Update README.md

ch06/README.md

@@ -360,4 +360,40 @@ int calc(const char*, const char*); calls lookup(const char *)
 
 illegal. both calls lookup(char*)
 
-## [Exercise 6.54 ~ 6.56](ex6_54_55_56_functionPointer.cpp)
+## Exercise 6.54
+```cpp
+int func(int a, int b);
+
+using pFunc1 = decltype(func) *;
+typedef decltype(func) *pFunc2;
+using pFunc3 = int (*)(int a, int b);
+using pFunc4 = int(int a, int b);
+typedef int(*pFunc5)(int a, int b);
+using pFunc6 = decltype(func);
+
+std::vector<pFunc1> vec1;
+std::vector<pFunc2> vec2;
+std::vector<pFunc3> vec3;
+std::vector<pFunc4*> vec4;
+std::vector<pFunc5> vec5;
+std::vector<pFunc6*> vec6; 
+```
+
+## Exercise 6.55
+```cpp
+int add(int a, int b) { return a + b; }
+int subtract(int a, int b) { return a - b; }
+int multiply(int a, int b) { return a * b; }
+int divide(int a, int b) { return b != 0 ? a / b : 0; }
+```
+
+## Exercise 6.56
+```cpp
+std::vector<decltype(func) *> vec{add, subtract, multiply, divide};
+for (auto f : vec)
+          std::cout << f(2, 2) << std::endl;
+```
+
+----
+
+see @Mooophy 's [complete codes](ex6_54_55_56.cpp).