C.con

C.con: Containers and other resource handles

A container is an object holding a sequence of objects of some type; std::vector is the archetypical container. A resource handle is a class that owns a resource; std::vector is the typical resource handle; its resource is its sequence of elements.

Summary of container rules:

• C.100: Follow the STL when defining a container
• C.101: Give a container value semantics
• C.102: Give a container move operations
• C.103: Give a container an initializer list constructor
• C.104: Give a container a default constructor that sets it to empty
• ???
• C.109: If a resource handle has pointer semantics, provide * and ->

See also: Resources

C.100: Follow the STL when defining a container

Reason

The STL containers are familiar to most C++ programmers and a fundamentally sound design.

Note

There are of course other fundamentally sound design styles and sometimes reasons to depart from the style of the standard library, but in the absence of a solid reason to differ, it is simpler and easier for both implementers and users to follow the standard.

In particular, std::vector and std::map provide useful relatively simple models.

Example

// simplified (e.g., no allocators):

template<typename T>
class Sorted_vector {
    using value_type = T;
    // ... iterator types ...

    Sorted_vector() = default;
    Sorted_vector(initializer_list<T>);    // initializer-list constructor: sort and store
    Sorted_vector(const Sorted_vector&) = default;
    Sorted_vector(Sorted_vector&&) noexcept = default;
    Sorted_vector& operator=(const Sorted_vector&) = default;     // copy assignment
    Sorted_vector& operator=(Sorted_vector&&) noexcept = default; // move assignment
    ~Sorted_vector() = default;

    Sorted_vector(const std::vector<T>& v);   // store and sort
    Sorted_vector(std::vector<T>&& v);        // sort and "steal representation"

    const T& operator[](int i) const { return rep[i]; }
    // no non-const direct access to preserve order

    void push_back(const T&);   // insert in the right place (not necessarily at back)
    void push_back(T&&);        // insert in the right place (not necessarily at back)

    // ... cbegin(), cend() ...
private:
    std::vector<T> rep;  // use a std::vector to hold elements
};

template<typename T> bool operator==(const Sorted_vector<T>&, const Sorted_vector<T>&);
template<typename T> bool operator!=(const Sorted_vector<T>&, const Sorted_vector<T>&);
// ...

Here, the STL style is followed, but incompletely. That's not uncommon. Provide only as much functionality as makes sense for a specific container. The key is to define the conventional constructors, assignments, destructors, and iterators (as meaningful for the specific container) with their conventional semantics. From that base, the container can be expanded as needed. Here, special constructors from std::vector were added.

Enforcement

???

C.101: Give a container value semantics

Reason

Regular objects are simpler to think and reason about than irregular ones. Familiarity.

Note

If meaningful, make a container Regular (the concept). In particular, ensure that an object compares equal to its copy.

Example

void f(const Sorted_vector<string>& v)
{
    Sorted_vector<string> v2 {v};
    if (v != v2)
        cout << "Behavior against reason and logic.\n";
    // ...
}

Enforcement

???

C.102: Give a container move operations

Reason

Containers tend to get large; without a move constructor and a copy constructor an object can be expensive to move around, thus tempting people to pass pointers to it around and getting into resource management problems.

Example

Sorted_vector<int> read_sorted(istream& is)
{
    vector<int> v;
    cin >> v;   // assume we have a read operation for vectors
    Sorted_vector<int> sv = v;  // sorts
    return sv;
}

A user can reasonably assume that returning a standard-like container is cheap.

Enforcement

???

C.103: Give a container an initializer list constructor

Reason

People expect to be able to initialize a container with a set of values. Familiarity.

Example

Sorted_vector<int> sv {1, 3, -1, 7, 0, 0}; // Sorted_vector sorts elements as needed

Enforcement

???

C.104: Give a container a default constructor that sets it to empty

Reason

To make it Regular.

Example

vector<Sorted_sequence<string>> vs(100);    // 100 Sorted_sequences each with the value ""

Enforcement

???

C.109: If a resource handle has pointer semantics, provide * and ->

Reason

That's what is expected from pointers. Familiarity.

Example

???

Enforcement

???