Update ex6_12.cpp
Update ex6_12.cpp

at86/CppPrimer,cseryp/CppPrimer,wgs111/CppPrimer,zhangzhizhongz3/CppPrimer,cheng668/CppPrimer,cseryp/CppPrimer,Sublele/CppPrimer,Sublele/CppPrimer,cheng668/CppPrimer,m0000re/CppPrimer,laohei889/CppPrimer,heamon7/CppPrimer,Milk-SF/CppPrimer,FingerSport/CppPrimer,pezy/CppPrimer,Milk-SF/CppPrimer,vzyw/CppPrimer,jwave/CppPrimer,lawlietye/CppPrimer,shuidong/CppPrimer,floristt/CppPrimer,jwave/CppPrimer,lawlietye/CppPrimer,pezy/CppPrimer,vzyw/CppPrimer,laohei889/CppPrimer,at86/CppPrimer,zhangzhizhongz3/CppPrimer,sheldon66/CppPrimer,sheldon66/CppPrimer,heamon7/CppPrimer,floristt/CppPrimer,m0000re/CppPrimer,shuidong/CppPrimer,wgs111/CppPrimer,FingerSport/CppPrimer,akanezorap/CppPrimer,akanezorap/CppPrimer