Initializing the size of a C++ vector

Question

What are the advantages (if any) of initializing the size of a C++ vector as well as other containers? Is there any reason to not just use the default no-arg constructor?

Answer 1

There are a few ways of creating a vector with n elements and I will even show some ways of populating a vector when you don't know the number of elements in advance.

But first

what NOT to do

std::vector phone_book;
for (std::size_t i = 0; i < n; ++i)
{
    phone_book[i] = entry; // <-- !! Undefined Behaviour !!
}

The default constructed vector, as in the example above creates an empty vector. Accessing elements outside of the range of the vector is Undefined Behavior. And don't expect to get a nice exception. Undefined behavior means anything can happen: the program might crash or might seem to work or might work in a wonky way. Please note that using reserve doesn't change the actual size of the vector, i.e. you can't access elements outside of the size of the vector, even if you reserved for them.

And now some options analyzed

default ctor + push_back (suboptimal)

std::vector phone_book;
for (std::size_t i = 0; i < n; ++i)
{
    phone_book.push_back(entry);
}

This has the disadvantage that reallocations will occur as you push back elements. This means memory allocation, elements move (or copy if they are non-movable, or for pre c++11) and memory deallocation (with object destruction). This will most likely happen more than once for an n decently big. It is worth noting that it is guaranteed "amortized constant" for push_back which means that it won't do reallocations after each push_back. Each reallocation will increase the size geometrically. Further read: std::vector and std::string reallocation strategy

Use this when you don't know the size in advance and you don't even have an estimate for the size.

"count default-inserted instances of T" ctor with later assignments (not recommended)

std::vector phone_book(n);
for (auto& elem : phone_book)
{
    elem = entry;
}

This does not incur any reallocation, but all n elements will be initially default constructed, and then copied for each push. This is a big disadvantage and the effect on the performance will most likely be measurable. (this is less noticeable for basic types).

Don't use this as there are better alternatives for pretty much every scenario.

"count copies of elements" ctor (recommended)

std::vector phone_book(n, entry);

This is the best method to use. As you provide all the information needed in the constructor, it will make the most efficient allocation + assignment. This has the potential to result in branchless code, with vectorized instructions for assignments if Entry has a trivial copy constructor.

default ctor + reserve + push_back (situational recommended)

vector phone_book;
phone_book.reserve(m);

while (some_condition)
{
     phone_book.push_back(entry);
}

// optional
phone_book.shrink_to_fit();

No reallocation will occur and the objects will be constructed only once until you exceed the reserved capacity. A better choice for push_back can be emplace_back.

Use this If you have a rough approximation of the size.

There is no magical formula for the reserve value. Test with different values for your particular scenarios to get the best performance for your application. At the end you can use shrink_to_fit.

default ctor + std::fill_n and std::back_inserter (situational recommended)

#include 
#include 

std::vector phone_book;

// at a later time
// phone_book could be non-empty at this time
std::fill_n(std::back_inserter(phone_book), n, entry);

Use this if you need to fill or add elements to the vector after its creation.

default ctor + std::generate_n and std::back_inserter (for different entry objects)

Entry entry_generator();

std::vector phone_book;
std::generate_n(std::back_inserter(phone_book), n, [] { return entry_generator(); });

You can use this if every entry is different and obtained from a generator