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

It doesn’t really answer why, but it is determined by the standard, §1.3.10

The information about a function that participates in overload resolution (13.3): the types of its parameters
and, if the function is a class member, the cv- qualifiers (if any) on the function itself and the class in which the member function is declared.

This just means the cv qualifiers of the arguments are ignored in the overload resolution.

A similar (but not equivalent) example with references works:

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

because here the types are different, the first case being a reference to char, the second a reference to const char (as opposed to a const reference to char).