C++ Primer, Fifth Edition

Appearance

14.5. Subscript Operator

Classes that represent containers from which elements can be retrieved by position often define the subscript operator, operator[].

INFO

Exercises Section 14.4

Exercise 14.20: Define the addition and compound-assignment operators for your Sales_data class.

Exercise 14.21: Write the Sales_data operators so that + does the actual addition and += calls +. Discuss the disadvantages of this approach compared to the way these operators were defined in § 14.3 (p. 560) and § 14.4 (p. 564).

Exercise 14.22: Define a version of the assignment operator that can assign a string representing an ISBN to a Sales_data.

Exercise 14.23: Define an initializer_list assignment operator for your version of the StrVec class.

Exercise 14.24: Decide whether the class you used in exercise 7.40 from § 7.5.1 (p. 291) needs a copy- and move-assignment operator. If so, define those operators.

Exercise 14.25: Implement any other assignment operators your class should define. Explain which types should be used as operands and why.

INFO

The subscript operator must be a member function.

To be compatible with the ordinary meaning of subscript, the subscript operator usually returns a reference to the element that is fetched. By returning a reference, subscript can be used on either side of an assignment. Consequently, it is also usually a good idea to define both const and nonconst versions of this operator. When applied to a const object, subscript should return a reference to const so that it is not possible to assign to the returned object.

TIP

Best Practices

If a class has a subscript operator, it usually should define two versions: one that returns a plain reference and the other that is a const member and returns a reference to const.

As an example, we’ll define subscript for StrVec (§ 13.5, p. 526):

c++
class StrVec {
public:
    std::string& operator[](std::size_t n)
        { return elements[n]; }
    const std::string& operator[](std::size_t n) const
        { return elements[n]; }
    // other members as in § 13.5 (p. 526)
private:
    std::string *elements;   // pointer to the first element in the array
};

We can use these operators similarly to how we subscript a vector or array. Because subscript returns a reference to an element, if the StrVec is nonconst, we can assign to that element; if we subscript a const object, we can’t:

c++
// assume svec is a StrVec
const StrVec cvec = svec; // copy elements from svec into cvec
// if svec has any elements, run the  string empty function on the first one
if (svec.size() && svec[0].empty())  {
    svec[0] = "zero"; // ok: subscript returns a reference to a string
    cvec[0] = "Zip";  // error: subscripting cvec returns a reference to const
}

INFO

Exercises Section 14.5

Exercise 14.26: Define subscript operators for your StrVec, String, StrBlob, and StrBlobPtr classes.