C++编程思想答案第七章其他章节请点击用户名找thinkinginC++annotatedsolutionguide(charpter7).doc

Viewing Hints Book Home Page Free Newsletter Seminars Seminars on CD ROM Consulting Annotated Solution GuideRevision 1.0for Thinking in C+, 2nd edition, Volume 1by Chuck Allison2001 MindView, Inc. All Rights Reserved. Previous Chapter Table of Contents Next Chapter Chapter 77-1Create a Text class that contains a string object to hold the text of a file. Give it two constructors: a default constructor and a constructor that takes a string argument that is the name of the file to open. When the second constructor is used, open the file and read the contents into the string member object. Add a member function contents( ) to return the string so (for example) it can be printed. In main( ), open a file using Text and print the contents.Solution:/: S07:Text.cpp#include #include #include using namespace std;class Text string text;public: Text() Text(const string& fname) ifstream ifs(fname.c_str(); string line; while (getline(ifs, line) text += line + n; string contents() return text; ;int main(int argc, char* argv) if (argc 1) Text t1; Text t2(argv1); cout t1 :n t1.contents() endl; cout t2 :n t2.contents() endl; /:When creating a Text object, the compiler guarantees that the text data member has its default constructor (string:string( ) executed before either Text constructor runs, hence the default Text constructor just builds an empty string. This program prints an empty string for t1 followed by the contents of the file named in the first command-line argument. Note the use of string:c_str( ) in the second constructor. Thats because the ifstream constructor takes a char* argument, not a string.7-2Create a Message class with a constructor that takes a single string with a default value. Create a private member string, and in the constructor simply assign the argument string to your internal string. Create two overloaded member functions called print( ): one that takes no arguments and simply prints the message stored in the object, and one that takes a string argument, which it prints in addition to the internal message. Does it make sense to use this approach instead of the one used for the constructor?Solution:/: S07:Message.cpp#include #include using namespace std;class Message string msg;public: Message(const string& s = MSG) : msg(s) void print() cout msg endl; void print(const string& suffix) cout msg suffix endl; ;int main() Message m1; Message m2(Error); m1.print(); m2.print(); m1.print(hello); m2.print(goodbye);/* Output:MSGErrorMSG helloError goodbye*/:Its usually more flexible to allow optional arguments in the call to print, since the text of a message is fixed when it is created. A common technique allows an optional prefix for messages, as the following example illustrates./: S07:MessageWithPrefix.cpp#include #include using namespace std;class Message string msg;public: Message(const string& s) : msg(s) void print() cout msg endl; void print(const string& prefix) cout prefix : msg endl; ;int main() Message m(This is a message); m.print(); m.print(Attention);/* Output:This is a messageAttention: This is a message*/:7-3Determine how to generate assembly output with your compiler, and run experiments to deduce the name-decoration scheme.(Left to the reader)7-4Create a class that contains four member functions, with 0, 1, 2, and 3 int arguments, respectively. Create a main( ) that makes an object of your class and calls each of the member functions. Now modify the class so it has instead a single member function with all the arguments defaulted. Does this change your main( )?Solution:Heres the first version:/: S07:ManyArgs.cpp#include using namespace std;class ManyArgs public: void f() cout n; void f(int i) cout i n; void f(int i, int j) cout i , j n; void f(int i, int j, int k) cout i , j , k n; ;int main() ManyArgs a; a.f(); a.f(1); a.f(1, 2); a.f(1, 2, 3);/* Output:11, 21, 2, 3*/:Now compare the output above to that from this default-argument version:/: S07:DefaultArgs.cpp#include using namespace std;class DefaultArgs public: void f(int i = 0, int j = 0, int k = 0) cout i , j , k n; ;int main() DefaultArgs a; a.f(); a.f(1); a.f(1, 2); a.f(1, 2, 3);/* Output:0, 0, 01, 0, 01, 2, 01, 2, 3*/:Although its true that the operations in main( ) did not change, the respective outputs suggest when each feature is appropriate. Use default arguments when there truly is a default value (like zero above). When you want no value at all in certain instances, then the functions are different enough that you need the overloads.7-5Create a function with two arguments and call it from main( ). Now make one of the arguments a “placeholder” (no identifier) and see if your call in main( ) changes.Solution:/: S07:NamelessArg.cpp#include using namespace std;void twoArgs(int i, float x) cout twoArgs( i , x )n;void placeHolder(int i, float) cout twoArgs( i ,)n;int main() twoArgs(1, 2); placeHolder(1, 2);/* Output:twoArgs(1, 2)twoArgs(1,)*/:Placeholders are useful in those rare occasions (often in maintaining code) when you need different versions of a function, but only the type, not the value, of the differentiating parameter is important.(Exercises 6 10 left to the reader)7-6Modify Stash3.h and Stash3.cpp to use default arguments in the constructor. Test the constructor by making two different versions of a Stash object.(Left to the reader)7-7Create a new version of the Stack class (from Chapter 6) that contains the default constructor as before, and a second constructor that takes as its arguments an array of pointers to objects and the size of that array. This constructor should move through the array and push each pointer onto the Stack. Test your class with an array of string.(Left to the reader)7-8Modify SuperVar so that there are #ifdefs around all the vartype code as described in the section on enum. Make vartype a regular and publicenumeration (with no instance) and modify print( ) so that it requires a vartype argument to tell it what to do.(Left to the reader)7-9Implement Mem2.h and make sure that the modified cla