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Tip of the Day
Home » Tip Bank » C++
Language: C++
Expertise: All
Jul 16, 1999



Building the Right Environment to Support AI, Machine Learning and Deep Learning

One More Reason to Avoid Macros

Even macros that look harmless can have detrimental effects. For example:

#define twice(x) ((x)+(x))

The macro, twice, is well parenthesized and performs a simple addition operation. Despite its simplicity, there can still be problems. When twice takes an expression with side effects as an argument, it yields unexpected results:

int n = 1;
int sum;
sum = twice(++n);  //guess what?

Since ++n equals 2, you might assume (rather naively) that sum would be 4, but it isn't. The expression twice(++n) is expanded as ((++n)+(++n)). However, if you had used an ordinary function instead of a macro, like this

inline int  twice(int x) { return x+x; }

The result will be 4, as expected.

Danny Kalev
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