emplace

来源： https://stackoverflow.com/questions/56483606/cannot-emplace-back-a-braced-initializer-on-a-vector-of-vectors

来源： https://stackoverflow.com/questions/56483606/cannot-emplace-back-a-braced-initializer-on-a-vector-of-vectors

问题 I was in the process of selecting one of two methods of putting things into an unordered_map: std::unordered_map<Key, Value> map; map.emplace( std::piecewise_construct, std::forward_as_tuple(a), std::forward_as_tuple(b, c, d)); vs std::unordered_map<Key, DifferentValue> map; auto& value = map[a]; if (value.isDefaultInitialized()) value = DifferentValue(b, c, d); I did some experiments to see which one would perform better to find that when inserting unique elements, the behaviour (as in

问题 I was in the process of selecting one of two methods of putting things into an unordered_map: std::unordered_map<Key, Value> map; map.emplace( std::piecewise_construct, std::forward_as_tuple(a), std::forward_as_tuple(b, c, d)); vs std::unordered_map<Key, DifferentValue> map; auto& value = map[a]; if (value.isDefaultInitialized()) value = DifferentValue(b, c, d); I did some experiments to see which one would perform better to find that when inserting unique elements, the behaviour (as in

问题 I want to construct an std::vector with some elements having these elements constructed by some particular constructor rather than the default constructor. In other words I want to emplace the elements while constructing the vector. How can I do that? Consider this: struct Item { Item(double) {} Item(const Item&) = delete; Item(Item&&) = delete; }; std::vector<Item> vv(10, 3.14); // Fails! Tries to copy while I want to emplace. 回答1: Your Item class doesn't support copies nor moves. This will

问题 I am implementing a simple circular vector class. I want to implement an emplace member function, but I am getting an error which I don't understand. It might be a simple fix for something I am doing wrong, but since I don't have much experience with variadic templates, I can't figure out what... The error I am getting is: main.cpp: In function 'int main()': main.cpp:104:50: error: no matching function for call to 'CircularVector<Item>::emplace(int, int, int, int, int, int, std::vector<int>,

问题 i have a std::vector<std::vector<double>> and would like to add some elements at the end of it so this was my trial: std::vector<std::vector<double> > vec; vec.emplace_back({0,0}); but this does not compile whereas the following will do: std::vector<double> vector({0,0}); Why can't emplace_back construct the element at this position? Or what am i doing wrong? Thanks for your help. 回答1: Template deduction cannot guess that your brace-enclosed initialization list should be a vector. You need to

问题 i have a std::vector<std::vector<double>> and would like to add some elements at the end of it so this was my trial: std::vector<std::vector<double> > vec; vec.emplace_back({0,0}); but this does not compile whereas the following will do: std::vector<double> vector({0,0}); Why can't emplace_back construct the element at this position? Or what am i doing wrong? Thanks for your help. 回答1: Template deduction cannot guess that your brace-enclosed initialization list should be a vector. You need to

问题 For C++11, is there still a performance difference between the following? (for std::map<Foo, std::vector<Bar> > as an example) map[key] = myVector and map.emplace(key, myVector) The part I'm not figuring out is the exact internal of operator[]. My understanding so far has been (when key doesn't exist): Create a new key and the associated empty default vector in place inside the map Return the reference of the associated empty vector Assign myVector to the reference??? The point 3 is the part

问题 The Problem I have a std::map<int, std::set<int>> named misi . I'm wondering why misi.emplace(2345, {6, 9}); and misi.emplace({2345, {6, 9}}); don't work as expected, as shown below. The Code #include <set> // std:set #include <map> // std::map #include <utility> // std::piecewise_construct, std::pair #include <tuple> // std::forward_as_tuple #include <iostream> // std::cout, std::endl int main() { // --- std::set initializer list constructor --- std::set<int> si({42, 16}); std::cout << "si