How much memory allocated with vector initialized with initializer lists and push_back

Question

If I have these 2 lines of code in C++

vector<int> vec = {3,4,5};
vec.push_back(6);

How much memory is allocated in total for the 2 lines and what assumption we need to make? I tried to look these up but can't find the definition for these anywhere.

Answer 1

Looking at llvm's libcxx library as an example, we could speculate that the capacity would be 6 ints in size.

vector<int> vec = {3,4,5}; allocates 3 ints on initialization __vallocate(3);

template <class _Tp, class _Allocator>
inline _LIBCPP_INLINE_VISIBILITY
vector<_Tp, _Allocator>::vector(initializer_list<value_type> __il)
{
#if _LIBCPP_DEBUG_LEVEL >= 2
    __get_db()->__insert_c(this);
#endif
    if (__il.size() > 0)
    {
        __vallocate(__il.size());
        __construct_at_end(__il.begin(), __il.end(), __il.size());
    }
}

vec.push_back(6); triggers re-allocation with a doubling in capacity 2*__cap

//  Precondition:  __new_size > capacity()
template <class _Tp, class _Allocator>
inline _LIBCPP_INLINE_VISIBILITY
typename vector<_Tp, _Allocator>::size_type
vector<_Tp, _Allocator>::__recommend(size_type __new_size) const
{
    const size_type __ms = max_size();
    if (__new_size > __ms)
        this->__throw_length_error();
    const size_type __cap = capacity();
    if (__cap >= __ms / 2)
        return __ms;
    return _VSTD::max<size_type>(2*__cap, __new_size);
}

https://github.com/llvm-mirror/libcxx/blob/78d6a7767ed57b50122a161b91f59f19c9bd0d19/include/vector

Update (based on ChrisD's comment)

Since the capacity multiplier could be in the range of 1.5 to 2, the size could be 4, 5, or 6 ints.