MFC Programmer's SourceBook : Thinking in C++
Bruce Eckel's Thinking in C++, 2nd Ed Contents | Prev | Next

Introduction to operators

You can think of operators as a special type of function (you’ll learn that C++ operator overloading treats operators precisely that way). An operator takes one or more arguments and produces a new value. The arguments are in a different form than ordinary function calls, but the effect is the same.

From your previous programming experience, you should be reasonably comfortable with the operators that have been used so far. The concepts of addition ( +), subtraction and unary minus ( -), multiplication ( *), division ( /) and assignment( =) all have essentialy the same meaning in any programming language. The full set of operators are enumerated later in this chapter.

Precedence

Operator precedence defines the order in which an expression evaluates when several different operators are present. C and C++ have specific rules to determine the order of evaluation. The easiest to remember is that multiplication and division happen before addition and subtraction. After that, if an expression isn’t transparent to you it probably won’t be for anyone reading the code, so you should use parentheses to make the order of evaluation explicit. For example:

A = X + Y - 2/2 + Z;

has a very different meaning from the same statement with a particular grouping of parentheses:

A = X + (Y - 2)/(2 + Z);

(Try evaluating the result with X = 1, Y = 2 and Z = 3.)

Auto increment and decrement

C, and therefore C++, is full of shortcuts. Shortcuts can make code much easier to type, and sometimes much harder to read. Perhaps the C language designers thought it would be easier to understand a tricky piece of code if your eyes didn’t have to scan as large an area of print.

One of the nicer shortcuts is the auto-increment and auto-decrement operators. You often use these to change loop variables, which control the number of times a loop executes.

The auto-decrement operator is -- and means “decrease by one unit.” The auto-increment operator is ++ and means “increase by one unit.” If A is an int, for example, the expression ++A is equivalent to ( A = A + 1 ). Auto-increment and auto-decrement operators produce the value of the variable as a result. If the operator appears before the variable, (i.e., ++A), the operation is first performed and the resulting value is produced. If the operator appears after the variable (i.e. A++), the current value is produced, and then the operation is performed. For example: 

//: C03:AutoIncrement.cpp
// Shows use of auto-increment
// and auto-decrement operators.
#include <iostream>
using namespace std;

int main() {
  int i = 0;
  int j = 0;
  cout << ++i << endl; // Pre-increment
  cout << j++ << endl; // Post-increment
  cout << --i << endl; // Pre-decrement
  cout << j-- << endl; // Post decrement
} ///:~

If you’ve been wondering about the name “C++,” now you understand. It implies “one step beyond C.”

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