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| cs:programming:cpp:cpp_primer:answers:chpt_7 [2024/01/14 13:46] – 移除 - 外部编辑 (未知日期) 127.0.0.1 | cs:programming:cpp:cpp_primer:answers:chpt_7 [2024/01/14 13:46] (当前版本) – ↷ 页面programming:cpp:cpp_primer:answers:chpt_7被移动至cs:programming:cpp:cpp_primer:answers:chpt_7 codinghare | ||
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| + | ======Chapter.7====== | ||
| + | 第七章的习题答案 | ||
| + | ---- | ||
| + | ====Ex.7.1-7.10==== | ||
| + | ==ex.7.1== | ||
| + | > | ||
| + | CODE: [[https:// | ||
| + | CODE: [[https:// | ||
| + | ==ex.7.2== | ||
| + | > | ||
| + | CODE: [[https:// | ||
| + | ==ex.7.3== | ||
| + | > | ||
| + | CODE: [[https:// | ||
| + | ==ex.7.4== | ||
| + | > | ||
| + | CODE: [[https:// | ||
| + | ==ex.7.5== | ||
| + | > | ||
| + | CODE: [[https:// | ||
| + | These functions should be declared as '' | ||
| + | ==ex.7.6== | ||
| + | > | ||
| + | CODE: [[https:// | ||
| + | ==ex.7.7== | ||
| + | > | ||
| + | CODE: [[https:// | ||
| + | ==ex.7.8== | ||
| + | > | ||
| + | Because the ' | ||
| + | ==ex.7.9== | ||
| + | > | ||
| + | ==ex.7.10== | ||
| + | > | ||
| + | <code cpp> | ||
| + | if (read(read(cin, | ||
| + | </ | ||
| + | Since the inner " | ||
| + | ====Ex.7.11-7.20==== | ||
| + | ==ex.7.11== | ||
| + | > | ||
| + | CODE: [[https:// | ||
| + | [[https:// | ||
| + | ==ex.7.12== | ||
| + | > | ||
| + | CODE: [[https:// | ||
| + | ==ex.7.13== | ||
| + | > | ||
| + | CODE: [[https:// | ||
| + | ==ex.7.14== | ||
| + | > | ||
| + | <code cpp> | ||
| + | Sales_data() : book_no("" | ||
| + | </ | ||
| + | ==ex.7.15== | ||
| + | > | ||
| + | CODE: [[https:// | ||
| + | [[https:// | ||
| + | ==ex.7.16== | ||
| + | > | ||
| + | There is no constraint on where and how often an access specifier may appear inside a class definition. Members belonging to a part of an interface, for example, the constructor or member function, should be defined after a public specifier. | ||
| + | ==ex.7.17== | ||
| + | > | ||
| + | The only difference between '' | ||
| + | ==ex.7.18== | ||
| + | > | ||
| + | Encapsulation is one of the most important aspects of class structure. It means that a class should be divided into two parts: interfaces and implementations. It allows implementers to focus on implementing the class, while the user may concentrate on using the interface' | ||
| + | ==ex.7.19== | ||
| + | > | ||
| + | |||
| + | * public members: constructors, | ||
| + | * private members: '' | ||
| + | ==ex.7.20== | ||
| + | >When are friends useful? Discuss the pros and cons of using friends. | ||
| + | The functions that are part of the class in aspect are not a part of the implementation of the class, but they need access to the encapsulated resource of the class. Declaring those functions as friends allows access to those resources. | ||
| + | * Pros: In some cases, the implementation requires data exchange between different classes or functions, but those data are not intended for public consumption. In that case, Friend works well. | ||
| + | * Cons: The encapsulation of a class will be lessened if it is friended. | ||
| + | ====Ex.7.21-7.30==== | ||
| + | ==ex.7.21== | ||
| + | > | ||
| + | CODE: [[https:// | ||
| + | [[https:// | ||
| + | ==ex.7.22== | ||
| + | > | ||
| + | CODE: [[https:// | ||
| + | [[https:// | ||
| + | ==ex.7.23== | ||
| + | > | ||
| + | CODE: [[https:// | ||
| + | [[https:// | ||
| + | ==ex.7.24== | ||
| + | > | ||
| + | CODE: [[https:// | ||
| + | [[https:// | ||
| + | ==ex.7.25== | ||
| + | > | ||
| + | As all of the members are related to '' | ||
| + | ==ex.7.26== | ||
