#include <cstddef>#include <deque>#include <iostream>#include <new> // 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";returnstatic_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>&){returntrue;}template<class T,class U>bool operator!=(const NAlloc<T>&,const NAlloc<U>&){returnfalse;} 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 bytesallocating 512 bytes Add 300 elements:allocating 512 bytesallocating 512 bytes Pop 100 elements: Run shrink_to_fit:allocating 64 bytesallocating 512 bytesallocating 512 bytesdeallocating 512 bytesdeallocating 512 bytesdeallocating 512 bytesdeallocating 64 bytes Destroy deque as it goes out of scope:deallocating 512 bytesdeallocating 512 bytesdeallocating 64 bytes