C++ overloading with one parameter const

Question

Why is following not allowed in C++

#include <iostream>

class Sample {
public:
  void Method(char x);
  void Method(char const x);
};

void Sample::Method(char x) {
  char y = x;
}

void Sample::Method(char const x) {
  char y = x;
}

int main() {
  Sample s;
  return 0;
}

Answer 1

Why is following not allowed in C++?
The reason is the very same that the compiler gives you as an compilation error:
Because they are ambiguous!

Why are these methods ambiguous?
Short answer: Because the C++ Standard says so.

What is the rationale behind these overloaded methods being ambiguous?
The compiler does not know whether the caller wants to treat the value of the passed argument as an const or not, there is no way for the compiler to determine that with the information at hand.

Note the emphasis on pass by value here, the argument is being passed by value, and hence the ambiguity. If the argument was passed by reference then the compiler knows for sure how the caller wants to treat the argument because then the actual object itself is being passed, and hence compiler can make a selection of the proper overload.

The following example gives a clearer idea to the explanation above.

Online Sample:

class Sample 
{
    public:
        void Method(char &x){}
        void Method(char const x){}
        void Method(char const &x){}
};


int main() 
{
     Sample s;
     return 0;
}