If we define a function to compute the absolute value of complex variable like
double Complex:: abs () { return sqrt(re*re + im*im);}
without the constant declaration and define thereafter a function
myabs as
double myabs (const Complex& c)
{ return c.abs(); } // Not ok because c.abs() is not a const func.
the compiler would not allow the c.abs() call in myabs
since Complex::abs is not a constant member function.