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.

Answer 1

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When you modify the vector, the iterators __begin and __end are no longer valid and the dereferencing __begin results in undefined behaviour.

Answer 2

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.