ch7 updated

7_16 to 7_19

Author: tsyw

ch7/7_16.cc

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+// the number and the position of access specifiers is not certain.
+// the members which you want to access at everywhere after the definition should be put after the public specifier.
+// the members which you want to access only at the class should be put after the private specifier.

ch7/7_17.cc

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+// The only difference between struct and class is the default access level.
+// struct: public
+// class: private

ch7/7_18.cc

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+// Encapsulation: the separation of implementation from interface. It hides the implementation details of a type.

ch7/7_19.cc

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+// class Person
+// {
+// public://the interface should be public
+// 	Person() = default;
+// 	Person(const string &s1, const string &s2) : name(s1), name(s2) { }
+// 	Person(std::istream &is)
+// 	{
+// 		read(is, *this);
+// 	}
+
+// 	string getName() const { return name; }
+// 	string getAddress() const { return address; }
+
+// private://the data should be private
+// 	string name;
+// 	string address;
+// };