Follow Techotopia on Twitter

On-line Guides
All Guides
eBook Store
iOS / Android
Linux for Beginners
Office Productivity
Linux Installation
Linux Security
Linux Utilities
Linux Virtualization
Linux Kernel
System/Network Admin
Programming
Scripting Languages
Development Tools
Web Development
GUI Toolkits/Desktop
Databases
Mail Systems
openSolaris
Eclipse Documentation
Techotopia.com
Virtuatopia.com
Answertopia.com

How To Guides
Virtualization
General System Admin
Linux Security
Linux Filesystems
Web Servers
Graphics & Desktop
PC Hardware
Windows
Problem Solutions
Privacy Policy

  




 

 

Thinking in C++
Prev Contents / Index Next

Turning variables and expressions into strings

When writing debugging code, it is tedious to write print expressions consisting of a character array containing the variable name, followed by the variable. Fortunately, Standard C includes the stringize operator ‘#’, which was used earlier in this chapter. When you put a # before an argument in a preprocessor macro, the preprocessor turns that argument into a character array. This, combined with the fact that character arrays with no intervening punctuation are concatenated into a single character array, allows you to make a very convenient macro for printing the values of variables during debugging:

#define PR(x) cout << #x " = " << x << "\n";

If you print the variable a by calling the macro PR(a), it will have the same effect as the code:

cout << "a = " << a << "\n";

This same process works with entire expressions. The following program uses a macro to create a shorthand that prints the stringized expression and then evaluates the expression and prints the result:

//: C03:StringizingExpressions.cpp
#include <iostream>
using namespace std;

#define P(A) cout << #A << ": " << (A) << endl;

int main() {
  int a = 1, b = 2, c = 3;
  P(a); P(b); P(c);
  P(a + b);
  P((c - a)/b);
} ///:~

You can see how a technique like this can quickly become indispensable, especially if you have no debugger (or must use multiple development environments). You can also insert an #ifdef to cause P(A) to be defined as “nothing” when you want to strip out debugging.

Thinking in C++
Prev Contents / Index Next

 
 
   Reproduced courtesy of Bruce Eckel, MindView, Inc. Design by Interspire