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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& operator= ( const A& that )
  13.     { i = that.i ; std::cout << "copy assignment i == " << i << '\n' ; return *this ; }
  14.  
  15.     int i ;
  16. };
  17.  
  18. A operator+ ( const A& a, const A& b )
  19. {
  20.     A temp( a.i + b.i ) ; // RVO applies, constructing 'temp'
  21.     return temp ; // and then copying temp is elided
  22.     // the function directly constructs the return value, 'temp' is elided
  23. }
  24.  
  25. int main()
  26. {
  27.     A a(6) ; // constructor i == 6
  28.     A b(8) ; // constructor i == 8
  29.     A c(0) ; // constructor i == 0
  30.     std::cout << "------------------------------\n" ;
  31.  
  32.     c = a+b ; // call operator+(a,b), RVO applies
  33.     // step one:
  34.     // construct anonymous temporary 'in place' - constructor i == 14
  35.     // no copy or move constructor is involved
  36.  
  37.     // step two:
  38.     // assign the anonymous temporary returned by operator+ to c
  39.     // assignment i == 14
  40.  
  41.     // step three:
  42.     // destroy the anonymous temporary returned by operator+
  43.     // destructor i == 14
  44.     std::cout << "------------------------------\n" ;
  45.  
  46.     a.i = 100 ;
  47.     b.i = 56 ;
  48.  
  49.     A d = a+b ; // call operator+(a,b), RVO applies
  50.     // this function returns a prvalue ("pure rvalue")
  51.     // a prvalue need not be associated with any object, it can be elided
  52.     // therefore, construct d 'in place'
  53.     // construct d - constructor i == 156
  54.     // no copy or move constructor is involved
  55.     std::cout << "------------------------------\n" ;
  56.  
  57.     std::cout << "press enter to destroy 'a', 'b', 'c' and 'd' and then quit\n" ;
  58.     std::cin.get() ;
  59.  
  60.     // destroy d - destructor i == 156
  61.     // destroy c - destructor i == 14
  62.     // destroy b - destructor i == 56
  63.     // destroy a - destructor i == 100
  64. }
  65.  
Success #stdin #stdout 0s 2732KB
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constructor i == 6
constructor i == 8
constructor i == 0
------------------------------
constructor i == 14
copy 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++ 4.3.2 (gcc-4.3.2)

					

					

						
created:
								
						
10 years ago

					

					

						
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