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C++ from the Ground Up
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Dynamic Initialization
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In C++, both local and global variables can be initialized at run time This process is sometimes referred to as dynamic initialization So far, most initializations that you have seen in this book have used constants However, under dynamic initialization, a variable can be initialized at run time using any C++ expression valid at the time the variable is declared This means that you can initialize a variable by using other variables and/or function calls, so long as the overall expression has meaning when the declaration is encountered For example, the following are all perfectly valid variable initializations in C++:
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int n = strlen(str); double arc = sin(theta); float d = 102 * count / deltax;
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Applying Dynamic Initialization to Constructors
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Like simple variables, objects can be initialized dynamically when they are created This feature allows you to create exactly the type of object you need, using information that is known only at run time To illustrate how dynamic initialization works, let s rework the timer program from the previous section Recall that in the first example of the timer program, there is little to be gained by overloading the timer( ) constructor, because all objects of its type are initialized using constants provided at compile time However, in cases where an object will be initialized at run time, there may be significant advantages to providing a variety of initialization formats This allows you, the programmer, the flexibility of using the constructor that most closely matches the format of the data available at the moment For example, in the following version of the timer program, dynamic initialization is used to construct two objects, b and c, at run time:
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// Demonstrate dynamic initialization #include <iostream> #include <cstdlib> #include <ctime> using namespace std; class timer{ int seconds; public: // seconds specified as a string timer(char *t) { seconds = atoi(t); } // seconds specified as integer timer(int t) { seconds = t; } // time specified in minutes and seconds timer(int min, int sec) { seconds = min*60 + sec; }
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A Closer Look at Classes
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void run(); } ; void timer::run() { clock_t t1; t1 = clock(); while((clock()/CLOCKS_PER_SEC - t1/CLOCKS_PER_SEC) < seconds); cout << "\a"; // ring the bell } int main() { timer a(10); arun(); cout << "Enter number of seconds: "; char str[80]; cin >> str; timer b(str); // initialize at run time brun(); cout << "Enter minutes and seconds: "; int min, sec; cin >> min >> sec; timer c(min, sec); // initialize at run time crun(); return 0; }
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As you can see, object a is constructed using an integer constant However, objects b and c are constructed using information entered by the user For b, since the user enters a string, it makes sense that timer( ) is overloaded to accept it In similar fashion, object c is also constructed at run time from user input In this case, since the time is entered as minutes and seconds, it is logical to use this format for constructing object c As the example shows, having a variety of initialization formats keeps you from having to perform conversions when initializing an object The point of overloading constructors is to help programmers handle greater complexity by allowing objects to be constructed in the most natural manner relative to their specific use Since there are three common methods of passing timing values to an object, it makes sense that timer( ) be overloaded to accept each method However, overloading timer( ) to accept days or nanoseconds is probably not a good idea Littering your code with constructors to handle seldom-used contingencies has a destabilizing influence on your program
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C++ from the Ground Up
EMEMBER: You must decide what constitutes valid constructor overloading and what is frivolous
Assigning Objects
If both objects are of the same type (that is, both are objects of the same class), then one object may be assigned to another It is not sufficient for the two classes to simply be physically similar their type names must be the same By default, when one object is assigned to another, a bitwise copy of the first object s data is copied to the second The following program demonstrates object assignment:
// Demonstrate object assignment #include <iostream> using namespace std; class myclass { int a, b; public: void setab(int i, int j) { a = i, b = j; } void showab(); }; void myclass::showab() { cout << "a is " << a << '\n'; cout << "b is " << b << '\n'; } int main() { myclass ob1, ob2; ob1setab(10, 20); ob2setab(0, 0); cout << "ob1 before assignment: \n"; ob1showab(); cout << "ob2 before assignment: \n"; ob2showab(); cout << '\n'; ob2 = ob1; // assign ob1 to ob2 cout << "ob1 after assignment: \n"; ob1showab(); cout << "ob2 after assignment: \n"; ob2showab(); return 0; }
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