| + | > | ||
| + | CODE: [[https:// | ||
| + | ==ex.7.27== | ||
| + | > | ||
| + | <code cpp> | ||
| + | Screen myScreen(5, | ||
| + | myScreen.move(4, | ||
| + | cout << " | ||
| + | myScreen.display(cout); | ||
| + | cout << " | ||
| + | </ | ||
| + | CODE: [[https:// | ||
| + | [[https:// | ||
| + | ==ex.7.28== | ||
| + | > | ||
| + | In the second call, As the return-by-value method returns a copy of the class, the '' | ||
| + | ==ex.7.29== | ||
| + | > | ||
| + | <code cpp> | ||
| + | //return by reference | ||
| + | XXXXXXXXXXXXXXXXXXXX# | ||
| + | XXXXXXXXXXXXXXXXXXXX# | ||
| + | //return by value | ||
| + | XXXXXXXXXXXXXXXXXXXX# | ||
| + | XXXXXXXXXXXXXXXXXXXXXXXXX | ||
| + | </ | ||
| + | ==ex.7.30== | ||
| + | > | ||
| + | If a member function has a local variable that hides a member, the '' | ||
| + | <code cpp> | ||
| + | class example { | ||
| + | int i = 0; | ||
| + | void func { | ||
| + | int i = 1; | ||
| + | cout << i; //print the local variable i | ||
| + | cout << this->a; //print the member i | ||
| + | } | ||
| + | }; | ||
| + | </ | ||
| + | Ref: | ||
| + | ====Ex.7.31-7.40==== | ||
| + | ==ex.7.31== | ||
| + | > | ||
| + | CODE: [[https:// | ||
| + | ==ex.7.32== | ||
| + | > | ||
| + | CODE: [[https:// | ||
| + | ==ex.7.33== | ||
| + | > | ||
| + | <code cpp> | ||
| + | pos Screen:: | ||
| + | return height * width; | ||
| + | } | ||
| + | </ | ||
| + | <code cpp> | ||
| + | Screen:: | ||
| + | Screen:: | ||
| + | return height * width; | ||
| + | } | ||
| + | </ | ||
| + | ==ex.7.34== | ||
| + | > | ||
| + | There will be an undeclare error since type names must appear before using. | ||
| + | ==ex.7.35== | ||
| + | > | ||
| + | <code cpp> | ||
| + | typedef string Type; | ||
| + | Type initVal(); //string | ||
| + | class Exercise { | ||
| + | public: | ||
| + | typedef double Type; | ||
| + | Type setVal(Type); | ||
| + | Type initVal(); //double | ||
| + | private: | ||
| + | int val; | ||
| + | }; | ||
| + | |||
| + | Type Exercise:: | ||
| + | val = parm + initVal(); | ||
| + | return val; | ||
| + | } | ||
| + | </ | ||
| + | Fix: | ||
| + | <code cpp> | ||
| + | Type Exercise:: | ||
| + | Exercise:: | ||
| + | </ | ||
| + | ==ex.7.36== | ||
| + | > | ||
| + | <code cpp> | ||
| + | struct X { | ||
| + | X (int i, int j): base(i), rem(base % j) { } | ||
| + | int rem, base; | ||
| + | }; | ||
| + | </ | ||
| + | In thi program, the constructor attempts to initialize '' | ||
| + | <code cpp> | ||
| + | struct X { | ||
| + | X (int i, int j): base(i), rem(i % j) { } | ||
| + | int rem, base; | ||
| + | }; | ||
| + | </ | ||
| + | ==ex.7.37== | ||
| + | > | ||
| + | <code cpp> | ||
| + | Sales_data first_item(cin); | ||
| + | int main() { | ||
| + | Sales_data next; // | ||
| + | Sales_data last(" | ||
| + | } | ||
| + | </ | ||
| + | ==ex.7.38== | ||
| + | > | ||
| + | <code cpp> | ||
| + | Sales_data(std:: | ||
| + | read(is, *this); | ||
| + | } | ||
| + | </ | ||
| + | ==ex.7.39== | ||
| + | > | ||
| + | No, it is illegal. If all of them take their default argument to initialize the members, they all become the default constructor, | ||
| + | ==ex.7.40== | ||
| + | > | ||
| + | (a) Book | ||
| + | (b) Date | ||
| + | (c) Employee | ||
| + | (d) Vehicle | ||
| + | (e) Object | ||
| + | (f) Tree\\ \\ | ||
| + | class //Book// should be the most realevant: | ||
| + | <code cpp> | ||
| + | Book() = Default; | ||
| + | Book(std:: | ||
| + | book_no(bn), | ||
| + | Book(std:: | ||
| + | | ||
| + | private: | ||
| + | std:: | ||
| + | unsigned book_sold{0}; | ||
| + | double book_price{0.0}; | ||
| + | std:: | ||
