最近同事轉貼了一篇

編譯器 LLVM 淺淺玩. 以動手實作來認識 The LLVM Compiler 寫得不錯，

裡面提到了 Cling 這個運用了 LLVM 架構寫的 Interactive C++ interpreter，

也就是 C++ 的直譯器，感覺很有趣，

就來玩玩看吧～

在 Mac 上可以用 Homebrew 安裝 Cling：

brew install cling

執行 cling 後，就會進到互動模式：

testuser@localhost ~ $ cling

****************** CLING ******************
* Type C++ code and press enter to run it *
*             Type .q to exit             *
*******************************************
[cling]$

執行 .help 可以看到可用的指令：

[cling]$ .help

 Cling (C/C++ interpreter) meta commands usage
 All commands must be preceded by a '.', except
 for the evaluation statement { }
 ==============================================================================
 Syntax: .Command [arg0 arg1 ... argN]

   .L <filename>		- Load the given file or library

   .(x|X) <filename>[args]	- Same as .L and runs a function with
                  signature: ret_type filename(args)

   .> <filename>		- Redirect command to a given file
      '>' or '1>'		- Redirects the stdout stream only
      '2>'			- Redirects the stderr stream only
      '&>' (or '2>&1')		- Redirects both stdout and stderr
      '>>'			- Appends to the given file

   .undo [n]			- Unloads the last 'n' inputs lines

   .U <filename>		- Unloads the given file

   .I [path]			- Shows the include path. If a path is given -
                  adds the path to the include paths

   .O <level>			- Sets the optimization level (0-3)
                  (not yet implemented)

   .class <name>		- Prints out class <name> in a CINT-like style

   .files 			- Prints out some CINT-like file statistics

   .fileEx 			- Prints out some file statistics

   .g 				- Prints out information about global variable
                  'name' - if no name is given, print them all

   .@ 				- Cancels and ignores the multiline input

   .rawInput [0|1]		- Toggle wrapping and printing the
                  execution results of the input

   .dynamicExtensions [0|1]	- Toggles the use of the dynamic scopes and the
                  late binding

   .printDebug [0|1]		- Toggles the printing of input's corresponding
                  state changes

   .storeState <filename>	- Store the interpreter's state to a given file

   .compareState <filename>	- Compare the interpreter's state with the one
                  saved in a given file

   .stats [name]		- Show stats for internal data structures
                  'ast'  abstract syntax tree stats
                  'asttree [filter]'  abstract syntax tree layout
                  'decl' dump ast declarations
                  'undo' show undo stack

   .help			- Shows this information

   .q				- Exit the program

像是我跑 printf，會因為還沒有引入相關標頭檔而失敗：

[cling]$ printf
input_line_4:2:2: error: use of undeclared identifier 'printf'
 printf
 ^

先跑 

就會變成有定義了，Cling 會秀出函式的宣告：

[cling]$ #include <stdio.h>

[cling]$ printf
(int (*)(const char *, ...)) Function @0x7fff202cb0a8
  at /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/stdio.h:170:
int	 printf(const char * __restrict, ...) __printflike(1, 2)

可以執行 

[cling]$ printf("this is a test")
this is a test(int) 14

這種互動模式最方便的地方，

就是拿來測試一些不熟的函式 (或語法)，可以直接觀察它的結果。

像是下面拿來測試一下 

[cling]$ strchr("abc", 'b')
(const char *) "bc"

[cling]$ strchr("abc", 'd')
(const char *) nullptr

當然也可以直接寫完整的 C++ 程式，來跑測試程式。

不過用這直譯器可以很方便的跑一些小敘述，算蠻實用的喔～

參考資料：GitHub – vgvassilev/cling: The interactive C++ interpreter Cling