C++ overload parenthesis operator to construct class/object after initialization?

Question

Is it possible to be "DRY" (do not repeat yourself")? I would like to declare my class and essentially construct it later.

this code works:

// default constructor        
UserInput::UserInput() {}; 

// other constructor
UserInput::UserInput(string title, string section, string subsection) :
    title(title), section(section), subsection(subsection) {
    SPLIT();
}

// i want to be "DRY", is it possible?
// this is the same code as the "other constrcutor"
void UserInput::operator()(string title, string section, string subsection) {
    this->title = title;
    this->section = section;
    this->subsection = subsection;
    SPLIT();
}

this does not work, the class ends up with blank strings:

void UserInput::operator()(string title, string section, string subsection) {
    UserInput(title, section, subsection);
}

It sounds like you might want a boost::optional, which lets you delay creation and makes no value a valid value, like null. — Qaz
– Qaz, Commented Dec 19, 2015 at 2:57

Sam Varshavchik · Accepted Answer · 2015-12-19 04:22:43Z

1

void UserInput::operator()(string title, string section, string subsection) {
    *this = UserInput(title, section, subsection);
}

This is not terribly efficient, but if performance is not an issue for you, this will do the trick.

void UserInput::operator()(const string &title, const string &section, const string &subsection) {
    *this = UserInput(title, section, subsection);
}

This will be a little bit efficient. You should also make the same change to the real constructor's parameters, as well.

EDIT: here' an alternatve approach referenced in the comments. This is a bit more "standard-ish". It is quite common for constructors to invoke additional class methods to finish constructing object. This only works when class members have default constructors. As mentioned earlier, there's some duplicated effort here -- the class members get default-constructed first, only to be fully constructed again. But, it does tend to eliminate code duplication, and is a bit cleaner when performance or efficiency isn't at the premium.

UserInput::UserInput(const string &title, const string &section, const string &subsection)
{
    operator()(title, section, subsection);
}

void UserInput::operator()(const string &title, const string &section, const string &subsection) {
    this->title = title;
    this->section = section;
    this->subsection = subsection;
    SPLIT();
}

edited Dec 19, 2015 at 4:22

answered Dec 19, 2015 at 2:50

Sam Varshavchik

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5 Comments

Elan Hickler Over a year ago

is what I'm doing non-standard, is there a common way of doing it? Should I be doing something like declaring the object then sending it a class later via a class constructor like this?: UserInput ui; ui = UserInput(params); edit: oh maybe this is exactly what you're doing in your answer.

Sam Varshavchik Over a year ago

Well, a slightly more "standard" approach would be to eliminate duplicate code the other way around, by invoking operator() from the constructor: UserInput::UserInput(...) { operator()(title, section, subsection); }

Elan Hickler Over a year ago

yes! that's what I wanted.. let me see if I can get that implemented.

Elan Hickler Over a year ago

Hey, could you add this explanation to your answer as a second alternative answer? Thank you!

Sam Varshavchik Over a year ago

Sure, added. Nothing really earth-shattering.

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