Passing between class methods

[romasi]

When passing non-const passed values from one overloaded method to the other causes a segfault but once they were changed to const it worked fine... curious as to how that worked?

main.cpp

int main() {

    Button *button = new Button;

    char *cstr[] = "Hello";
    button->setCaption( cstr );

    string str = "Howdy";
    button->setCaption( str );
    ...
}

button.cpp (with error)

void Button::setCaption( char * cstr ) {

    string button_text = cstr;    // showing type string for info purposes.
    RenderText( button_text );
}
void Button::setCaption( string str ) {

    setCaption( str.c_str() );    // ERROR: Segfault!
}

button.cpp (fixed)

void Button::setCaption( const char * cstr ) {

    string button_text = cstr;    // showing string type for info purposes.
    RenderText( button_text );
}
void Button::setCaption( const string &str ) {

    setCaption( str.c_str() );    // Works fine!
}

Note: void Button::setCaption( const string str ) and void Button::setCaption( const string &str ) worked. However passing it by reference would probably be the best method. Any additional improvements would be appreciated.

[m4ster r0shi]

The first approach causes a problem because setCaption(string) enters an infinite recursion when called. You see, str.c_str() returns a const char *, so setCaption(str.c_str()); doesn't call setCaption(char *). Instead, a temporary string object is created (because a string object can be created from a const char * object), and setCaption(string) is called again. And then again... And then again... And there you have your seg-fault :P

Once you add the const keyword the problem is solved because now setCaption(str.c_str()); calls setCaption(const char *) as you want it.

[romasi]

How would I stop str.c_str() from creating a temporary string object without using const? Create a temporary string variable inside the function, assign the value and then pass that variable?

[Athar]

You could do it by creating a temporary C string and use strcpy to copy str.c_str().
That would be a rather stupid way to do it, though. Changing setCaption to accept const char* instead is the correct way.

[Disch]

How would I stop str.c_str() from creating a temporary string object without using const?

You're basically asking this: "How do I solve this problem without solving the problem?"

Using const is the proper solution here. setCaption(const char* cstr) is the way this function should be formed.

Any solution that doesn't involve const (like Athar's suggestion of creating a temporary buffer) will work, but will be hackish, error prone, and yield worse performance.

Just be const correct. It's the easiest solution here.

[romasi]

It was more of a information question =). I try to be const correct at first when possible. Other times I get the code to work first, then convert it over to const correctness afterwards.

Thanks for the responses.

[jsmith]

I would strongly recommend that you get into the habit of writing const correct code
from the beginning. Const correctness is not something that is necessarily easy to
introduce later.