cpp_casting.rst

Casting

Table of Contents

• C Style Casting
• Const Casting
• Reinterpret Casting
• Static Casting
• Dynamic Casting

C Style Casting

#include <iostream>
#include <cmath>

int main(int argc, char *argv[]) {
  double x = M_PI;
  int xx = (int) x;
  std::cout << xx << "\n";

  long z = LONG_MAX;
  int zz = (int) z;  // dangerous. overflow
  std::cout << zz << "\n";
}

Const Casting

#include <iostream>

void f(const int &x) {
  const_cast<int &>(x) = 0;
}

int main(int argc, char *argv[]) {
  int x = 123;
  f(x);
  std::cout << x << "\n";
}

Reinterpret Casting

#include <iostream>
#include <iomanip>

struct A { int x; int y; };

int main(int argc, char *argv[]) {
  A a{1, 2};

  // convert a struct to a byte array
  char *buf = reinterpret_cast<char *>(&a);
  for (int i = 0; i < sizeof(A); ++i) {
    std::cout << static_cast<int>(buf[i]) << " ";
  }
  // output: 1 0 0 0 2 0 0 0
}

Static Casting

#include <iostream>
#include <memory>

struct A {
  virtual void f() { std::cout << __func__ << "\n"; }
  virtual ~A() = default;
};

struct B : public A {;
  B(int *x) : x_{x} {}
  int *g() { return x_; }
  int *x_;
};

int main(int argc, char *argv[]) {
  auto a = std::make_unique<A>();
  // downcasting may be dangerous
  auto b = static_cast<B *>(a.get());
  auto x = b->g();
  std::cout << *x << "\n";
}

Dynamic Casting

#include <iostream>
#include <memory>

struct A {
  virtual void f() { std::cout << __func__ << "\n"; }
};

struct B : public A {
   void f() override { std::cout << __PRETTY_FUNCTION__ << "\n"; }
};

int main(int argc, char *argv[]) {
  auto a = std::make_unique<A>();
  auto b = std::make_unique<B>();

  // downcast
  auto bb = dynamic_cast<B *>(a.get());
  std::cout << "Is dynamic_cast(*a) to *b success? " << !!bb << "\n";
  // output: Is dynamic_cast(*a) to *b success? 0

  // upcast
  auto aa = dynamic_cast<A *>(b.get());
  std::cout << "Is dynamic_cast(*b) to *a success? " << !!aa << "\n";
  // output: Is dynamic_cast(*a) to *b success? 1
}