const string which cause segmentation fault - any way to detect this bug during compile time?

[yccheok]

Hi, consider the following bug at

#include <stdio.h>

class Drink {
public:
Drink( char *name_ ) : name( name_ ) {}
char *name;
char & get(int i) { return name; }
};

void stir( char &ch ) { ch++; }

int main() {
Drink Martini("Martini");
printf("%c\n", Martini.get(2));
stir(Martini.get(2));
return 0;
}

which will dump out output:

r
Segmentation fault

I was understand that it is because "Martini" is a const string.
Run-time error will occur while trying to modify const string content.


Now, the challenge here is, is there any way to detect this bug during
compile time? Or is there any good programming practice to prevent
this?

My initial thought is that, re-design Drink object to use std::string.
What if the Drink code is copyrighted to others and I does not have
right to modify it? Is there anywhere I can improve it from client code
side (main function)??

Thank you very much!

-cheok

 

[Mike Wahler]

Hi, consider the following bug at

#include <stdio.h>

class Drink {
public:
Drink( char *name_ ) : name( name_ ) {}
char *name;
char & get(int i) { return name; }
};

void stir( char &ch ) { ch++; }

int main() {
Drink Martini("Martini");
printf("%c\n", Martini.get(2));
stir(Martini.get(2));
return 0;
}

which will dump out output:

r
Segmentation fault

I was understand that it is because "Martini" is a const string.
Run-time error will occur while trying to modify const string content.

It might, it might not. The behavior is undefined.

Now, the challenge here is, is there any way to detect this bug during
compile time? Or is there any good programming practice to prevent
this?

Sure. Make 'get()' a const member function:

char &get(int i) const { return name; }

Any member function which is not intended to modify
the object's state should be declared const.

My initial thought is that, re-design Drink object to use std::string.

That would allow you to modify the string, yes (unless the
string member were declared const). IMO std::string should
be preferred over 'C-style strings' anyway.

What if the Drink code is copyrighted to others and I does not have
right to modify it? Is there anywhere I can improve it from client code
side (main function)??

If you can't modify the code, then all you can do is
'follow the rules' (i.e. don't invoke undefined behavior).

-Mike