Code:

#include "dot.h"
namespace dot {
	const char* leader(char* s){
		for (int i=0;i<ML;i++)
			s[i]=".";                
		s[ML]="\0";
		return s;
	}
}

It works with single quotes. I just want to know why double quotes dont work.

Because single quotes indicate a char value. Double quotes are interpreted by the compiler as null terminated C strings, which cannot be assigned to a single char.

thank you

Why not use std::string?

Part of the job of a compiler is type-checking data types and making sure that you don't do something stupid like try to divide an apple by an orange... metaphorically speaking. In this case, the double-quoted string literal is likely of type const char* (the address of the string literal in read-only memory) which you are attempting to assign to a char. From the compiler's standpoint, it sees you doing something questionable and rightly complains about it. As mentioned by MK27, single-quoted values represent individual characters and it is completely acceptable to assign a char to a char.

Well, to be pedantic, the type is actually const char[n] where n is the length of the array (the length of the string + 1).