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void shrink_to_fit(); | (constexpr since C++20) | |
Requests the removal of unused capacity.
It is a non-binding request to reducecapacity() tosize(). It depends on the implementation whether the request is fulfilled.
If reallocation occurs, all iterators (including theend() iterator) and all references to the elements are invalidated. If no reallocation occurs, no iterators or references are invalidated.
If | (since C++11) |
Contents |
At most linear in the size of the container.
ExceptionsIf an exception is thrown other than by the move constructor of a non-CopyInsertable | (since C++11) |
In libstdc++,shrink_to_fit() isnot available in C++98 mode.
#include <iostream>#include <vector> int main(){std::vector<int> v;std::cout<<"Default-constructed capacity is "<< v.capacity()<<'\n'; v.resize(100);std::cout<<"Capacity of a 100-element vector is "<< v.capacity()<<'\n'; v.resize(50);std::cout<<"Capacity after resize(50) is "<< v.capacity()<<'\n'; v.shrink_to_fit();std::cout<<"Capacity after shrink_to_fit() is "<< v.capacity()<<'\n'; v.clear();std::cout<<"Capacity after clear() is "<< v.capacity()<<'\n'; v.shrink_to_fit();std::cout<<"Capacity after shrink_to_fit() is "<< v.capacity()<<'\n';for(int i=1000; i<1300;++i) v.push_back(i);std::cout<<"Capacity after adding 300 elements is "<< v.capacity()<<'\n'; v.shrink_to_fit();std::cout<<"Capacity after shrink_to_fit() is "<< v.capacity()<<'\n';}
Possible output:
Default-constructed capacity is 0Capacity of a 100-element vector is 100Capacity after resize(50) is 100Capacity after shrink_to_fit() is 50Capacity after clear() is 50Capacity after shrink_to_fit() is 0Capacity after adding 300 elements is 512Capacity after shrink_to_fit() is 300
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
| DR | Applied to | Behavior as published | Correct behavior |
|---|---|---|---|
| LWG 755 | C++98 | std::vector lacked explicit shrink-to-fit operations | provided |
| LWG 2033 | C++98 C++11 | 1. the complexity requirement was missing (C++98) 2. T was not required to beMoveInsertable (C++11) | 1. added 2. required |
| LWG 2223 | C++98 C++11 | 1. references, pointers, and iterators were not invalidated (C++98) 2. there was no exception safety guarantee (C++11) | 1. they may be invalidated 2. added |
| returns the number of elements (public member function)[edit] | |
| returns the number of elements that can be held in currently allocated storage (public member function)[edit] |
