#include  
#include  

The class X 
{
Public: 
Virtual ~ X () {} 
}; 
Class Y: public {X}; 
Int main (int arg c, char * argv []) 
{
STD: : vector A; 
A.e mplace_back (); 
STD: : unique_ptr & lt; Y> & A=a, [0]. 
A.e mplace_back (); 
STD: : unique_ptr & lt; Y> & B=A, [1]. 
A.r eset (new Y ()); 
B.r eset (new Y ()); 
return 0; 
} 

The above code to compile successfully, but the runtime for mistakes. But after switching lines 14 and 15 won't appear mistake.