regarding array declaration in c++ -

int main(){          int n;          scanf("%d",&n);          int a[n]; } 

in above space array a[] , allocated ? in stack or heap ?

if compiler compiles that, it's going on stack. in standard parlance, if care apply construct that's not standard-conforming, has automatic storage duration, meaning don't have clean , become invalid @ end of scope.

what have there vla (variable length array), construct c allows have arrays dimensions known @ runtime. usually, way work similar "function" alloca, decreases stack pointer amount known @ runtime , "returns" pointer it. put "function" in quotes because doing requires low-level hackery that's not provided normal function scope semantics.

vlas don't exist in c++, you're using compiler extension, , precise semantics of vlas in extension depend on compiler. since gcc, i'll leave link the relevant part of documentation.