关于implicit_cast的作用

[yinghaoyu]

相关文件muduo/base/Types.h

template<typename To, typename From>     // use like this: down_cast<T*>(foo);
inline To down_cast(From* f)                     // so we only accept pointers
{
  // Ensures that To is a sub-type of From *.  This test is here only
  // for compile-time type checking, and has no overhead in an
  // optimized build at run-time, as it will be optimized away
  // completely.
  if (false)
  {
    implicit_cast<From*, To>(0);
  }

#if !defined(NDEBUG) && !defined(GOOGLE_PROTOBUF_NO_RTTI)
  assert(f == NULL || dynamic_cast<To>(f) != NULL);  // RTTI: debug mode only!
#endif
  return static_cast<To>(f);
}

我试过很多情况，这里的implicit_cast并不能在编译期间发现问题，都在运行时dynamic_cast才检查出了错误。g++ version 9.4。

[yinghaoyu]

@chenshuo 陈硕大大可以来回答一下么

[chenshuo]

implicit_cast 的作用是

// Ensures that *To* is a sub-type of *From*. 

它是静态类型检查，当然不能完全代替 dynamic_cast，它连参数 f 都没用到。

它的作用是让编译器在 From 不是 To 的 base class 的时候报错。

[yinghaoyu]

我试了一下这个例子：

#include <assert.h>
#include <sched.h>
#include <cstring>
#include <iostream>

using namespace std;

template <typename To, typename From>
inline To implicit_cast(From const& f) {
  return f;
}

template <typename To, typename From>  // use like this: down_cast<T*>(foo);
inline To down_cast(From* f)           // so we only accept pointers
{
  // Ensures that To is a sub-type of From *.  This test is here only
  // for compile-time type checking, and has no overhead in an
  // optimized build at run-time, as it will be optimized away
  // completely.
  if (false) {
    implicit_cast<From*, To>(0);
  }

#if !defined(NDEBUG) && !defined(GOOGLE_PROTOBUF_NO_RTTI)
  assert(f == NULL || dynamic_cast<To>(f) != NULL);  // RTTI: debug mode only!
#endif
  return static_cast<To>(f);
}

class Animal {
 public:
  virtual ~Animal() {}
};

class Dog : public Animal {
 public:
  void Bark() {}
};

class Cat : public Animal {
 public:
  void Meow() {}
};
int main() {
  Animal* animal = new Cat;
  Cat* cat = new Cat;
  Dog* dog = down_cast<Dog*>(animal);  // 1
  Dog* dogy = down_cast<Dog*>(cat);  // 2
  return 0;
}

g++ version 9.4这两步分别测试如下：

1. 编译时不出错，只有在运行时出错。
2. 编译时static_cast就能检测出错误，implicit_cast虽然也能检测出错，但好像多此一举？

[chenshuo]

我没有细想，按你的实验结果，可能确实是多此一举。