C++ Primer Plus

The difference between using bracket notation and the at() member function is that if you use at(), an invalid index is caught during runtime and the program, by default, aborts. This added checking does come at the cost of increased run time, which is why C++ gives you the option of using either notation. More than that, these classes offer ways of using objects that reduce the chances of accidental range errors. For example, the classes have begin() and end() member functions that let you delimit the range without accidentally exceeding the bounds. But we’ll save that discussion until Chapter 16.

Summary

Arrays, structures, and pointers are three C++ compound types. An array can hold several values, all of the same type, in a single data object. By using an index, or subscript, you can access the individual elements in an array.

A structure can hold several values of different types in a single data object, and you can use the membership operator (.) to access individual members. The first step in using structures is to create a structure template that defines what members the structure holds. The name, or tag, for this template then becomes a new type identifier. You can then declare structure variables of that type.

A union can hold a single value, but it can be of a variety of types, with the member name indicating which mode is being used.

Pointers are variables that are designed to hold addresses. We say a pointer points to the address it holds. The pointer declaration always states to what type of object a pointer points. Applying the dereferencing operator (*) to a pointer yields the value at the location to which the pointer points.

A string is a series of characters terminated by a null character. A string can be represented by a quoted string constant, in which case the null character is implicitly understood. You can store a string in an array of char, and you can represent a string with a pointer-to-char that is initialized to point to the string. The strlen() function returns the length of a string, not counting the null character. The strcpy() function copies a string from one location to another. When using these functions, you include the cstring or the string.h header file.

The C++ string class, supported by the string header file, offers an alternative, more user-friendly means to deal with strings. In particular, string objects are automatically resized to accommodate stored strings, and you can use the assignment operator to copy a string.

The new operator lets you request memory for a data object while a program is running. The operator returns the address of the memory it obtains, and you can assign that address to a pointer. The only means to access that memory is to use the pointer. If the data object is a simple variable, you can use the dereferencing operator (*) to indicate a value. If the data object is an array, you can use the pointer as if it were an array name to access the elements. If the data object is a structure, you can use the pointer dereferencing operator (->) to access structure members.

Pointers and arrays are closely connected. If ar is an array name, then the expression ar[i] is interpreted as *(ar + i), with the array name interpreted as the address of the first element of the array. Thus, the array name plays the same role as a pointer. In turn, you can use a pointer name with array notation to access elements in an array allocated by new.

The new and delete operators let you explicitly control when data objects are allocated and when they are returned to the memory pool. Automatic variables, which are those declared within a function, and static variables, which are defined outside a function or with the keyword static, are less flexible. An automatic variable comes into being when the block containing it (typically a function definition) is entered, and it expires when the block is left. A static variable persists for the duration of a program.

The Standard Template Library (STL), added by the C++98 standard, provides a vector template class that provides an alternative to do-it-yourself dynamic arrays. C++11 provides an array template class that offers an alternative to fixed-sized built-in arrays.

Chapter Review

1. How would you declare each of the following?

a. actors is an array of 30 char.

b. betsie is an array of 100 short.

c. chuck is an array of 13 float.

d. dipsea is an array of 64 long double.

2. Does Chapter Review Question 1 use the array template class instead of built-in arrays.

3. Declare an array of five ints and initialize it to the first five odd positive integers.

4. Write a statement that assigns the sum of the first and last elements of the array in Question 3 to the variable even.

5. Write a statement that displays the value of the second element in the float array ideas.

6. Declare an array of char and initialize it to the string "cheeseburger".

7. Declare a string object and initialize it to the string "Waldorf Salad".