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Thinking in C++
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More preprocessor features

Earlier, I said that you almost always want to use inline functions instead of preprocessor macros. The exceptions are when you need to use three special features in the C preprocessor (which is also the C++ preprocessor): stringizing, string concatenation, and token pasting. Stringizing, introduced earlier in the book, is performed with the # directive and allows you to take an identifier and turn it into a character array. String concatenation takes place when two adjacent character arrays have no intervening punctuation, in which case they are combined. These two features are especially useful when writing debug code. Thus,

#define DEBUG(x) cout << #x " = " << x << endl

This prints the value of any variable. You can also get a trace that prints out the statements as they execute:

#define TRACE(s) cerr << #s << endl; s

The #s stringizes the statement for output, and the second s reiterates the statement so it is executed. Of course, this kind of thing can cause problems, especially in one-line for loops:

for(int i = 0; i < 100; i++)

Because there are actually two statements in the TRACE( ) macro, the one-line for loop executes only the first one. The solution is to replace the semicolon with a comma in the macro.

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