Question

How can I change the value of a boolean macro when I run my program through the command line? For instance, suppose I have the following macro in my cpp file, call it MyCpp.cpp

#define DEBUG 1

How can I change this when I run my program? through the command line:

g++ -Wall -Wextra -o MyCpp MyCpp.cpp

I am pretty sure you specify some kind of command line option, does this ring any bells?
Also, I do NOT want to use argv[]

Answer 1

First, change your source code:

#ifndef DEBUG
#  define DEBUG 1
#endif

Now you can say on the command line:

g++ -Wall -Wextra -o MyCpp MyCpp.cpp -DDEBUG=5
#                                    ^^^^^^^^^

The command line argument -DFOO=bar has the same effect as putting #define FOO bar in your source code; you need the #ifndef guard to avoid an illegal redefinition of the macro.

---

Sometimes people use an auxiliary macro to prevent the definition of another macro:

#ifndef SUPPRESS_FOO
#  define FOO
#endif

// ... later

#ifdef FOO
// ...
#endif

Now say -DSUPPRESS_FOO to not define FOO in the code…