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<h1 id="use-castings"><a class="header" href="casting.html#use-castings">use castings</a></h1>
<p>c++ casting <strong>functions</strong> : yes they're functions so anything is valid with any function is valid for them too !</p>
<ul>
<li>static_cast</li>
<li>dynamic_cast</li>
<li>reinterpret_cast</li>
<li>const_cast</li>
</ul>
<blockquote>
<p>these are special and they are not topic of this note</p>
</blockquote>
<ul>
<li><del>bit_cast</del></li>
<li><del>any_cast</del></li>
<li><del>saturate_cast</del></li>
<li><del>duration_cast</del></li>
</ul>
<blockquote>
<p>reference to reference casting is very different than reference to value or value to value casting. you don't create a new item when doing ref to ref casting</p>
</blockquote>
<blockquote>
<p>call calls are methods and they are not magical stuff</p>
</blockquote>
<p>what happens when you call static_cast with T, T&... or assign it to T& or T. well samethings happen when you call any function like:</p>
<pre><code class="language-c++">
struct base {};
struct derived: base {};
template <typename To, typename From>
To my_static_cast(From data) {
//if To and From has & data it won't call any ctor or cctor or ohter...
// like any function if you use static_cast<T> then you will create a copy when returning
// if you assign it to a T then you call for cctor or cop from returned value
// but if you assign it to a T& or cast to a <T&> you will not call any special function = like you do with any function ! No magic !
}
int main() {
derived d;
base& b = d; // upcasting
derived& d = static_cast<derived&>(b); // downcasting : no call to cctor, cop
derived d = static_cast<derived&>(b); // called cop
derived d = static_cast<derived>(b); // called ctor(base&) and cop !!! you can try all...
...
}
</code></pre>
<h2 id="static_cast"><a class="header" href="casting.html#static_cast">static_cast</a></h2>
<blockquote>
<p>supports pointer <T*> and reference <T&> casts</p>
</blockquote>
<blockquote>
<p>static_cast: doesn't use rtti and type checking is done at compilation - doesn't check it at runtime.</p>
</blockquote>
<blockquote>
<p>down-casting can be problematic if the inner type of the rvalue is not derived but a real base type which means you don't have derived type memory space and static_cast will ignore it and give you a derived type which will have <strong>undefined</strong> stuff in it so best be cautious with this</p>
</blockquote>
<p>a comprehensive example with all possibilities:</p>
<pre><code class="language-c++">
class base {
public:
virtual ~base() = default;
};
class derived : public base {
public:
void foo() {}
};
int main() {
int ival = 3;
// value type casting
float fval = static_cast<float>(ival);
derived dobj;
base& bobj = static_cast<base&>(dobj); // upcasting - works without static_cast too
derived& dref = static_cast<derived&>(bobj); // downcasting
dref.foo();
base* bptr = &dobj; // upcasting
derived* dptr = static_cast<derived*>(bptr);
dptr->foo();
void* vptr = static_cast<void*>(dptr);
dptr = static_cast<derived*>(vptr);
dptr->foo();
// undefined behaviour
{
base* b = new base{};
derived* d = static_cast<derived*>(b);
d->foo(); /// ? this doesn't exist => UB
}
}
</code></pre>
<h2 id="const_cast"><a class="header" href="casting.html#const_cast">const_cast</a></h2>
<blockquote>
<p>supports pointer <T*> and reference <T&> casts</p>
</blockquote>
<blockquote>
<p>const_cast: can add or remove const from result.</p>
</blockquote>
<pre><code class="language-c++">void foo(int const& x) {
int& y = const_cast<int&>(x);
y = 2;
}
int main() {
int x = 0;
foo(x);
assert(x == 2);
return 0;
</code></pre>
<h2 id="dynamic_cast"><a class="header" href="casting.html#dynamic_cast">dynamic_cast</a></h2>
<blockquote>
<p>supports pointer <T*> and reference <T&> casts but ony for polymorphic (virtual implemented) types - not for normal inheritance which you can use static_cast instead</p>
</blockquote>
<blockquote>
<p>dynamic_cast: rtti based - runtime casting. it can success or fail. If it fails it returns nullptr if it's pointer casting or throws std::bad_cast exception if it's reference casting. <strong>be careful</strong></p>
</blockquote>
<pre><code class="language-c++">
struct base {
virtual ~base() = default;
};
struct derived : base {};
struct unrelated {};
int main() {
base* bptr = new derived();
derived* dptr = dynamic_cast<derived*>(bptr);
assert(dptr != nullptr);
derived d;
base& b = d;
derived& dref = dynamic_cast<derived&>(b);
try {
derived d;
base& b = d;
unrelated& u = dynamic_cast<unrelated&>(b);
} catch (std::exception const& e) { // std::bad_cast
std::cout << e.what() << std::endl;
}
return 0;
</code></pre>
<h2 id="reinterpret_cast"><a class="header" href="casting.html#reinterpret_cast">reinterpret_cast</a></h2>
<blockquote>
<p>supports pointer <T*> and reference <T&> casts</p>
</blockquote>
<blockquote>
<p>reinterpret_cast: I call this byte_cast. it casts low-level (bit level)- it's in your control and it's open to UB (undefined behaviour) so be sure types are compatible.</p>
</blockquote>
<pre><code class="language-c++">
using std::byte;
struct base {
int x;
};
struct derived : base {
int y;
};
int main() {
// byteing
{
int x = 101;
byte* bptr = reinterpret_cast<byte*>(&x);
for (auto i = 0; i < 4; i++) {
std::cout << int(*(bptr + i)) << ' ';
}
std::cout << std::endl;
}
// static_cast job
{
derived d;
d.x = 101;
d.y = 102;
base& b = reinterpret_cast<base&>(d);
std::cout << b.x << std::endl;
derived& dref = reinterpret_cast<derived&>(b);
std::cout << dref.x << ":" << dref.y << std::endl;
}
// void*
{
char name[] = "hakan gedek";
void* vptr = reinterpret_cast<void*>(name);
std::cout << "address: " << vptr << std::endl;
const char* cptr = reinterpret_cast<char const*>(vptr);
std::cout << strlen(cptr) << std::endl;
std::cout << cptr << std::endl;
}
</code></pre>
<p>check also <strong>how to use casting correctly</strong> note</p>
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