Member functions 21/11/2018. Accessing member variables. Accessing member variables

Transcription

1 ECE 150 Fundamentals of Programming Outline 2 In this lesson, we will: Describe member functions Discuss their usage Explain why this is necessary and useful Prof. Hiren Patel, Ph.D. Douglas Wilhelm Harder, M.Math. LEL hdpatel@uwaterloo.ca dwharder@uwaterloo.ca 2018 by Douglas Wilhelm Harder and Hiren Patel. Some rights reserved. 3 4 Suppose we go back to our string class: class String; // Constructors ; std::size_t array_capacity; std::size_t string_length; char *character_array; We can create them and destroy them, but that s about it If you tried to access a member variable, you d get a compile-time error: String str{10; std::cout << "The capacity is " << str.array_capacity << std::endl; example.cpp: In function 'int main()': example.cpp:15:21: error: 'std::size_t String::array_capacity' is private std::size_t array_capacity; ^ example.cpp:39:44: error: within this context std::cout << "The capacity is " << str.array_capacity ^ 1

2 5 6 You may not want the user to directly access the member variables because: They may incorrectly place the '\0' They may not update string_length They may go beyond the end of the array They may reassign member variables We can add publicly visible member functions: // Constructors and destructor // char get_char( std::size_t n ) const; void set_char( std::size_t n, char ch ); ; std::size_t array_capacity; std::size_t string_length; char *character_array; 7 8 We must specify their functionality: char get_char( std::size_t n ) const; This function will not change any member variables (const) This member function will either Return the n th character if n < array_length, Otherwise it will return the null character '\0' Getting a character is straight-forward: char String::get_char( std::size_t n ) const { return ( n < string_length )? character_array[n] : '\0'; char set_char( std::size_t n, char ch ); This member function will either Set the n th character to ch if n < array_length, Update the n th character to ch if n >= array_length && n < (array_capacity - 1) but also update array_length and the position of '\0', filling in all intermediate spaces with the new character, as well 2

3 9 Calling member functions 10 Setting a character is more difficult: void String::set_char( std::size_t n, char ch ) { if ( n < string_length ) { character_array[n] = ch; else if ( n < array_capacity ) { // Set all entries after the last to 'ch' for ( std::size_t k{string_length; k <= n; ++k ) { character_array[k] = ch; string_length = n + 1; character_array[string_length] = '\0'; else { throw n; When we worked with structures, we implemented functions as follows: string_t str_1, str_2; // Allocate memory for strings of max length 16 and 32 string_init( str_1, 16 ); string_init( str_2, 32 ); The instance was always passed as the first argument by reference void string_init( string_t &str, std::size_t max_len ) { str.capacity = max_len + 1; str.character_array = new char[str.capacity]; str.character_array[0] = '\0'; str.length = 0; Calling member functions 11 Calling member functions 12 With classes, we call member functions on an instance the exact same way we access member variables: String str{11; str.set_char( 0, 'H' ); str.set_char( 1, 'i' ); str.set_char( 10, '!' ); for ( std::size_t k{0; k <= 10; ++k ) { std::cout << str.get_char( k ); std::cout << std::endl; H i!!!!!!!!! \0 Output: Hi!!!!!!!!! If we are dealing with pointers, we use the arrow operator: String p_str{new String{11; p_str->set_char( 0, 'H' ); p_str->set_char( 1, 'i' ); p_str->set_char( 10, '!' ); for ( std::size_t k{0; k <= 10; ++k ) { std::cout << p_str->get_char( k ); std::cout << std::endl; delete p_str; p_str = nullptr; Output: Hi!!!!!!!!! 3

4 13 14 There may be other questions we may ask about our string: // Constructor // Destructor // Member function declarations char get_char( std::size_t n ) const; std::size_t length() const; std::size_t max_length() const; There may be other questions we may ask about our string: std::size_t String::length() const { return string_length; std::size_t String::max_length() const { return array_capacity - 1; void set_char( std::size_t n, char ch ); ; // Private member variables listed here Suppose we want to return the character array: We cannot just return the value of the member variable char *String::get_character_array() const { return character_array; We could declare the return type to be constant: This prevents the user from changing the result char const *String::get_character_array() const { return character_array; Why? 4

