14.4. Assignment Operators

In addition to the copy- and move-assignment operators that assign one object of the class type to another object of the same type (§ 13.1.2, p. 500, and § 13.6.2, p. 536), a class can define additional assignment operators that allow other types as the right-hand operand.

As one example, in addition to the copy- and move-assignment operators, the library vector class defines a third assignment operator that takes a braced list of elements (§ 9.2.5, p. 337). We can use this operator as follows:

vector<string> v;
v = {"a", "an", "the"};

We can add this operator to our StrVec class (§ 13.5, p. 526) as well:

class StrVec {
public:
    StrVec &operator=(std::initializer_list<std::string>);
    // other members as in § 13.5 (p. 526)
};

To be consistent with assignment for the built-in types (and with the copy- and move-assignment operators we already defined), our new assignment operator will return a reference to its left-hand operand:

StrVec &StrVec::operator=(initializer_list<string> il)
{
    // alloc_n_copy allocates space and copies elements from the given range
    auto data = alloc_n_copy(il.begin(), il.end());
    free();   // destroy the elements in this object and free the space
    elements = data.first; // update data members to point to the new space
    first_free = cap = data.second;
    return *this;
}

As with the copy- and move-assignment operators, other overloaded assignment operators have to free the existing elements and create new ones. Unlike the copy-and move-assignment operators, this operator does not need to check for self-assignment. The parameter is an initializer_list<string> (§ 6.2.6, p. 220), which means that il cannot be the same object as the one denoted by this.