Use the compiler to use macro variables

Link: http://www.cppblog.com/jialisoft/archive/2012/10/04/192742.html

1. # Include <stdio. h>
2. IntMain (IntArgc,Char** Argv)
3. {
4. # Ifdef my_mac
5. Printf ("hello-D. \ n ");
6. # Else
7. Printf ("my_mac was not defined. \ n ");
8. # Endif
9. Return 0;
10. }

The aboveCodeMy_mac macro is used,

 

[] $ G ++-dmy_mac-O Dtest. c

Execution result:

 

1. [] $./Dtest
2. Hello-D.
3. [] $

If the-D option is not set, re-compile and execute the result:

 

1. [] $ G ++./Dtest. C-o Dtest
2. [] $./Dtest
3. My_mac was not defined.

We can use the compiler option to debug ourProgramAO.

 

What if my macro represents a constant ??

Check the modified Code:

 

1. # Include <stdio. h>
2. IntMain (IntArgc,Char** Argv)
3. {
4. # Ifdef my_mac
5. Printf ("hello-D. % d \ n", my_mac );
6. # Else
7. Printf ("my_mac was not defined. \ n ");
8. # Endif
9. Return 0;
10. }

I output the macro value.

 

Run the following command on the terminal:

 

1. [] $ G ++-Dmy_mac=5./Dtest. C-o Dtest
2. [] $./Dtest
3. Hello-D. 5
4. [] $

 

If the program has multiple macros, You can compile them like this.

 

1. [] $ G ++-Dmac1=5 -Dmac2=6Soucefile. c

 

 

 

^ _ ^, Are you very excited ?? In this way, we do not need to modify macro variables in the code ..