5 // Class declaration class String; // Function declaration int main(); // Class definition ; // Constructor // Destructor // Member function declarations char get_char( std:: size_t n ) const; std::size_t length() const; std::size_t max_length() const; char const *get_character_array() const; char *copy_character_array() const; void set_char( std :: size_t n, char ch ); std:: size_t array_capacity; std:: size_t string_length; char *character_array; // Constructor definition String:: String( std::size_t max_length ): array_capacity{max_length + 1, string_length{0, character_array{new char[array_capacity] { character_array[0] = '\0'; std::cout << "Finished calling 'String( " << max_length << " )" << std::endl; // Destructor definition String:: ~String() { std::cout << "Calling ~String() with " << array_capacity << std :: endl; delete[] character_array; character_array = nullptr; char String:: get_char( std::size_t n ) const { return ( n < string_length )? character_array[n] : '\0'; void String:: set_char( std::size_t n, char ch ) { if ( n < string_length ) { character_array[n] = ch; else if ( n < array_capacity ) { // Set all entries after the last to 'ch ' for ( std::size_t k{string_length; k <= n; ++k ) { character_array[k] = ch; string_length = n + 1; character_array[string_length] = '\0'; else { throw n; std :: size_t String:: length() const { return string_length; std :: size_t String:: max_length() const { return array_capacity - 1; char const *String:: get_character_array() const { return character_array; char *String::copy_character_array() const { char *return_array{new char[string_length + 1]; for ( std :: size_t k{ 0; k <= string_length; ++k ) { return_array[k] = character_array[k]; return return_array; String str{256; String *p_str{new String{; str.set_char( 0, 'H' ); str.set_char( 1, 'i' ); std::cout << "The array length is now " << str.length() << std::endl; std::cout << str.get_character_array() << std::endl; p_str->set_char( 0, 'B' ); p_str->set_char( 1, 'y' ); p_str->set_char( 2, 'e' ); std::cout << "The array length is now " << p_str->length() << std::endl; std::cout << p_str->get_character_array() << std::endl; delete p_str; p_str = nullptr; 21/11/ Alternatively, we could create a new array This requires us to copy everything over char *String::copy_character_array() const { char *copied_array{new char[string_length + 1]; for ( std::size_t k{0; k <= string_length; ++k ) { copied_array[k] = character_array[k]; Consider this program: String str{256; String *p_str{new String{; str.set_char( 0, 'H' ); str.set_char( 1, 'i' ); Example Output: Finished calling 'String( 256 ) Finished calling 'String( 32 ) The array length is now 2 Hi The array length is now 3 Bye Calling ~String() with 33 Calling ~String() with 257 std::cout << "The array length is now " << str.length() << std::endl; std::cout << str.get_character_array() << std::endl; return copied_array; p_str->set_char( 0, 'B' ); p_str->set_char( 1, 'y' ); p_str->set_char( 2, 'e' ); std::cout << "The array length is now " << p_str->length() << std::endl; std::cout << p_str->get_character_array() << std::endl; delete p_str; p_str = nullptr; A String class Summary Try this yourself #include <iostream> Following this lesson, you now Understand how member functions are defined Understand they are called on the There is a lot of time spent swapping, which can be reduced significantly The run time is still approximately the same as selection sort 5

6 References 21 Colophon 22 [1] No references? These slides were prepared using the Georgia typeface. Mathematical equations use Times New Roman, and source code is presented using Consolas. The photographs of lilacs in bloom appearing on the title slide and accenting the top of each other slide were taken at the Royal Botanical Gardens on May 27, 2018 by Douglas Wilhelm Harder. Please see for more information. Disclaimer 23 These slides are provided for the ECE 150 Fundamentals of Programming course taught at the University of Waterloo. The material in it reflects the authors best judgment in light of the information available to them at the time of preparation. Any reliance on these course slides by any party for any other purpose are the responsibility of such parties. The authors accept no responsibility for damages, if any, suffered by any party as a result of decisions made or actions based on these course slides for any other purpose than that for which it was intended. 6