update dir

const/extern_const/const_file1.cpp

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+extern const int ext=12;

const/extern_const/const_file2.cpp

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+#include<iostream>
+/**
+ * by 光城
+ * compile: g++ -o file const_file2.cpp const_file1.cpp
+ * execute: ./file
+ */
+extern const int ext;
+int main(){
+
+    std::cout<<ext<<std::endl;
+}

const/extern_const/file1.cpp

@@ -0,0 +1 @@
+int ext;

const/extern_const/file2.cpp

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+#include<iostream>
+/**
+ * by 光城
+ * compile: g++ -o file file2.cpp file1.cpp
+ * execute: ./file
+ */
+extern int ext;
+int main(){
+
+    std::cout<<(ext+10)<<std::endl;
+}