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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 as close as possible to the position just beforehint.
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)....
No iterators or references are invalidated.
Contents |
| hint | - | iterator to the position before which the new element will be inserted |
| 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).
Logarithmic in the size of the container in general, but amortized constant if the new element is inserted just beforehint.
#include <chrono>#include <cstddef>#include <functional>#include <iomanip>#include <iostream>#include <map> constint n_operations=100'500'0; std::size_t map_emplace(){std::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::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::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::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::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:
365.08ms for plain emplace (ratio: 1.00) 96.54ms for emplace with correct hint (ratio: 3.78)409.40ms for emplace with wrong hint (ratio: 0.89) 85.57ms for corrected emplace (ratio: 4.27) 84.31ms for emplace using returned iterator (ratio: 4.33)
(C++11) | constructs element in-place (public member function)[edit] |
| inserts elementsor nodes(since C++17) (public member function)[edit] |
