CODEWITHSUNDEEP
c++memoryvectormemory-managementdynamic-memory-allocation
shjnlee
shjnlee

Reputation: 243

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

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.

Upvotes: 1

Views: 373

Answers (2)

SargeATM
SargeATM

Reputation: 2841

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.

Upvotes: 1

ralex
ralex

Reputation: 48

I'm not 100% sure, but as I know std::vector initial capacity is implementation-defined. In most compilers it is of size of initializer lists. Size of additional allocations ( if we have not enough capacity to push_back(6) ) is also implementation-defined ( in most compiler current capacity is just doubles ). So, if you want to know exactly how large your vector's capacity is you should use reserve() function. In the example below there are only 1 allocation of vector's capacity( of size 4 * sizeof(int) bytes ) :

std::vector<int> vec;
vec.reserve(4); // reserve memory for 4 ints
push_back(3);
push_back(4);
push_back(5);
push_back(6);
// no additional allocations

Upvotes: 0

Related Questions