f2&nbsp;=&nbsp;f1;</strong></p> <p style="margin-left:0pt;">&nbsp;&nbsp;return&nbsp;0;</p> <p style="margin-left:0pt;">}</p> </td> 
  </tr> 
 </tbody> 
</table>

上列代码在C++11、C++14和C++17均会报错。不过如规则所示，C++20（含C++2a）上则可以正常编译：

<table style="width:426.1pt;"> 
 <tbody> 
  <tr> 
   <td style="vertical-align:top;width:426.1pt;"> <p style="margin-left:0pt;">a3.cpp:&nbsp;In&nbsp;function&nbsp;'int&nbsp;main()':</p> <p style="margin-left:0pt;">a3.cpp:4:8:&nbsp;error:&nbsp;use&nbsp;of&nbsp;deleted&nbsp;function&nbsp;'main()::&lt;lambda()&gt;&amp;&nbsp;main()::&lt;lambda()&gt;::operator=(const&nbsp;main()::&lt;lambda()&gt;&amp;)'</p> <p style="margin-left:0pt;">&nbsp;&nbsp;&nbsp;&nbsp;4&nbsp;|&nbsp;&nbsp;&nbsp;f2&nbsp;=&nbsp;f1;</p> <p style="margin-left:0pt;">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;|&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;^~</p> <p style="margin-left:0pt;">a3.cpp:2:14:&nbsp;note:&nbsp;a&nbsp;lambda&nbsp;closure&nbsp;type&nbsp;has&nbsp;<strong>a&nbsp;</strong><span style="color:#0000ff;"><strong>deleted&nbsp;</strong></span><strong>copy&nbsp;assignment&nbsp;operator</strong></p> <p style="margin-left:0pt;">&nbsp;&nbsp;&nbsp;&nbsp;2&nbsp;|&nbsp;&nbsp;&nbsp;auto&nbsp;f1&nbsp;=&nbsp;[]{};</p> <p style="margin-left:0pt;">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;|&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;^</p> </td> 
  </tr> 
 </tbody> 
</table>

希望通过本文，对理解Lambda有所帮助。

# **8. 参考资料** #

1) [https://zh.cppreference.com/w/cpp/language/lambda][https_zh.cppreference.com_w_cpp_language_lambda]

2) [https://docs.microsoft.com/en-us/cpp/cpp/lambda-expressions-in-cpp?view=vs-2019][https_docs.microsoft.com_en-us_cpp_cpp_lambda-expressions-in-cpp_view_vs-2019]

3) [https://en.cppreference.com/w/cpp/language/lambda][https_en.cppreference.com_w_cpp_language_lambda]

4) [https://stackoverflow.com/questions/7627098/what-is-a-lambda-expression-in-c11][https_stackoverflow.com_questions_7627098_what-is-a-lambda-expression-in-c11]

5) [https://www.cprogramming.com/c++11/c++11-lambda-closures.html][https_www.cprogramming.com_c_11_c_11-lambda-closures.html]

[GCC-8.3.0]: https://blog.csdn.net/Aquester/article/details/89735466
[https_zh.cppreference.com_w_cpp_language_lambda]: https://zh.cppreference.com/w/cpp/language/lambda
[https_docs.microsoft.com_en-us_cpp_cpp_lambda-expressions-in-cpp_view_vs-2019]: https://docs.microsoft.com/en-us/cpp/cpp/lambda-expressions-in-cpp?view=vs-2019
[https_en.cppreference.com_w_cpp_language_lambda]: https://en.cppreference.com/w/cpp/language/lambda
[https_stackoverflow.com_questions_7627098_what-is-a-lambda-expression-in-c11]: https://stackoverflow.com/questions/7627098/what-is-a-lambda-expression-in-c11
[https_www.cprogramming.com_c_11_c_11-lambda-closures.html]: https://www.cprogramming.com/c++11/c++11-lambda-closures.html