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  1. #include <iostream>
  2.  
  3. struct A
  4. {
  5.     A( int ii ) : i(ii) { std::cout << "constructor i == " << i << '\n' ; }
  6.  
  7.     A( const A& that ) : i(that.i)
  8.     { std::cout << "copy constructor i == " << i << '\n' ; }
  9.  
  10.     ~A() { std::cout << "destructor i == " << i << '\n' ; }
  11.  
  12.     A( A& that ) : i(that.i)
  13.     { std::cout << "move constructor i == " << i << '\n' ; }
  14.  
  15.     A& operator= ( const A& that )
  16.     { i = that.i ; std::cout << "copy assignment i == " << i << '\n' ; return *this ; }
  17.  
  18.     A& operator= ( A&& that )
  19.     { i = that.i ; std::cout << "move assignment i == " << i << '\n' ; return *this ; }
  20.  
  21.     int i ;
  22. };
  23.  
  24. A operator+ ( const A& a, const A& b )
  25. {
  26.     A temp( a.i + b.i ) ; // RVO applies, constructing 'temp'
  27.     return temp ; // and then copying temp is elided
  28.     // the function directly constructs the return value, 'temp' is elided
  29. }
  30.  
  31. int main()
  32. {
  33.     A a(6) ; // constructor i == 6
  34.     A b(8) ; // constructor i == 8
  35.     A c(0) ; // constructor i == 0
  36.     std::cout << "------------------------------\n" ;
  37.  
  38.     c = a+b ; // call operator+(a,b), RVO applies
  39.     // step one:
  40.     // construct anonymous temporary 'in place' - constructor i == 14
  41.     // no copy or move constructor is involved
  42.  
  43.     // step two:
  44.     // assign the anonymous temporary returned by operator+ to c
  45.     // move assignment i == 14
  46.  
  47.     // step three:
  48.     // destroy the anonymous temporary returned by operator+
  49.     // destructor i == 14
  50.     std::cout << "------------------------------\n" ;
  51.  
  52.     a.i = 100 ;
  53.     b.i = 56 ;
  54.  
  55.     A d = a+b ; // call operator+(a,b), RVO applies
  56.     // this function returns a prvalue ("pure rvalue")
  57.     // a prvalue need not be associated with any object, it can be elided
  58.     // therefore, construct d 'in place'
  59.     // construct d - constructor i == 156
  60.     // no copy or move constructor is involved
  61.     std::cout << "------------------------------\n" ;
  62.  
  63.     std::cout << "press enter to destroy 'a', 'b', 'c' and 'd' and then quit\n" ;
  64.     std::cin.get() ;
  65.  
  66.     // destroy d - destructor i == 156
  67.     // destroy c - destructor i == 14
  68.     // destroy b - destructor i == 56
  69.     // destroy a - destructor i == 100
  70. }
  71.  
Success #stdin #stdout 0s 3344KB
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constructor i == 6
constructor i == 8
constructor i == 0
------------------------------
constructor i == 14
move assignment i == 14
destructor i == 14
------------------------------
constructor i == 156
------------------------------
press enter to destroy 'a', 'b', 'c' and 'd' and then quit
destructor i == 156
destructor i == 14
destructor i == 56
destructor i == 100


	

		
	 
	
	
			

					
			
			
			
			
			
			
			
			

			
			
			
		

	
		
		

			

			                        







			
			

				

	
					
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language:

						
C++14 (gcc 8.3)

					

					

						
created:
								
						
10 years ago

					

					

						
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