ch7 completed

7-56 to 7-58

Author: tsyw

ch7/7_56.cc

@@ -0,0 +1,3 @@
+// static member: member of a class, which is related to the class correctly, not the specific object.
+// advantage: you can change the value of a static member once and change all the value in all objects.
+// differences: firstly, we can use it as an incomplete type. Secondly, it can be used as a default argument.

ch7/7_57.h

@@ -0,0 +1,37 @@
+#ifndef EX7_57_H
+#define EX7_57_H
+
+#include <iostream>
+#include <string>
+
+class Account
+{
+public:
+	Account() = default;
+	Accoutn(std::istream &is){ is >> owner >> amount; }
+	Account(double a, std::string o) : amount(a), owner(o) { }
+
+	void calculate() { amount += amount * interestRate; }
+	static double rate() { return interestRate; }
+	static void rate(double);
+
+private:
+	double amount;
+	std::string owner;
+	static double interestRate;
+	static double initRate();
+};
+
+void Account::rate(double newRate)
+{
+	interestRate = newRate;
+}
+
+double Account::initRate()
+{
+	return 0.0;
+}
+
+double Account::interestRate = initRate();
+
+#endif

ch7/7_58.cc

@@ -0,0 +1,13 @@
+//  example.h
+class Example
+{
+public:
+	static double rate = 6.5;// must be constexpr
+	static const int vecSize = 20;
+	static vector<double> vec(vecSize);// error, in-class initializer should use = or {}
+};
+
+// example.C
+#include "example.h"
+double Example::rate;
+vector<double> Example::vec;// vector<double> Example::vec(vecSize);