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<h1 id="define-is-and-has-relationships"><a class="header" href="is_and_has_relationships.html#define-is-and-has-relationships">define is and has relationships</a></h1>
<p>Based on the using the features of the relationship.</p>
<p>Is: public inheritance.</p>
<p>Has: private or protected inheritance (private is better).</p>
<p>By using public inheritance we can provide all features from base to derived. Using derived pointers we can access them freely - no restrictions outside.</p>
<p>private/protected inheritance restricts external usage so it's not a <code>is-a</code> relationship but can provide <code>has-a</code> relationship. In this inheritance derived types can still own base's features and access them or we can create an instance of them and put it into fields.o</p>
<p>a simple is-a relationship</p>
<pre><code class="language-c++">
struct base {
int id_{};
};
struct derived: base {
std::string name_{};
};
derived d {0, "hakan gedek"};
base* bptr = &d;
std::cout << bptr->id_ << std::endl;
derived* dptr = static_cast<derived*>(bptr);
std::cout << dptr->name_ << std::endl;
</code></pre>
<p>using concepts <code>convertible_to</code> and <code>derived_from</code> to check if it's a <code>is-a</code> relationship.</p>
<p>public_derived => base: is-a relationship</p>
<pre><code class="language-c++">
struct base {
int id{};
};
struct public_derived : base {
std::string name{};
};
struct private_derived : private base {
std::string name{};
}
int main() {
std::cout << std::boolalpha
<< std::derived_from<public_derived, base> << std::endl;
std::cout << std::boolalpha
<< std::derived_from<private_derived, base> << std::endl;
std::cout << std::boolalpha
<< std::convertible_to<public_derived, base> << std::endl;
std::cout << std::boolalpha
<< std::convertible_to<base, public_derived> << std::endl;
std::cout << std::boolalpha
<< std::convertible_to<private_derived, base> << std::endl;
std::cout << std::boolalpha
}
</code></pre>
<pre><code class="language-c++">true
false
true
false
false
false
</code></pre>
<p>has-a ?</p>
<ul>
<li>if you prefer private inheritance you can hide base contents from outside and they can be only accessible by deriveds</li>
<li>if you prefer having an instance in fields; you should either expose all (public access) or define a friendship</li>
</ul>
<pre><code class="language-c++">
// forward-decl
struct owner;
// this type will not be used for creating any instance
struct hidden_base {
protected:
int id_;
std::string name_;
hidden_base(int id, std::string name) : id_(id), name_(name) {}
friend owner;
};
// has-a relationship
struct derived : private hidden_base {
derived(int id, std::string name) : hidden_base(id, name) {}
void print() const { std::cout << id_ << " " << name_ << std::endl; }
};
struct owner {
hidden_base obj;
owner(int id, std::string name) : obj{id, name} {}
void print() const {
std::cout << obj.id_ << " " << obj.name_ << std::endl;
}
};
int main()
{
derived d{0, "hakan gedek"};
d.print();
owner u{1, "hakan gedek"};
u.print();
}
</code></pre>
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