Add elements to a vector during range-based loop c++11

Question

I have used the new range-based for loop provided by C++11 standard and I came up with the following question: suppose that we iterate over a vector<> using the range-based for, and we add some element in the end of the vector during this iteration. Thus, when do the loop end?

For instance, see this code:

#include <iostream> #include <vector> using namespace std; int main() { vector<unsigned> test({1,2,3}); for(auto &num : test) { cout << num << " "; if(num % 2) test.push_back(num + 10); } cout << "\n"; for(auto &num : test) cout << num << " "; return 0; }

I tested G++ 4.8 and Apple LLVM version 4.2 (clang++) with "-std=c++11" flag, and the output is (for both):

1 2 3 1 2 3 11 13

Note that the first loop terminates in the end of original vector, although we add other elements to it. It seems that the for-range loop evaluate the container end in beginning only. Is this, in fact, the correct behavior of range-for? Is it specified by the committee? Can we trust in this behavior?

Note that if we change the first loop by

for(vector<unsigned>::iterator it = test.begin(); it != test.end(); ++it)

with invalid the iterators and come up with a segmentation fault.

though adding isn't erasing, this is a duplicate of Erasing an element from a container while inside a range-based for loop because the answer there shows you what the standard says the range-for does - end is determined and saved before the loop starts, and that saved value is used on subsequent iterations. — Kate Gregory
– Kate Gregory, Commented Jun 12, 2013 at 22:53
@KateGregory I would argue this is not a duplicate because removing the current element would be undefined behavior for all containers, but adding an element is only undefined behavior for std::vector, std::deque, std::unordered_set and std::unorderd_map. — David Brown
– David Brown, Commented Jun 12, 2013 at 23:24
I had seem that question, by the behavior is slightly different. Anyway, thanks. — an_drade
– an_drade, Commented Jun 12, 2013 at 23:30
@KateGregory This is not a duplicate. The answer is different - see mine below. — Paul Jurczak
– Paul Jurczak, Commented Feb 17, 2016 at 21:13

David Brown · Accepted Answer · 2013-06-12 23:02:23Z

21

No you cannot rely on this behaviour. Modifying the vector inside the loop results in undefined behaviour because the iterators used by the loop are invalidated when the vector is modified.

The range based for loop

for ( range_declaration : range_expression) loop_statement

is essentially equivalent to

{ auto && __range = range_expression ; for (auto __begin = std::begin(__range), __end = std::end(__range); __begin != __end; ++__begin) { range_declaration = *__begin; loop_statement } }

When you modify the vector, the iterators __begin and __end are no longer valid and the dereferencing __begin results in undefined behaviour.

answered Jun 12, 2013 at 23:02

David Brown

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4 Comments

an_drade Over a year ago

Thanks David. It is interesting note that my two pieces of code, although essentially equivalent, come up with very different results.

Massa Over a year ago

actually, you are missing the disctinction between for(auto _begin = r.begin(); _begin != r.end(); ++begin) and for(auto _begin = r.begin(), _end = r.end(); _begin != _end; ++_begin) (the cacheing of r.end()) And that is why your program does the impression of working! the iterator is pointing to an old version of the vector. Put a destructor in the contained objects and hilarity will ensue... :-D

Paul Jurczak Over a year ago

iterators used by the loop are invalidated when the vector is modified Not necessarily. If vector's capacity is large enough not to require reallocation, then all iterators remain valid.

Ivan Over a year ago

it would be nice to have a compilation error if you try to modify the vector in the loop

Community · Accepted Answer · 2020-06-20 09:12:55Z

You can rely on this behavior as long as vector's capacity is large enough, so reallocation is not required. This would guarantee validity of all iterators and references other than past-the-end iterator after call to push_back(). This is fine since end() was calculated only once at the beginning of the loop (see std::vector::push_back).

In your case, you have to increase capacity of test vector to guarantee this behavior:

vector<unsigned> test({1,2,3}); test.reserve(5);

EDIT

Based on responses to Is it legal to add elements to a preallocated vector in a range-based for loop over that vector?, this is a case of undefined behavior. Release version built with gcc, clang and cl runs fine, but a debug version built with cl (Visual Studio) will throw an exception.

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