NAME¶

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

Synopsis¶

void reserve( size_type new_cap ); (constexpr 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 end() iterator,
and all references to the elements are invalidated. Otherwise, no iterators or
references are invalidated.

After a call to reserve(), insertions will not trigger reallocation unless the
insertion would make the size of the vector greater than the value of capacity().

Parameters¶

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

Type requirements¶

-
T must meet the requirements of MoveInsertable into *this. (since C++11)

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

Defect reports

The following behavior-changing defect reports were applied retroactively to
previously published C++ standards.

DR Applied to Behavior as published Correct behavior
reallocation might be triggered if an
insertion only triggers if the size
LWG 329 C++98 makes the size of the vector greater of the vector becomes
than the size greater than capacity()
specified in the most recent call to
reserve()
LWG 2033 C++11 T was not required to be required
MoveInsertable

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
(DR*) (public member function)