| + | </ | ||
| + | ====Ex.7.41-7.50==== | ||
| + | ==ex.7.41== | ||
| + | > | ||
| + | CODE: [[https:// | ||
| + | [[https:// | ||
| + | ==ex.7.42== | ||
| + | > | ||
| + | <code cpp> | ||
| + | //base constructor | ||
| + | Book(std:: | ||
| + | book_no(bn), | ||
| + | // | ||
| + | Book(): | ||
| + | Book(std:: | ||
| + | is >> book_no >> book_sold >> book_price >> sold_start_date; | ||
| + | private: | ||
| + | std:: | ||
| + | unsigned book_sold{0}; | ||
| + | double book_price{0.0}; | ||
| + | std:: | ||
| + | </ | ||
| + | ==ex.7.43== | ||
| + | > | ||
| + | <code cpp> | ||
| + | class NoDefault { | ||
| + | public: | ||
| + | NoDefault(int i){}; | ||
| + | }; | ||
| + | |||
| + | class C{ | ||
| + | public: | ||
| + | C(): | ||
| + | private: | ||
| + | NoDefault non_d_obj; | ||
| + | }; | ||
| + | </ | ||
| + | ==ex.7.44== | ||
| + | > | ||
| + | < | ||
| + | vector< | ||
| + | </ | ||
| + | It is illegal. As an initialization with the form of '' | ||
| + | ==ex.7.45== | ||
| + | > | ||
| + | It is legal beucase the class C has a default consturctor. | ||
| + | ==ex.7.46== | ||
| + | > | ||
| + | |||
| + | **//a) A class must provide at least one constructor. //** \\ | ||
| + | Literally, yes. It doesn' | ||
| + | |||
| + | **//b) A default constructor is a constructor with an empty parameter list.//** \\ | ||
| + | |||
| + | No. Default constructors are defined by their behavior; they are used for default initialization and value initialization. The opposite is a constructor that has default arguments for all its parameters. It operates as a default constructor if it is not supplied with any arguments.\\ \\ | ||
| + | |||
| + | **//c) If there are no meaningful default values for a class, the class should not provide a default constructor.// | ||
| + | No. As a default constructor is required for both default initialization and value initialization, | ||
| + | |||
| + | **//d) If a class does not define a default constructor, | ||
| + | No. The compiler must find no custom defined constructor in the class definition before creating a synthesized default constructor. | ||
| + | ==ex.7.47== | ||
| + | > | ||
| + | In the Sales_data class, it should be OK if there is no explicit constructor. In the class, an implicit conversion will result in a book that has its ISBN but no copies sold, and an unfilled price. This makes sense.\\ \\ | ||
| + | // // | ||
| + | Pros: | ||
| + | * It restricts the workflow. In order to perform the operation, the user must supply a Sales_data type. By avoiding implicit conversion, the potential side effects can be avoided. | ||
| + | Cons: | ||
| + | * Lost flexablity. | ||
| + | * The keyword explict only works for constructors with one argument. | ||
| + | ==ex.7.48== | ||
| + | > | ||
| + | <code cpp> | ||
| + | string null_isbn(" | ||
| + | Sales_data item1(null_isbn); | ||
| + | Sales_data item2(" | ||
| + | </ | ||
| + | //**What happens if the Sales_data constructors are explicit? | ||
| + | The keyword '' | ||
| + | ==ex.7.49== | ||
| + | > | ||
| + | <code cpp> | ||
| + | (a) Sales_data & | ||
| + | //Ok. | ||
| + | //The combine is asking a Sale_data type parameter. We feed a string which is converted to a temporary Sale_data obj. | ||
| + | |||
| + | (b) Sales_data & | ||
| + | //Error. The combine is kasing a reference to non-const, we cannot bind a temporary object to it. | ||
| + | |||
| + | (c) Sales_data & | ||
| + | // | ||
| + | </ | ||
| + | >//p259: Because combine’s parameter is a reference to const, we can pass a temporary to that parameter.// | ||
| + | ==ex.7.50== | ||
| + | > | ||
