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CS253 Increment Decrement Methods

Why

If you’re writing a class that is numeric or pointery, then it needs:

• Preincrement (++obj)
• Postincrement (obj++)
• Predecrement (--obj)
• Postdecrement (obj--)

None of these come for free. You have to write them all.

Preincrement

Preincrement is easy:

    // Change the state of the object
    // Return the object by reference
    Whatever &operator++() {	    // No argument means preincrement
        // Do what is needed to change the object
        // which might be *this += 1;
        return *this;
    }

• We have to return the value after it’s incremented.
• The return type is Whatever &, a reference to the current object.
• Fast & efficient—no copying is required upon return, since we’re just returing a reference.

Postincrement

Postincrement is harder, but is the same for nearly all classes:

    // Save a copy of the object
    // Change the state of the object
    // Return the copy by value
    Whatever operator++(int) {	    // Dummy int arg means postincrement
        const auto save = *this;
        ++*this;		    // Call the preincrement method
        return save;
    }

• We have to return the old value, before it was incremented.
• Therefore, make a copy, and return that.
• It’s a shame that we have to make a copy, but there’s no alternative.
• Can’t return a reference to the short-lived value on the stack.

Decrement

Pre-/post-decrement is essentially the same. Get pre-/post-increment working, and then (I can’t believe that I’m saying this) use copy & paste.

General Guidelines

• Unless you have a really peculiar class, use the postincrement boilerplate given above. It’s the same for every class!
• Often, operator++ should use operator+= to accomplish the increment. Less duplication of information—good!
• When you have a choice, use preincrement rather than postincrement, because postincrement makes a copy, whereas preincrement doesn’t.
• For built-in integers, it doesn’t matter—the compiler will do the right thing. Follow whatever local coding standards are in force, or use whichever one looks better to you.