statcast.cpp

Think in C++ 第二版源码

//: C24:Statcast.cpp
// From Thinking in C++, 2nd Edition
// Available at http://www.BruceEckel.com
// (c) Bruce Eckel 1999
// Copyright notice in Copyright.txt
// Examples of static_cast

class Base { /* ... */ };
class Derived : public Base {
public:
  // ...
  // Automatic type conversion:
  operator int() { return 1; }
};

void func(int) {}

class Other {};

int main() {
  int i = 0x7fff; // Max pos value = 32767
  long l;
  float f;
  // (1) typical castless conversions:
  l = i;
  f = i;
  // Also works:
  l = static_cast<long>(i);
  f = static_cast<float>(i);

  // (2) narrowing conversions:
  i = l; // May lose digits
  i = f; // May lose info
  // Says "I know," eliminates warnings:
  i = static_cast<int>(l);
  i = static_cast<int>(f);
  char c = static_cast<char>(i);

  // (3) forcing a conversion from void* :
  void* vp = &i;
  // Old way produces a dangerous conversion:
  float* fp = (float*)vp;
  // The new way is equally dangerous:
  fp = static_cast<float*>(vp);

  // (4) implicit type conversions, normally
  // Performed by the compiler:
  Derived d;
  Base* bp = &d; // Upcast: normal and OK
  bp = static_cast<Base*>(&d); // More explicit
  int x = d; // Automatic type conversion
  x = static_cast<int>(d); // More explicit
  func(d); // Automatic type conversion
  func(static_cast<int>(d)); // More explicit

  // (5) Static Navigation of class hierarchies:
  Derived* dp = static_cast<Derived*>(bp);
  // ONLY an efficiency hack. dynamic_cast is
  // Always safer. However:
  // Other* op = static_cast<Other*>(bp);
  // Conveniently gives an error message, while
  Other* op2 = (Other*)bp;
  // Does not.
} ///:~