| + | This constructor should be explicit. | ||
| + | <code cpp> | ||
| + | Person(std:: | ||
| + | </ | ||
| + | ====Ex.7.51-7.58==== | ||
| + | ==ex.7.51== | ||
| + | > | ||
| + | Because converting a number to a vector doesn' | ||
| + | <code cpp> | ||
| + | void print(vector< | ||
| + | print(1); //There is no relationship between a number and a vector. | ||
| + | void print(std:: | ||
| + | print(" | ||
| + | </ | ||
| + | ==ex.7.52== | ||
| + | > | ||
| + | <code cpp> | ||
| + | Sales_data item = {" | ||
| + | </ | ||
| + | The class Sale_data | ||
| + | <code cpp> | ||
| + | struct Sales_data { | ||
| + | std::string bookNo; | ||
| + | unsigned units_sold = 0; | ||
| + | double revenue = 0.0; | ||
| + | }; | ||
| + | </ | ||
| + | is not an aggregate class because an aggregate class doesn' | ||
| + | <code cpp> | ||
| + | struct Sales_data { | ||
| + | std::string bookNo; | ||
| + | unsigned units_sold; | ||
| + | double revenue; | ||
| + | }; | ||
| + | </ | ||
| + | ==ex.7.53== | ||
| + | > | ||
| + | CODE: [[https:// | ||
| + | [[https:// | ||
| + | PS: | ||
| + | There might be a problem with the code in the book: | ||
| + | <code cpp> | ||
| + | constexpr Debug(bool b = true): hw(b), io(b), other(b) {} | ||
| + | constexpr Debug(bool h, bool i, bool o): hw(h), io(i), other(o) {} | ||
| + | constexpr bool any() { return hw || io || other; } | ||
| + | </ | ||
| + | As the Debug constructor is a constexpr constructor, | ||
| + | The keyword constexpr only specifiy the return type. To get a const member function, we need to add const after the parameter list: | ||
| + | <code cpp> | ||
| + | constexpr bool any() const { return hw || io || other; } | ||
| + | </ | ||
| + | PPS: The problem is caused by the new standard, using '' | ||
| + | |||
| + | ==ex.7.54== | ||
| + | > | ||
| + | No. A constexpr function can only have one executable statement, which is '' | ||
| + | ==ex.7.55== | ||
| + | > | ||
| + | No. The //Data// class has a member with // | ||
| + | ==ex.7.56== | ||
| + | > | ||
| + | < | ||
| + | A static class member is a member of a class that does not belong to its object.\\ \\ | ||
| + | < | ||
| + | A static class member is a member of a class that does not belong to its object. Since it is an independent property of the class object, it can be shared by all existing class objects. In this way, we can take advantage of static class members by using them in something that is very common across all objects and maintain them much easier.\\ \\ | ||
| + | < | ||
| + | * a static member can be an incomplete type | ||
| + | * a static member can be a default argument | ||
| + | ==ex.7.57== | ||
| + | > | ||
| + | CODE: [[https:// | ||
| + | [[https:// | ||
| + | ==ex.7.58== | ||
| + | > | ||
| + | <code cpp> | ||
| + | // example.h | ||
| + | class Example { | ||
| + | public: | ||
| + | //The static members below should be initialized and defined outside of the class. | ||
| + | static double rate = 6.5; | ||
| + | static const int vecSize = 20; //This is legal, but it would be better to initialize it again outside of the class. | ||
| + | static vector< | ||
| + | }; | ||
| + | // example.C | ||
| + | #include " | ||
| + | double Example:: | ||
| + | vector< | ||
| + | </ | ||
| + | Fix: | ||
| + | <code cpp> | ||
| + | // example.h | ||
| + | class Example { | ||
| + | public: | ||
| + | static double rate; | ||
| + | static const int vecSize; | ||
| + | static vector< | ||
| + | }; | ||
| + | // example.C | ||
| + | #include " | ||
| + | double Example:: | ||
| + | const int Example:: | ||
| + | std:: | ||
| + | </ | ||