how can set different function signature to the same function pointer?

How can I set a function pointer depending on some condiction to functions with different signature?

example

short int A() 
{
    return 0;
}

long int B()
{
    return 0;
}

void main()
{
   std::function<short int()> f = A;
   f();
   if(true)
   {
      //error
      f = B;
   }
}

How can use the same function pointer for two function with different signature? is it possible?

If is not, there is an efficient way to call the appropriate function depending on behavior instead of use a variable and split the whole code with if statements?

thanks.