c++ - reference to a non-existent variable will be a error, but why doesn't this cause any error? -

i got question function below may result in run time error. why? code :

int& sub(int& , int& b){ int c = - b ; return c ; }

how can write code in main there run-time error?? thanks!!

as it's undefined behaviour, there no guaranteed portable error.

but here , example, betting on gfact nested calls produce referred result overwritten.

int = 5, b=4, c=2;  int r = msub(a, msub(b,c)); cout << "should 3: "<<r<<endl; // output depends on compiler. received 0, incorect !

needless such errors extremely nasty ! happens here ?

the compiler first calls msub(b,c) resutl temporary reference former local variable on stack. @ moment, there's high probability, computed value of 2 still there, if temporary variable doesn't exist anymore.
then compiler calls msub(a, ...) using reference. call change stack, overwriting value referred to.
so there's no segfault, no horror (in simple compiler specific case), computed value inacurate.

i've tried describe general principle of problem:

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