Читаем C++ Primer Plus полностью

The i < n test stops the loop after n characters have been copied. The second part of the test, the expression str[i], is the code for the character about to be copied. If the loop reaches the null character, the code is 0, and the loop terminates. The final while loop terminates the string with the null character and then sets the rest of the allocated space, if any, to null characters.

Another approach for setting the size of the new string is to set n to the smaller of the passed value and the string length:

int len = strlen(str);

n = (n < len) ? n : len;    // the lesser of n and len

char * p = new char[n+1];

This ensures that new doesn’t allocate more space than what’s needed to hold the string. That can be useful if you make a call such as left("Hi!", 32767). The first approach copies the "Hi!" into an array of 32767 characters, setting all but the first 3 characters to the null character. The second approach copies "Hi!" into an array of 4 characters. But by adding another function call (strlen()), it increases the program size, slows the process, and requires that you remember to include the cstring (or string.h) header file. C programmers have tended to opt for faster running, more compact code and leave a greater burden on the programmer to use functions correctly. However, the C++ tradition places greater weight on reliability. After all, a slower program that works correctly is better than a fast program that works incorrectly. If the time taken to call strlen() turns out to be a problem, you can let left() determine the lesser of n and the string length directly. For example, the following loop quits when m reaches n or the end of the string, whichever comes first:

int m = 0;

while (m <= n && str[m] != '\0')

      m++;

char * p = new char[m+1]:

// use m instead of n in rest of code

Remember, the expression str[m] != '\0' evaluates to true when str[m] is not the null character and to false when it is the null character. Because nonzero values are converted to true in an && expression and zero is converted to false, the while test also can be written this way:

while (m<=n && str[m])

Function Overloading

Function polymorphism is a neat C++ addition to C’s capabilities. Whereas default arguments let you call the same function by using varying numbers of arguments, function polymorphism, also called function overloading, lets you use multiple functions sharing the same name. The word polymorphism means having many forms, so function polymorphism lets a function have many forms. Similarly, the expression function overloading means you can attach more than one function to the same name, thus overloading the name. Both expressions boil down to the same thing, but we’ll usually use the expression function overloading—it sounds harder working. You can use function overloading to design a family of functions that do essentially the same thing but using different argument lists.

Overloaded functions are analogous to verbs having more than one meaning. For example, Miss Piggy can root at the ball park for the home team, and she can root in soil for truffles. The context (one hopes) tells you which meaning of root is intended in each case. Similarly, C++ uses the context to decide which version of an overloaded function is intended.

The key to function overloading is a function’s argument list, also called the function signature. If two functions use the same number and types of arguments in the same order, they have the same signature; the variable names don’t matter. C++ enables you to define two functions by the same name, provided that the functions have different signatures. The signature can differ in the number of arguments or in the type of arguments, or both. For example, you can define a set of print() functions with the following prototypes:

void print(const char * str, int width);  // #1

void print(double d, int width);          // #2

void print(long l, int width);            // #3

void print(int i, int width);             // #4

void print(const char *str);              // #5

When you then use a print() function, the compiler matches your use to the prototype that has the same signature:

print("Pancakes", 15);         // use #1

print("Syrup");                // use #5

print(1999.0, 10);             // use #2

print(1999, 12);               // use #4

print(1999L, 15);              // use #3

For example, print("Pancakes", 15) uses a string and an integer as arguments, and it matches Prototype #1.

When you use overloaded functions, you need to be sure you use the proper argument types in the function call. For example, consider the following statements:

unsigned int year = 3210;

print(year, 6);           // ambiguous call

Перейти на страницу:

Все книги серии Developer's Library

C++ Primer Plus
C++ Primer Plus

C++ Primer Plus is a carefully crafted, complete tutorial on one of the most significant and widely used programming languages today. An accessible and easy-to-use self-study guide, this book is appropriate for both serious students of programming as well as developers already proficient in other languages.The sixth edition of C++ Primer Plus has been updated and expanded to cover the latest developments in C++, including a detailed look at the new C++11 standard.Author and educator Stephen Prata has created an introduction to C++ that is instructive, clear, and insightful. Fundamental programming concepts are explained along with details of the C++ language. Many short, practical examples illustrate just one or two concepts at a time, encouraging readers to master new topics by immediately putting them to use.Review questions and programming exercises at the end of each chapter help readers zero in on the most critical information and digest the most difficult concepts.In C++ Primer Plus, you'll find depth, breadth, and a variety of teaching techniques and tools to enhance your learning:• A new detailed chapter on the changes and additional capabilities introduced in the C++11 standard• Complete, integrated discussion of both basic C language and additional C++ features• Clear guidance about when and why to use a feature• Hands-on learning with concise and simple examples that develop your understanding a concept or two at a time• Hundreds of practical sample programs• Review questions and programming exercises at the end of each chapter to test your understanding• Coverage of generic C++ gives you the greatest possible flexibility• Teaches the ISO standard, including discussions of templates, the Standard Template Library, the string class, exceptions, RTTI, and namespaces

Стивен Прата

Программирование, программы, базы данных

Похожие книги

1С: Бухгалтерия 8 с нуля
1С: Бухгалтерия 8 с нуля

Книга содержит полное описание приемов и методов работы с программой 1С:Бухгалтерия 8. Рассматривается автоматизация всех основных участков бухгалтерии: учет наличных и безналичных денежных средств, основных средств и НМА, прихода и расхода товарно-материальных ценностей, зарплаты, производства. Описано, как вводить исходные данные, заполнять справочники и каталоги, работать с первичными документами, проводить их по учету, формировать разнообразные отчеты, выводить данные на печать, настраивать программу и использовать ее сервисные функции. Каждый урок содержит подробное описание рассматриваемой темы с детальным разбором и иллюстрированием всех этапов.Для широкого круга пользователей.

Алексей Анатольевич Гладкий

Программирование, программы, базы данных / Программное обеспечение / Бухучет и аудит / Финансы и бизнес / Книги по IT / Словари и Энциклопедии
1С: Управление торговлей 8.2
1С: Управление торговлей 8.2

Современные торговые предприятия предлагают своим клиентам широчайший ассортимент товаров, который исчисляется тысячами и десятками тысяч наименований. Причем многие позиции могут реализовываться на разных условиях: предоплата, отсрочка платежи, скидка, наценка, объем партии, и т.д. Клиенты зачастую делятся на категории – VIP-клиент, обычный клиент, постоянный клиент, мелкооптовый клиент, и т.д. Товарные позиции могут комплектоваться и разукомплектовываться, многие товары подлежат обязательной сертификации и гигиеническим исследованиям, некондиционные позиции необходимо списывать, на складах периодически должна проводиться инвентаризация, каждая компания должна иметь свою маркетинговую политику и т.д., вообщем – современное торговое предприятие представляет живой организм, находящийся в постоянном движении.Очевидно, что вся эта кипучая деятельность требует автоматизации. Для решения этой задачи существуют специальные программные средства, и в этой книге мы познакомим вам с самым популярным продуктом, предназначенным для автоматизации деятельности торгового предприятия – «1С Управление торговлей», которое реализовано на новейшей технологической платформе версии 1С 8.2.

Алексей Анатольевич Гладкий

Финансы / Программирование, программы, базы данных