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, for example, from some container or list.
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[edit] Syntax
for ( range_declaration : range_expression) loop_statement
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[edit] Explanation
The above syntax produces code similar to the following (__range
, __begin
and __end
are for exposition only):
{
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The range_expression is evaluated to determine the sequence or range will be iterated over. Each element of the sequence is dereferenced, and assigned to the variable using the type and name given in the range_declaration.
The begin_expr
and end_expr
are defined to be either:
- (__range) and (__range + __bound) for array types, where
__bound
is the array bound - begin(__range) and end(__range), which are found based on argument-lookup rules. For standard containers this ends up being equivalent to std::begin and std::end which in turn calls __range.begin() and __range.end().
If range_expression returns a temporary, its lifetime is extended until the end of the loop, as indicated by binding to the rvalue reference __range
.
Just as with a traditional loop, break statement can be used to exit the loop early and continue statement can be used to restart the loop with the next element.
[edit] Keywords
[edit] Example
#include <iostream> #include <vector> int main() { std::vector<int> v = {0, 1, 2, 3, 4, 5}; for (int &i : v) // access by reference (const allowed) std::cout << i << ' '; std::cout << '\n'; for (auto i : v) // compiler uses type inference to determine the right type std::cout << i << ' '; std::cout << '\n'; for (int i : v) // access by value as well std::cout << i << ' '; std::cout << '\n'; }
Output:
0 1 2 3 4 5 0 1 2 3 4 5 0 1 2 3 4 5