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V803. Decreased performance. It is more effective to use the prefix form of ++it. Replace iterator++ with ++iterator.
The analyzer detected a construct which may be optimized. An iterator is changed in the program code by the increment/decrement postfix operator. Since the previous iterator's value is not used, you may replace the postfix operator with the prefix one. In some cases, the prefix operator will work faster than the postfix one, especially in Debug-versions.
Example:
std::vector<size_t>::const_iterator it;
for (it = a.begin(); it != a.end(); it++)
{ ... }This code is faster:
std::vector<size_t>::const_iterator it;
for (it = a.begin(); it != a.end(); ++it)
{ ... }The prefix increment operator changes the object's state and returns itself already changed. The prefix operator in the iterator's class to handle std::vector might look as follows:
_Myt& operator++()
{ // preincrement
++_Myptr;
return (*this);
}The situation with the postfix increment operator is more complicated. The object's state must change but it is the previous state which is returned. So an additional temporary object is created:
_Myt operator++(int)
{ // postincrement
_Myt _Tmp = *this;
++*this;
return (_Tmp);
}If we want only to increment the iterator's value, it appears that the prefix version is preferable. So here you are one of the tips on micro-optimization of software: write "for (it = a.begin(); it != a.end(); ++it)" instead of "for (it = a.begin(); it != a.end(); it++)". In the latter case, an unnecessary temporary object is created, which reduces performance.
To study all these questions in detail, refer to the book by Scott Meyers "Efficient use of C++. 35 new recommendations on improving your programs and projects" (Rule 6. Distinguish between prefix increment and decrement operators) [1].
You may also study the results of speed measurements in the post "Is it reasonable to use the prefix increment operator ++it instead of postfix operator it++ for iterators?" [2].
References
- Meyers, Scott. More Effective C++: 35 New Ways to Improve Your Programs and Designs. Addison-Wesley, Reading, Mass., 1996. ISBN-10: 020163371X. ISBN-13: 9780201633719.
- Andrey Karpov. Is it reasonable to use the prefix increment operator ++it instead of postfix operator it++ for iterators? http://www.viva64.com/en/b/0093/
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