Do Virtual Functions in C Significantly Impact Performance?

Virtual Functions and Performance in C

The use of abstract classes and virtual functions in class design has prompted concerns regarding potential performance implications. It's a common belief that virtual functions introduce a performance hit. However, the extent of this impact remains a matter of debate.

Performance Impact in Practice

To assess the performance influence of virtual functions, a benchmarking test was conducted on a 3GHz PowerPC CPU. A simple 4D vector class with getters and setters was created.

The test involved arrays of 1024 vectors, with each vector added to another 1000 times. The functions were defined as inline, virtual, and regular function calls.

Benchmarking Results

The results revealed significant differences in execution times:

• Inline: 8ms (0.65ns per call)
• Direct: 68ms (5.53ns per call)
• Virtual: 160ms (13ns per call)

Analysis and Conclusion

The test confirmed that virtual functions incurred a performance penalty, approximately 20 times slower than inline calls. However, this overhead is insignificant in most practical scenarios.

Considering that each loop iteration invoked 12,288 function calls, the additional cost per virtual function call amounted to only 7 nanoseconds. This suggests that unless an application requires millions of virtual function calls per second, the impact on performance should be negligible.

Recommendation

While virtual functions may introduce a slight performance overhead, it's crucial to prioritize premature optimization. In most real-world applications, the benefits of using abstract classes and virtual functions outweigh the potential performance impact. Therefore, it is generally recommended to use virtual functions as needed, without excessive concern about performance implications.

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