create exercise7_1.cpp, update common_bugs.md

c++ note/chp7/chp7answers.md

@@ -0,0 +1,6 @@
+Solutions to Exercises in Chapter Seven
+=======================================
+
+> Exercise 7.1: Write a version of the transaction-processing program from §1.6 (p. 24) using the Sales_data class you defined for the exercises in §2.6.1 (p. 72).
+
+[exercise7_1.cpp](exercise7_1.cpp)

c++ note/chp7/exercise7_1.cpp

@@ -0,0 +1,56 @@
+#include <iostream>
+#include <string>
+using namespace std;
+
+struct Sales_data{
+	string bookNo;
+	unsigned units_sold = 0;
+	double revenue = 0.0;
+
+	string isbn() const { return bookNo; }
+	void combine(Sales_data const &data){
+		units_sold += data.units_sold;
+		revenue    += data.revenue;
+	}
+};
+
+void add(Sales_data one, Sales_data two){
+	one.combine(two);
+}
+
+istream& read(istream &is, Sales_data &data){
+	is >> data.bookNo;
+	is >> data.units_sold;
+	is >> data.revenue;
+	return is;
+}
+
+ostream& print(ostream &os, Sales_data const data){
+	os << "book number: "   << data.bookNo << endl;
+	os << "units sold: "    << data.units_sold << endl;
+	os << "total revenue: " << data.revenue << endl;
+	return os;
+}
+
+int main(){
+	Sales_data total;							// variable to hold the running sum
+	if (read(cin, total)) {						// read the first transaction
+		Sales_data trans;						// variable to hold data for the next transaction
+		while (read(cin, trans)) {				// read the remaining transactions
+			if (total.isbn() == trans.isbn())	// check the isbns
+				total.combine(trans);			// update the running total
+			else {
+				print(cout, total) << endl;		// print the results
+				total = trans;					// process the next book
+			}
+		}
+		print(cout, total) << endl;				// print the last transaction
+	}
+	else {										// there was no input
+		cerr << "No data?!" << endl;			// notify the user
+	}
+
+	system("pause");
+	return 0;
+}
+

c++ note/common_bugs.md

@@ -5,4 +5,9 @@ Some Common Bugs
 
 > 在调用了构造函数，进行了必要的初始化，之后的操作却出现了空指针？
 
-因为我嵌套调用了构造函数。在构造函数中调用其他构造函数，只会产生一个匿名的对象，并不会帮助你初始化当前的对象。
+因为我嵌套调用了构造函数。在构造函数中调用其他构造函数，只会产生一个匿名的对象，并不会帮助你初始化当前的对象。
+
+> 写了一个`read`函数，接受一个`istream`以及一个对象作为参数，即`read(istream is, myClass object)`。在函数体中利用输入流将数据读入对象中，并返回该`istream`作为进一步的判断或者使用。结果`object`没有得到赋值？！
+
+
+因为我是个傻逼。传入的`object`肯定要是引用啊。