c++ - What's happening with the snippet below in a release build? -

the code below generates dangling reference can seen in warning emitted compiler , fact destructor a object in function g() invoked before function returns. 1 can verify in main() after "using stack", returned reference has garbage, @ least in debug build. couldn't reproduce same behavior in release build. why that? kind of optimization compiler doing here give impression reference r ok?

#include <iostream>  struct a{     a(int i) : i(i) { std::cout << "ctor\n"; }     a(const a& a) { = a.i; std::cout << "copy ctor\n"; }     ~a() { std::cout << "dtor\n"; }     int i; };  a& g(int i) { x(i); return x; }  int main() {     const a& r = g(1);     std::cout << "using stack\n";          std::cout << r.i << '\n';   // r.i has garbage in debug, not in release build. } 

ps. argue against nrvo, function doesn't return a object.

edit: in response mark tolonen. if include these expressions after const a& r = g(1); release build doesn't show garbage in std::cout << r.i << '\n';

std::cout << "using stack ...................................................................................................................\n"; std::cout << "using stack ...................................................................................................................\n"; std::cout << "using stack ...................................................................................................................\n"; std::cout << "using stack ...................................................................................................................\n"; 

here's speed-optimized (/o2 compiler switch) release build of visual studio 2012 64-bit when runs code , prints out one:

int main() { 000000013f7c7e50  sub         rsp,28h       const a& r = g(1); 000000013f7c7e54  lea         rdx,[string "ctor\n" (013f83da4ch)]   000000013f7c7e5b  lea         rcx,[std::cout (013f85faa0h)]   000000013f7c7e62  call        std::operator<<<std::char_traits<char> > (013f7c1500h)   000000013f7c7e67  lea         rdx,[string "dtor\n" (013f83da54h)]   000000013f7c7e6e  lea         rcx,[std::cout (013f85faa0h)]   000000013f7c7e75  call        std::operator<<<std::char_traits<char> > (013f7c1500h)       std::cout << "using stack\n";      000000013f7c7e7a  lea         rdx,[string "using stack\n" (013f83da60h)]   000000013f7c7e81  lea         rcx,[std::cout (013f85faa0h)]   000000013f7c7e88  call        std::operator<<<std::char_traits<char> > (013f7c1500h)       std::cout << r.i << '\n';   // r.i has garbage in debug, not in release build. 000000013f7c7e8d  lea         rcx,[std::cout (013f85faa0h)]   000000013f7c7e94  mov         edx,1   000000013f7c7e99  call        std::basic_ostream<char,std::char_traits<char> >::operator<< (013f7c1384h)   000000013f7c7e9e  mov         dl,0ah   000000013f7c7ea0  mov         rcx,rax   000000013f7c7ea3  call        std::operator<<<std::char_traits<char> > (013f7c10ebh)   

note doesn't bother create , destroy a object. call cout 4 times. each time, rdx holds object print. first 3 print strings "ctor\n", "dtor\n", , "using stack\n". last 1 looks prints integer in edx 1.

the compiler can undefined behavior. prints besides 1 space-optimized (/o1 compiler switch), or op found, not-optimized (/od).