std::deque shrink_to_fit() method
- od C++11
// Non const version only
void shrink_to_fit();
Requests to reduce the unused capacity to size()
.
It is implementation-defined whether the capacity will be reduced.
If reallocation occurs, all iterators, including the past-the-end iterator, and all references to the elements are invalidated.
If no reallocation takes place, no iterators or references are invalidated.
Parameters
(none)
Type requirements
T
(the container's element type) must meet the requirements ofMoveInsertable
.
Return value
(none)
Complexity
At most linear in the size of the container - O(size()).
Exceptions
If an exception is thrown other than by T
's move constructor, this function has no effect (strong exception guarantee).
Example
#include <iostream>
#include <new>
#include <deque>
// 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);
std::cout << "allocating " << n << " bytes\n";
return static_cast<Tp*>(::operator new(n));
}
void deallocate(Tp* p, std::size_t n)
{
std::cout << "deallocating " << n*sizeof*p << " bytes\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() {
/* std::queue has no capacity() function (like std::vector),
* because of this we use a custom allocator to show
* the working of shrink_to_fit. */
std::cout << "Default-construct deque:\n";
std::deque<int, NAlloc<int>> deq;
std::cout << "\nAdd 300 elements:\n";
for (int i = 1000; i < 1300; ++i)
deq.push_back(i);
std::cout << "\nPop 100 elements:\n";
for (int i = 0; i < 100; ++i)
deq.pop_front();
std::cout << "\nRun shrink_to_fit:\n";
deq.shrink_to_fit();
std::cout << "\nDestroy deque as it goes out of scope:\n";
}
Default-construct deque:
allocating 64 bytes
allocating 512 bytes
Add 300 elements:
allocating 512 bytes
allocating 512 bytes
Pop 100 elements:
Run shrink_to_fit:
allocating 64 bytes
allocating 512 bytes
allocating 512 bytes
deallocating 512 bytes
deallocating 512 bytes
deallocating 512 bytes
deallocating 64 bytes
Destroy deque as it goes out of scope:
deallocating 512 bytes
deallocating 512 bytes
deallocating 64 bytes
Hover to see the original license.