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Thinking in C++
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Runtime debugging flags

In some situations it is more convenient to turn debugging flags on and off during program execution, especially by setting them when the program starts up using the command line. Large programs are tedious to recompile just to insert debugging code.

To turn debugging code on and off dynamically, create bool flags:

//: C03:DynamicDebugFlags.cpp
#include <iostream>
#include <string>
using namespace std;
// Debug flags aren't necessarily global:
bool debug = false;

int main(int argc, char* argv[]) {
  for(int i = 0; i < argc; i++)
    if(string(argv[i]) == "--debug=on")
      debug = true;
  bool go = true;
  while(go) {
    if(debug) {
      // Debugging code here
      cout << "Debugger is now on!" << endl;
    } else {
      cout << "Debugger is now off." << endl;
    cout << "Turn debugger [on/off/quit]: ";
    string reply;
    cin >> reply;
    if(reply == "on") debug = true; // Turn it on
    if(reply == "off") debug = false; // Off
    if(reply == "quit") break; // Out of 'while'
} ///:~

This program continues to allow you to turn the debugging flag on and off until you type “quit” to tell it you want to exit. Notice it requires that full words are typed in, not just letters (you can shorten it to letter if you wish). Also, a command-line argument can optionally be used to turn debugging on at startup – this argument can appear anyplace in the command line, since the startup code in main( ) looks at all the arguments. The testing is quite simple because of the expression:


This takes the argv[i] character array and creates a string, which then can be easily compared to the right-hand side of the ==. The program above searches for the entire string --debug=on. You can also look for --debug= and then see what’s after that, to provide more options. Volume 2 (available from devotes a chapter to the Standard C++ string class.

Although a debugging flag is one of the relatively few areas where it makes a lot of sense to use a global variable, there’s nothing that says it must be that way. Notice that the variable is in lower case letters to remind the reader it isn’t a preprocessor flag.

Thinking in C++
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   Reproduced courtesy of Bruce Eckel, MindView, Inc. Design by Interspire