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I was looking over some ugly code (that was modifying the underlying sequence while iterating), and to explore the definition of the range-based for loop, I went to cppreference.
There I noticed something strange:
The range based for loop changed in C++17, but I do not see the reason for the change, and the code looks the same to me (just "refactored").
So the old one was:
{
auto && __range = range_expression;
for (auto __begin = begin_expr, __end = end_expr; __begin != __end; ++__begin) {
range_declaration = *__begin;
loop_statement
}
}
The new one is
{
auto && __range = range_expression;
auto __begin = begin_expr;
auto __end = end_expr;
for ( ; __begin != __end; ++__begin) {
range_declaration = *__begin;
loop_statement
}
}
Why was this change made, and does it make any legal C++14 programs exhibit undefined behavior (UB) in C++17?
579
Range-based for loop in C++ Range-based for loop in C++ is added since C++ 11. It 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.
No it doesn't. The first statement in the for loop is executed only once. Once i is set, changing the value of the variable that was used to set i does not effect i . You are allowed to change the value of i explicitly within the for loop body though.
A range-based for loop terminates when one of these in statement is executed: a break , return , or goto to a labeled statement outside the range-based for loop. A continue statement in a range-based for loop terminates only the current iteration.
Range-for is as fast as possible since it caches the end iterator[citation provided], uses pre-increment and only dereferences the iterator once. Then, yes, range-for may be slightly faster, since it's also easier to write there's no reason not to use it (when appropriate).
Using
auto __begin = begin_expr, __end = end_expr;
requires both begin_expr and end_expr to return the same type. This means you cannot have a sentinel iterator type that is different from the beginning type. Using
auto __begin = begin_expr ;
auto __end = end_expr ;
fixes that issue while proving full backwards compatibility with C++14.
77
It is explained later in the "notes":
As of C++17, the types of the begin_expr and the end_expr do not have to be the same ...
and you can't have that with:
auto __begin = begin_expr, __end = end_expr;
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