有没有办法检查在编译时是否声明了变量？

Question

I mean case like this (this is of course false code /idea only/ as I know it doesn't work like that):

int main() {
  int my_int = 42;
  #ifdef my_int
    my_int = 43;
  #else
    int my_int = 43;
}

I know that c/c++ program cannot see the names of the variables and cannot tell if it was set or not but compiler can do it, and I ask here is there any way to direct compiler to a given action like omitting or inserting a snippet of code when it encounters a situation when there are duplicate declarations in the same block.

Answer 1

No. The only thing that can happen when you have duplicate declarations, is a compiler error. It's always ill-formed. And because the pre-processor has no knowledge of variables, you can't use that either.

Answer 2

No, there is no way. And even if there were, using it would go down the path towards madness.

If you know that the declaration of a symbol is conditional on some defined constant, use an #ifdef on that constant to decide what to do.

If you don't know what the declaration of a symbol is conditional on, you really should not be messing around with that symbol because obviously it's external to your project.