c++ rvo vs std::move

To summarize, RVO is a compiler optimization technique, while std::move is just an rvalue cast, which also instructs the compiler that it's eligible to move the object. The price of moving is lower than copying but higher than RVO, so never apply std::move to local objects if they would otherwise be eligible for the RVO.

 

#include 
     
      #include 
      
       #include 
       
        class BigObject {public:    BigObject() {        std::cout << "constructor. " << std::endl;    }    ~BigObject() {        std::cout << "destructor."<< std::endl;    }    BigObject(const BigObject&) {        std::cout << "copy constructor." << std::endl;    }    BigObject(BigObject&&) {        std::cout << "move constructor"<< std::endl;    }};struct info{    std::string str;    std::unordered_map 
        
          umap;};int64_t get_current_time_ns(){    std::chrono::nanoseconds ss = std::chrono::high_resolution_clock::now().time_since_epoch();    int64_t tt = ss.count();    std::cout<<"current time:"<
         <
          
           (6, "eggs")); return ifo;}info get_info_v2(){ info ifo; ifo.str = "ttppppppppppppppppppppppppppppppppppppppppppppppppppppppppp"; ifo.umap.insert(std::make_pair
           
            (6, "eggs")); return std::move(ifo);}BigObject foo(int n) { BigObject localObj; return localObj;}int main() { auto f = foo(1); int64_t t_1= get_current_time_ns(); std::cout<<"test rvo:"<
           
          
        
       
      
     

 

Result

constructor. current time:1568273863513694551test rvo:current time:1568273863517139874v1 time cost:3445323test move:current time:1568273863521213442v2 time cost:4073568info test rvo:current time:1568273863574775754info v1 time cost:53562312info test move:current time:1568273863641223923info v2 time cost:66448169destructor.

 

Reference