This time we converted "Hello World" in a modular program. One module contains the message, and one sends the greeting.
Well, that's the way it's supposed to work, something funny is happening.
// Message to say hello const char[] message = "Hello World!\n";
#include <stdio.h>
extern const char* const message;
int main()
{
printf(message);
return(0);
}
Source code for the string Source code for main
Hint 1: C is not typesafe.
Hint 2: The types char* and char[] are almost interchangeable.
Hint 3: "Almost" is a very significant word.
Hint 4: What's located at memory location 0x48656C6C?
Hint 5: The value of "H" is 0x48. The value of "Hell" is 0x48656C6C. 0x486656C6C is not a good pointer.
Hint 6: What does "Hello World!\n" and 0x48656C6C have in common? The same thing that char* and char[] do.
Answer: C++ is not completely typesafe. This is one of those cases where things break down. In the module the string is defined as a character array:
const char[] message = "Hello World!\n";
In the main it's referenced as a character pointer. That means that C (in main) thinks that the variable holds a 4 byte pointer, not a bunch of characters. So the program grabs the first 4 bytes of the array, treats them as a pointer and tries to print. The result is a mess.