Printing values in DEBUG mode in C++

By: Emiley J in C++ Tutorials on 2007-09-17

In addition to asserting that something is true using the assert() macro, you may want to print the current value of pointers, variables, and strings. This can be very helpful in checking your assumptions about the progress of your program, and in locating off-by-one bugs in loops. Listing below illustrates this idea.

Printing values in DEBUG mode.

1:     // Printing values in DEBUG mode
2:     #include <iostream.h>
3:     #define DEBUG
4:
5:     #ifndef DEBUG
6:     #define PRINT(x)
7:     #else
8:     #define PRINT(x) \
9:        cout << #x << ":\t" << x << endl;
10:    #endif
11:
12:    enum BOOL { FALSE, TRUE } ;
13:
14:    int main()
15:    {
16:       int x = 5;
17:       long y = 73898l;
18:       PRINT(x);
19:       for (int i = 0; i < x; i++)
20:       {
21:          PRINT(i);
22:       }
23:
24:       PRINT (y);
25:       PRINT("Hi.");
26:       int *px = &x;
27:       PRINT(px);
28:       PRINT (*px);
29:     return 0;
30: }

Output: x:      5
i:      0
i:      1
i:      2
i:      3
i:      4
y:      73898
"Hi.":  Hi.
px:       0x2100 (You may receive a value other than 0x2100)
*px:    5

Analysis: The macro on lines 5-10 provides printing of the current value of the supplied parameter. Note that the first thing fed to cout is the stringized version of the parameter; that is, if you pass in x, cout receives "x".

Next, cout receives the quoted string ":\t", which prints a colon and then a tab. Third, cout receives the value of the parameter (x), and then finally, endl, which writes a new line and flushes the buffer.