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CS253 Typename

Requirement

• I want to create a templated class that contains a value.
• The value should be the same type as set::size_type, an alias for the type that set::size() returns, which is often, but not necessarily size_t.
• E.g., set<bool>::size() is always 0, 1, or 2, so a template specialization for set<bool> could use a small (perhaps, 16-bit) unsigned short to store the size.

set<bool> sb;
auto &id = typeid(sb.size());
if (id == typeid(unsigned short)) cout << "unsigned short\n";
if (id == typeid(unsigned))       cout << "unsigned int\n";
if (id == typeid(0UL))            cout << "unsigned long\n";
if (id == typeid(0ULL))           cout << "unsigned long long\n";
if (id == typeid(size_t))         cout << "size_t\n";

unsigned long
size_t

• set<bool>::size() could return a different type than set<string>::size().

First pass

template <class T>
struct Answer {
    set<T>::size_type value = 42;  // 🦡
};

int main() {
    Answer<short> doug;
    cout << doug.value;
}

c.cc:3: error: need ‘typename’ before ‘std::set<T>::size_type’ because 
   ‘std::set<T>’ is a dependent scope

• The compiler doesn’t know that, for all possible types T, set<T>::size_type will be a type, and not a method or a variable.
• Sure, we know that ::size_type is always an unsigned integer type, but we’re smart.

Second pass

template <class T>
struct Answer {
    typename set<T>::size_type value = 42;
};

int main() {
    Answer<short> doug;
    cout << doug.value;
}

42

• The keyword typename, meaning “trust me, it’s a type”, is needed for a dependent type, i.e., a type that depends on a template parameter T in set<T>::size_type.
• This also applies to new types defined by typedef or using:

     using newtype = typename oldtype;
     typedef typename oldtype newtype;

• Perhaps, someday, typename will no longer be required.

Why?

• OK, fine, but nobody would ever want to actually do that.
  • Well, sometimes, you do.
• Consider a class that’s exactly like a set, execpt that it’s fussy.
  • Specifically, like a fussy child, it just doesn’t like one particular food.
  • It doesn’t like … ice cream! 😲

Practical use

template <class T>
class fussyset {
    set<T> store;
  public:
    using iterator = set<T>::iterator;  // 🦡
    void insert(T val) { if (val != "ice cream") store.insert(val); }
    iterator begin() const { return store.begin(); }
    iterator end() const { return store.end(); }
};

int main() {
    fussyset<string> fs;
    for (auto food : {"apple", "cereal", "ice cream", "apple"})
        fs.insert(food);
    for (auto val : fs)
        cout << val << '\n';
}

c.cc:5: error: need ‘typename’ before ‘std::set<T>::iterator’ because 
   ‘std::set<T>’ is a dependent scope

Practical use

template <class T>
class fussyset {
    set<T> store;
  public:
    using iterator = typename set<T>::iterator;
    void insert(T val) { if (val != "ice cream") store.insert(val); }
    iterator begin() const { return store.begin(); }
    iterator end() const { return store.end(); }
};

int main() {
    fussyset<string> fs;
    for (auto food : {"apple", "cereal", "ice cream", "apple"})
        fs.insert(food);
    for (auto val : fs)
        cout << val << '\n';
}

apple
cereal

Has-a vs. Is-a

• The previous class did not have an is-a relationship.
• Instead, a fussyset has-a set<T>, with forwarding methods.
• It can be done using is-a, however.

Is-a

template <class T>
class fussyset : private set<T> {   // PRIVATE inheritance‽
    using super = set<T>;
  public:
    using iterator = typename super::iterator;
    void insert(T val) { if (val != "ice cream") super::insert(val); }
    iterator begin() const { return super::begin(); }
    iterator end() const { return super::end(); }
};

int main() {
    fussyset<string> fs;
    for (auto food : {"apple", "cereal", "ice cream", "apple"})
        fs.insert(food);
    for (auto val : fs)
        cout << val << '\n';
}

apple
cereal

Those forwarding methods are still a pain, and WET, repeating the return types, constness, and arguments.

Is-a

template <class T>
class fussyset : private set<T> {
    using super = set<T>;
  public:
    using super::iterator, super::begin, super::end;  // public
    void insert(T val) { if (val != "ice cream") super::insert(val); }
};

int main() {
    fussyset<string> fs;
    for (auto food : {"apple", "cereal", "ice cream", "apple"})
        fs.insert(food);
    for (auto val : fs)
        cout << val << '\n';
}

apple
cereal

A new use of using! Much better. This way, we know that the types & arguments of the fussyset methods are the same as those of the set<T> methods. No typename, either!