Cast base to derived to implement extending base API

Question

Currently, I am do a project to implement Unified API. For example for following code

class Helloworld {
 public:
  const std::string & str() const { return str_; }
  std::string & str() { return str_; }
  void str(const std::string & s) { str_ = s; }
 private:
  std::stringstr_;
};

Apparently, all APIs in Helloworld are very strange, I want to add following

const std::string & GetCRefStr() const { return str_; }
std::string & GetMutableStr() { return str_; }
void SetStr(const std::string & s) { str_ = s; }

For this goal, I use cast from base to derived

class HelloworldWrap : public Helloworld {
public:
  const std::string & GetCRefStr() const { return Helloworld::str(); }
  std::string & GetMutableStr() { return Helloworld::str(); }
  void SetStr(const std::string & s) { Helloworld::str(s); }

  // No any member in HelloworldWrap, this class just extend APIs for Helloworld
};

And for using

Helloworld kHW;

HelloworldWrap * GetMutableHelloworld() {
  return static_cast<HelloworldWrap *>(&kHW);
}

const HelloworldWrap & GetCRefHelloworld() {
  return *(static_cast<HelloworldWrap*>(&kHW));
}

int main() {
  GetMutableHelloworld()->SetStr("helloworld");
  // balabala
}

It works ok! Does this approach correct??

With `static_cast(&kHW)` you break [*strict aliasing*](https://stackoverflow.com/questions/98650/what-is-the-strict-aliasing-rule). The object `kHW` is *not* a `HelloworldWrap` object, and can't really be used as one. What you do leads to *undefined behavior*. — Some programmer dude, Jul 10 '23 at 06:24
as long as `HelloworldWrap` has no members this will probably work with most compilers but it is undefined behaviour, making an actual wrapper class rather than deriving would be a better approach — Alan Birtles, Jul 10 '23 at 07:08
**making an actual wrapper class rather than deriving** looks good, however it will bring performance issue. Consider this case: ```C++ HelloworldWrap * GetMutableHelloworld() { return an instance of HelloworldWrap, Yes, but, where to place it, as class member? For more complicated case, if return value is HelloworldWrap array } ``` I found this issue can not be solved in C++ except **Cast from base to derived** approach — mac.ma, Jul 10 '23 at 07:53
If it's implemented with inline functions it should be optimised away by the compiler — Alan Birtles, Jul 10 '23 at 10:29

Cast base to derived to implement extending base API

0 Answers0