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Given this C++11 example code:
for ( const auto &foo : bar() ) { // ... do something with foo... } Is it guaranteed by the standard that the expression bar() in this example is evaluated only once?
Or could it end up being called at every iteration of the loop?
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Range-based for loop (since C++11) Executes a for loop over a range. Used as a more readable equivalent to the traditional for loop operating over a range of values, such as all elements in a container.
Range-for is as fast as possible since it caches the end iterator[citationprovided], uses pre-increment and only dereferences the iterator once.
To loop through a set of code a specified number of times, we can use the range() function, The range() function returns a sequence of numbers, starting from 0 by default, and increments by 1 (by default), and ends at a specified number.
Foreach performance is approximately 6 times slower than FOR / FOREACH performance. The FOR loop without length caching works 3 times slower on lists, comparing to arrays. The FOR loop with length caching works 2 times slower on lists, comparing to arrays.
It is evaluated only once. The standard says that the range-based for loop is equivalent to this:
§6.5.4 The range-based for statement [stmt.ranged]
{ auto && __range = range-init; for ( auto __begin = begin-expr, __end = end-expr; __begin != __end; ++__begin ) { for-range-declaration = *__begin; statement } } with range-init being equivalent to ( bar() ) in your case (the expression you specify, surrounded by parenthesis). That expression is only evaluated once as you can see.
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