I currently am migrating a large code base from
to
and now, my tests fail in strange places - I get index out of bounds crashes and other strange changes in behavior. Upon digging I noticed that the following code:
#include <iostream>
#include <vector>
int main()
{
std::vector<std::vector<int>> nestedVector;
nestedVector.insert(nestedVector.begin(), {});
std::cout << nestedVector.size() << " elements";
}
produces a one-element vector in VS2013 but an empty vector in VS2019.
Other ways of inserting
nestedVector.insert(nestedVector.begin(), std::vector<int>{});
nestedVector.insert(nestedVector.begin(), std::vector<int>());
work in both setups and properly add a new element. What is happening here?
As was already mentioned in the comments, in your original program the call of nestedVector.insert(nestedVector.begin(), {}) selects the overloaded method:
iterator insert( const_iterator pos, std::initializer_list<T> ilist );
see https://en.cppreference.com/w/cpp/container/vector/insert.
Since the initializer list is empty, nothing is really inserted and the vector size is zero at the end. It is the same behavior in all compilers, demo: https://godbolt.org/z/9nnET5ren
Apparently, VS2013 did not implement this overload of insert method, so another one called:
iterator insert( const_iterator pos, const T& value );
creating one element in the vector. This is wrong behavior.