添加#undef和条件编译

data/3.C语言高阶/4.预处理器/3.#undef指令/config.json

@@ -6,5 +6,7 @@
     "C语言"
   ],
   "children": [],
-  "export": []
+  "export": [
+      "undef.json"
+  ]
 }
\ No newline at end of file

data/3.C语言高阶/4.预处理器/3.#undef指令/undef.json

+{
+    "type": "code_options",
+    "author": "幻灰龙",
+    "source": "undef.md"
+}
\ No newline at end of file

data/3.C语言高阶/4.预处理器/3.#undef指令/undef.md

+# 使用 `#undef`
+
+C语言的宏定义可以通过`#undef`取消定义，下面程序的输出是哪个选项？
+```c
+#include <stdio.h>
+
+#define next(x) x+1
+#undef next
+
+int main(int argc, char** args){
+    printf("%d", next(10));
+    return 0;
+}
+```
+
+## 答案
+
+```c
+error: use of undeclared identifier 'next'
+```
+
+## 选项
+
+### 直接定义宏字符串常量
+
+```c
+10
+```
+
+### 两个字符串常量组合
+
+```c
+11
+```
+
+### 宏直接拼接
+
+```c
+12
+```
\ No newline at end of file

data/3.C语言高阶/4.预处理器/4.条件编译/config.json

@@ -6,5 +6,8 @@
     "C语言"
   ],
   "children": [],
-  "export": []
+  "export": [
+      "ifdef01.json",
+      "ifdef02.json"
+  ]
 }
\ No newline at end of file

data/3.C语言高阶/4.预处理器/4.条件编译/ifdef01.json

+{
+    "type": "code_options",
+    "author": "幻灰龙",
+    "source": "ifdef01.md"
+}
\ No newline at end of file

data/3.C语言高阶/4.预处理器/4.条件编译/ifdef01.md

+# 使用 `#undef`
+
+C语言的宏定义可以通过`#ifdef`,`#else`,`#endif`包含条件编译，下面输出11的程序是？
+
+## 答案
+
+```c
+#include <stdio.h>
+
+#define add_encode(x) x+1
+#define sub_encode(x) x-1
+#define unit_encode(x) x
+
+
+#define USE_INC
+
+#ifdef USE_INC
+    #define encode add_encode
+#else
+    #ifdef USE_SUB
+        #define encode sub_encode
+    #else
+        #define encode unit_encode
+    #endif
+#endif
+
+int main(int argc, char** args){
+    printf("%d", encode(10));
+    return 0;
+}
+```
+
+## 选项
+
+### 直接定义宏字符串常量
+
+```c
+#include <stdio.h>
+
+#define add_encode(x) x+1
+#define sub_encode(x) x-1
+#define unit_encode(x) x
+
+
+#define USE_SUB
+
+#ifdef USE_INC
+    #define encode add_encode
+#else
+    #ifdef USE_SUB
+        #define encode sub_encode
+    #else
+        #define encode unit_encode
+    #endif
+#endif
+
+int main(int argc, char** args){
+    printf("%d", encode(10));
+    return 0;
+}
+```
+
+### 两个字符串常量组合
+
+```c
+#include <stdio.h>
+
+#define add_encode(x) x+1
+#define sub_encode(x) x-1
+#define unit_encode(x) x
+
+
+#define USE_UNIT
+
+#ifdef USE_INC
+    #define encode add_encode
+#else
+    #ifdef USE_SUB
+        #define encode sub_encode
+    #else
+        #define encode unit_encode
+    #endif
+#endif
+
+int main(int argc, char** args){
+    printf("%d", encode(10));
+    return 0;
+}
+```
+
+### 宏直接拼接
+
+```c
+#include <stdio.h>
+
+#define add_encode(x) x+1
+#define sub_encode(x) x-1
+#define unit_encode(x) x
+
+#ifdef USE_INC
+    #define encode add_encode
+#else
+    #ifdef USE_SUB
+        #define encode sub_encode
+    #else
+        #define encode unit_encode
+    #endif
+#endif
+
+int main(int argc, char** args){
+    printf("%d", encode(10));
+    return 0;
+}
+```
\ No newline at end of file

data/3.C语言高阶/4.预处理器/4.条件编译/ifdef02.json

+{
+    "type": "code_options",
+    "author": "幻灰龙",
+    "source": "ifdef02.md"
+}
\ No newline at end of file

data/3.C语言高阶/4.预处理器/4.条件编译/ifdef02.md

+# 使用 `#undef`
+
+C语言的宏定义可以通过`#ifdef`,`#else`,`#endif`包含条件编译，下面输出10的程序是？
+
+## 答案
+
+```c
+#include <stdio.h>
+
+#define DEBUG
+
+int main(int argc, char** args){
+    #ifdef DEBUG
+        printf("%d", encode(10));
+    #else
+        printf("%d", encode(11));
+    #endif
+    return 0;
+}
+```
+
+## 选项
+
+### 直接定义宏字符串常量
+
+```c
+#include <stdio.h>
+
+int main(int argc, char** args){
+    #ifdef DEBUG
+        printf("%d", encode(10));
+    #else
+        printf("%d", encode(11));
+    #endif
+    return 0;
+}
+```
+
+### 两个字符串常量组合
+
+```c
+#include <stdio.h>
+
+#define DEBUG
+#undef DEBUG
+
+int main(int argc, char** args){
+    #ifdef DEBUG
+        printf("%d", encode(10));
+    #else
+        printf("%d", encode(11));
+    #endif
+    return 0;
+}
+```
+
+### 宏直接拼接
+
+```c
+#include <stdio.h>
+
+#define DEBUG
+
+int main(int argc, char** args){
+    #ifdef DEBUG
+        printf("%d", encode(11));
+    #else
+        printf("%d", encode(10));
+    #endif
+    return 0;
+}
+```
\ No newline at end of file