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<h1 id="cast-stdshared_ptrs"><a class="header" href="casting_shared_ptr.html#cast-stdshared_ptrs">cast std::shared_ptrs</a></h1>
<p>std library has necessary casting methods</p>
<ul>
<li>static_pointer_cast</li>
<li>dynamic_pointer_cast</li>
<li>const_pointer_cast</li>
<li>reinterpret_pointer_cast</li>
</ul>
<blockquote>
<p>the result will share ownership with arg. if casting fails dynamic_pointer_cast returns nullptr like dynamic_cast so ownership fails either.</p>
</blockquote>
<pre><code class="language-c++">
class base {
public:
base() = default;
base(int x) : x(x) {}
virtual ~base() = default;
virtual void print() const {
std::cout << "base..." << "x: " << x << std::endl;
}
int x = 0;
};
class derived : public base {
public:
derived() = default;
~derived() = default;
derived(int x, int y) : base(x), y(y) {}
void print() const override {
std::cout << "derived..." << "x: " << x << " y: " << y << std::endl;
}
int y = 0;
};
int main() {
using std::shared_ptr;
{
shared_ptr<const derived> csp = std::make_shared<const derived>(1, 2);
shared_ptr<derived> sp = const_pointer_cast<derived>(csp);
sp->print();
}
{
{
// origin: derived
// results: derived
shared_ptr<base> bp = std::make_shared<derived>(1, 2);
bp->print();
auto dp = static_pointer_cast<derived>(bp);
dp->print();
}
{
// origin: base
// results: base
shared_ptr<base> bp = std::make_shared<base>(1);
bp->print();
auto dp = static_pointer_cast<derived>(bp);
dp->print();
}
}
{
{
// origin: derived
// results: ok & derived
shared_ptr<base> bp = std::make_shared<derived>(1, 2);
bp->print();
if (auto dp = dynamic_pointer_cast<derived>(bp); dp) {
dp->print();
}
}
{
// origin: base
// results: fail & base
shared_ptr<base> bp = std::make_shared<base>(1);
bp->print();
// fails
if (auto dp = dynamic_pointer_cast<derived>(bp); dp) {
dp->print();
}
}
}
{
{
// origin: derived
// results: ok & derived
shared_ptr<base> bp = std::make_shared<derived>(1, 2);
bp->print();
if (auto dp = reinterpret_pointer_cast<derived>(bp); dp) {
dp->print();
}
}
{
// origin: base
// results: ok & base
shared_ptr<base> bp = std::make_shared<base>(1);
bp->print();
// fails => dp acts as base type and derived part sliced
if (auto dp = reinterpret_pointer_cast<derived>(bp); dp) {
dp->print();
}
}
}
}
</code></pre>
<blockquote>
<p>shared_ptr has a special ctor for these casting calls. in cppreference page you can find how they are implemented but in short, they get the raw pointer and use static_cast, const_cast, dynamic_cast ar reinterpret_cast on it. then create another shared_ptr to share ownership and returns the result...important point here is you should never touch inner raw pointer like deleting etc... you have to keep alive it because shared_ptr will use it for value reading/writing</p>
</blockquote>
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