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| Deduction guides(C++17) | ||||||
template<class...Args> iterator emplace_hint( const_iterator hint, Args&&...args); | (since C++11) (constexpr since C++26) | |
Inserts a new element into the container, usinghint as a suggestion where the element should go.
The constructor ofvalue_type (i.e.,std::pair<const Key, T>) is called with exactly the same arguments as supplied to the function, forwarded withstd::forward<Args>(args)....
If after the operation the new number of elements is greater than oldmax_load_factor() * bucket_count() a rehashing takes place.
If rehashing occurs (due to the insertion), all iterators are invalidated. Otherwise (no rehashing), iterators are not invalidated.
Contents |
| hint | - | iterator, used as a suggestion as to where to insert the new element |
| args | - | arguments to forward to the constructor of the element |
An iterator to the inserted element, or to the element that prevented the insertion.
If an exception is thrown for any reason, this function has no effect (strong exception safety guarantee).
Amortized constant on average, worst case linear in the size of the container.
#include <chrono>#include <cstddef>#include <functional>#include <iomanip>#include <iostream>#include <unordered_map> constint n_operations=100'500'0; std::size_t map_emplace(){std::unordered_map<int,char> map;for(int i=0; i< n_operations;++i) map.emplace(i,'a');return map.size();} std::size_t map_emplace_hint(){std::unordered_map<int,char> map;auto it= map.begin();for(int i=0; i< n_operations;++i){ map.emplace_hint(it, i,'b'); it= map.end();}return map.size();} std::size_t map_emplace_hint_wrong(){std::unordered_map<int,char> map;auto it= map.begin();for(int i= n_operations; i>0;--i){ map.emplace_hint(it, i,'c'); it= map.end();}return map.size();} std::size_t map_emplace_hint_corrected(){std::unordered_map<int,char> map;auto it= map.begin();for(int i= n_operations; i>0;--i){ map.emplace_hint(it, i,'d'); it= map.begin();}return map.size();} std::size_t map_emplace_hint_closest(){std::unordered_map<int,char> map;auto it= map.begin();for(int i=0; i< n_operations;++i) it= map.emplace_hint(it, i,'e');return map.size();} double time_it(std::function<std::size_t()> map_test,std::string what="",double ratio=0.0){constauto start=std::chrono::system_clock::now();conststd::size_t map_size= map_test();constauto stop=std::chrono::system_clock::now();std::chrono::duration<double,std::milli> time= stop- start;if(what.size()&& map_size)std::cout<<std::setw(8)<< time<<" for "<< what<<" (ratio: "<<(ratio==0.0?1.0: ratio/ time.count())<<")\n";return time.count();} int main(){std::cout<<std::fixed<<std::setprecision(2); time_it(map_emplace);// cache warmupconstauto x= time_it(map_emplace,"plain emplace"); time_it(map_emplace_hint,"emplace with correct hint", x); time_it(map_emplace_hint_wrong,"emplace with wrong hint", x); time_it(map_emplace_hint_corrected,"corrected emplace", x); time_it(map_emplace_hint_closest,"emplace using returned iterator", x);}
Possible output:
143.48ms for plain emplace (ratio: 1.00)164.78ms for emplace with correct hint (ratio: 0.87)171.22ms for emplace with wrong hint (ratio: 0.84)166.55ms for corrected emplace (ratio: 0.86)167.41ms for emplace using returned iterator (ratio: 0.86)
| constructs element in-place (public member function)[edit] | |
| inserts elementsor nodes(since C++17) (public member function)[edit] |
