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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
        *this += 1;		    // just an example
        return *this;
    }

• Use other methods to do the real work, if you can—DRY!
• We have to both increment the value and return the modified value.
• The return type is Whatever &, a reference to the current object.
• Fast & efficient—no copying is required upon return, since we’re just returning 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 & Guilt

• 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.

Example

class ID {
  public:
    ID &operator++() {
        if (++s[1] > '5') {
            s[0]++; s[1] = '1';
        }
        return *this;
    }
    string s = "A1";
};

ID ident;
for (int i=0; i<12; i++)
    cout << (++ident).s << ' ';

A2 A3 A4 A5 B1 B2 B3 B4 B5 C1 C2 C3 

This class creates an ID object, and increments it several times. Since it’s preincrement, we never see A1. That requires postincrement.

Example using postincrement

class ID {
  public:
    ID &operator++() {
        if (++s[1] > '5') {
            s[0]++; s[1] = '1';
        }
        return *this;
    }
    string s = "A1";
};

ID ident;
for (int i=0; i<12; i++)
    cout << (ident++).s << ' ';

c.cc:14: error: no 'operator++(int)' declared for postfix '++'

Oh, yeah—we’ve got to actually write the postincrement method.

Example with postincrement

class ID {
  public:
    ID &operator++() {
        if (++s[1] > '5') {
            s[0]++; s[1] = '1';
        }
        return *this;
    }
    ID operator++(int) {
        const auto save = *this;
        ++*this;  // DRY!
        return save;
    }
    string s = "A1";
};

ID ident;
for (int i=0; i<12; i++)
    cout << (ident++).s << ' ';

A1 A2 A3 A4 A5 B1 B2 B3 B4 B5 C1 C2 

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.