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Classes that represent containers from which elements can be retrieved by position often define the subscript operator, operator[].
Exercises Section 14.4
Exercise 14.20: Define the addition and compound-assignment operators for your
Sales_dataclass.Exercise 14.21: Write the
Sales_dataoperators 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
stringrepresenting an ISBN to aSales_data.Exercise 14.23: Define an
initializer_listassignment operator for your version of theStrVecclass.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.
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.
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
constmember and returns a reference toconst.
As an example, we’ll define subscript for StrVec (§ 13.5, p. 526):
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:
// 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
}
Exercises Section 14.5
Exercise 14.26: Define subscript operators for your
StrVec,String,StrBlob, andStrBlobPtrclasses.
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Note
Best Practices