std::vector::reserve(3)

NAME
std::vector::reserve - std::vector::reserve

Synopsis
   void reserve( size_type new_cap ); (until C++20)
   constexpr void reserve( size_type new_cap ); (since C++20)
   Increase the capacity of the vector (the total number of elements that the vector
   can hold without requiring reallocation) to a value that's greater or equal to
   new_cap. If new_cap is greater than the current capacity(), new storage is
   allocated, otherwise the function does nothing.
   reserve() does not change the size of the vector.
   If new_cap is greater than capacity(), all iterators, including the past-the-end
   iterator, and all references to the elements are invalidated. Otherwise, no
   iterators or references are invalidated.

Parameters
   new_cap - new capacity of the vector, in number of elements

Type requirements
   -
   T must meet the requirements of MoveInsertable.

Return value
   (none)

Exceptions
     * std::length_error if new_cap > max_size().
     * any exception thrown by Allocator::allocate() (typically std::bad_alloc)
   If an exception is thrown, this function has no effect (strong exception guarantee).
   If T's move constructor is not noexcept and T is not CopyInsertable
   into *this, vector will use the throwing move constructor. If it (since C++11)
   throws, the guarantee is waived and the effects are unspecified.

Complexity
   At most linear in the size() of the container.

Notes
   Correctly using reserve() can prevent unnecessary reallocations, but inappropriate
   uses of reserve() (for instance, calling it before every push_back() call) may
   actually increase the number of reallocations (by causing the capacity to grow
   linearly rather than exponentially) and result in increased computational complexity
   and decreased performance. For example, a function that receives an arbitrary vector
   by reference and appends elements to it should usually not call reserve() on the
   vector, since it does not know of the vector's usage characteristics.
   When inserting a range, the range version of insert() is generally preferable as it
   preserves the correct capacity growth behavior, unlike reserve() followed by a
   series of push_back()s.
   reserve() cannot be used to reduce the capacity of the container; to that end
   shrink_to_fit() is provided.

Example
// Run this code
 #include <cstddef>
 #include <iostream>
 #include <new>
 #include <vector>
 // minimal C++11 allocator with debug output
 template <class Tp>
 struct NAlloc {
     typedef Tp value_type;
     NAlloc() = default;
     template <class T> NAlloc(const NAlloc<T>&) {}
     Tp* allocate(std::size_t n)
     {
         n *= sizeof(Tp);
         Tp* p = static_cast<Tp*>(::operator new(n));
         std::cout << "allocating " << n << " bytes @ " << p << '\n';
         return p;
     }
     void deallocate(Tp* p, std::size_t n)
     {
         std::cout << "deallocating " << n*sizeof*p << " bytes @ " << p << "\n\n";
         ::operator delete(p);
     }
 };
 template <class T, class U>
 bool operator==(const NAlloc<T>&, const NAlloc<U>&) { return true; }
 template <class T, class U>
 bool operator!=(const NAlloc<T>&, const NAlloc<U>&) { return false; }
 int main()
 {
     constexpr int max_elements = 32;
     std::cout << "using reserve: \n";
     {
         std::vector<int, NAlloc<int>> v1;
         v1.reserve( max_elements ); // reserves at least max_elements * sizeof(int) bytes
         for(int n = 0; n < max_elements; ++n)
             v1.push_back(n);
     }
     std::cout << "not using reserve: \n";
     {
         std::vector<int, NAlloc<int>> v1;
         for(int n = 0; n < max_elements; ++n) {
             if(v1.size() == v1.capacity()) {
                 std::cout << "size() == capacity() == " << v1.size() << '\n';
             }
             v1.push_back(n);
         }
     }
 }

Possible output:
 using reserve:
 allocating 128 bytes @ 0xa6f840
 deallocating 128 bytes @ 0xa6f840
 not using reserve:
 size() == capacity() == 0
 allocating 4 bytes @ 0xa6f840
 size() == capacity() == 1
 allocating 8 bytes @ 0xa6f860
 deallocating 4 bytes @ 0xa6f840
 size() == capacity() == 2
 allocating 16 bytes @ 0xa6f840
 deallocating 8 bytes @ 0xa6f860
 size() == capacity() == 4
 allocating 32 bytes @ 0xa6f880
 deallocating 16 bytes @ 0xa6f840
 size() == capacity() == 8
 allocating 64 bytes @ 0xa6f8b0
 deallocating 32 bytes @ 0xa6f880
 size() == capacity() == 16
 allocating 128 bytes @ 0xa6f900
 deallocating 64 bytes @ 0xa6f8b0
 deallocating 128 bytes @ 0xa6f900

See also
                 returns the number of elements that can be held in currently allocated
   capacity storage
                 (public member function)
   max_size returns the maximum possible number of elements
                 (public member function)
   resize changes the number of elements stored
                 (public member function)
   shrink_to_fit reduces memory usage by freeing unused memory
   (C++11) (public member function)