7.5. Local Objects

In C++, names have scope, and objects have lifetimes. To understand how functions operate, it is important to understand both of these concepts. The scope of a name is the part of the program's text in which that name is known. The lifetime of an object is the time during the program's execution that the object exists.

The names of parameters and variables defined within a function are in the scope of the function: The names are visible only within the function body. As usual, a variable's name can be used from the point at which it is declared or defined until the end of the enclosing scope.

Exercises Section 7.4.1 Exercise 7.24: Which, if any, of the following declarations are errors? Why? (a) int ff(int a, int b = 0, int c = 0); (b) char *init(int ht = 24, int wd, char bckgrnd); Exercise 7.25: Given the following function declarations and calls, which, if any, of the calls are illegal? Why? Which, if any, are legal but unlikely to match the programmer's intent? Why? // declarations char *init(int ht, int wd = 80, char bckgrnd = ' '); (a) init(); (b) init(24,10); (c) init(14, '*'); Exercise 7.26: Write a version of make_plural with a default argument of 's'. Use that version to print singular and plural versions of the words "success" and "failure".

7.5.1. Automatic Objects

By default, the lifetime of a local variable is limited to the duration of a single execution of the function. Objects that exist only while a function is executing are known as automatic objects. Automatic objects are created and destroyed on each call to a function.

The automatic object corresponding to a local variable is created when the function control path passes through the variable's definition. If the definition contains an initializer, then the object is given an initial value each time the object is created. Uninitialized local variables of built-in type have undefined values. When the function terminates, the automatic objects are destroyed.

Parameters are automatic objects. The storage in which the parameters reside is created when the function is called and is freed when the function terminates.

Automatic objects, including parameters, are destroyed at the end of the block in which they were defined. Parameters are defined in the function's block and so are destroyed when the function terminates. When a function exits, its local storage is deallocated. After the function exits, the values of its automatic objects and parameters are no longer accessible.

7.5.2. Static Local Objects

It is can be useful to have a variable that is in the scope of a function but whose lifetime persists across calls to the function. Such objects are defined as static.

A static local object is guaranteed to be initialized no later than the first time that program execution passes through the object's definition. Once it is created, it is not destroyed until the program terminates; local statics are not destroyed when the function ends. Local statics continue to exist and hold their value across calls to the function. As a trivial example, consider a function that counts how often it is called:

     size_t count_calls()
     {
          static size_t ctr = 0; // value will persist across calls
          return ++ctr;
     }
     int main()
     {
         for (size_t i = 0; i != 10; ++i)
             cout << count_calls() << endl;
         return 0;
     }

This program will print the numbers from 1 through 10 inclusive.

Before count_calls is called for the first time, ctr is created and given an initial value of 0. Each call increments ctr and returns its current value. Whenever count_calls is executed, the variable ctr already exists and has whatever value was in the variable the last time the function exited. Thus, on the second invocation, the value is 1, on the third it is 2, and soon.

Exercises Section 7.5.2 Exercise 7.27: Explain the differences between a parameter, a local variable and a static local variable. Give an example of a program in which each might be useful. Exercise 7.28: Write a function that returns 0 when it is first called and then generates numbers in sequence each time it is called again.