Is This Legal C++? [CRTP]

This is a discussion on Is This Legal C++? [CRTP] within the C++ Programming forums, part of the General Programming Boards category; Is this usage of the Curiously Recurring Template Pattern legal? Can I call a derived member function from a base ...

  1. #1
    Even death may die... Dante Shamest's Avatar
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    Is This Legal C++? [CRTP]

    Is this usage of the Curiously Recurring Template Pattern legal?

    Can I call a derived member function from a base class constructor like this?

    Code:
    #include <cstdio>
    
    using std::printf;
    
    template <typename T>
    class Base {
    public:
      Base() { static_cast<T*>(this)->echo(); }
      void echo() { printf("Base"); }
    };
    
    class Derived: public Base<Derived> {
    public:
      void echo() { printf("Derived"); }
    };
    
    int main()
    {
      Derived derived;
    }
    My output is "Derived".

  2. #2
    Registered User Codeplug's Avatar
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    Yes. The object is fully constructed once inside the constructor body. You can then up or down cast this, as long as it's done in a safe manner.

    gg

  3. #3
    Even death may die... Dante Shamest's Avatar
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    Thanks for responding Codeplug.

    My assumption is that the key reason this technique works, is because Base knows Derived exists, and just uses a Derived pointer, and not a concrete Derived object.

    So without templates, I might have tried something like this?

    Code:
    #include <cstdio>
    
    class Base{
    public:
      Base();
      void echo() { printf( "Echo" ); }
    };
    
    class Derived : public Base{
    public:
      void echo(){ printf("Derived"); }
    };
    
    Base::Base(){
      (static_cast<Derived *>(this))->echo();
    }
    
    int main() {
      Derived derived;
    }

  4. #4
    Even death may die... Dante Shamest's Avatar
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    I've emailed Bjarne Stroustrup. He says my template version is totally illegal, it may work on most implementations, but it's not guaranteed.

    Details can be found here.

  5. #5
    Skunkmeister Stoned_Coder's Avatar
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    it is illegal because you are using a derived object before it is fully constructed. Sure it may work especially if derived adds no data members but it is definately illegal.
    make the fuction virtual. Notice the difference. Now explain why base::echo() would be called if the function was virtual.
    Free the weed!! Class B to class C is not good enough!!
    And the FAQ is here :- http://faq.cprogramming.com/cgi-bin/smartfaq.cgi

  6. #6
    Even death may die... Dante Shamest's Avatar
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    make the fuction virtual. Notice the difference. Now explain why base::echo() would be called if the function was virtual.
    Because derived is not constructed yet.

    I'm aware of this, that's why I tried to cheat using templates instead of virtual.

  7. #7
    Skunkmeister Stoned_Coder's Avatar
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    then you answered your own question
    Free the weed!! Class B to class C is not good enough!!
    And the FAQ is here :- http://faq.cprogramming.com/cgi-bin/smartfaq.cgi

  8. #8
    Even death may die... Dante Shamest's Avatar
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    My question was about templates, not virtual functions.

    And my reason for asking: the template code works on the compiler I tested.

  9. #9
    Registered User Codeplug's Avatar
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    >> The object is fully constructed once inside the constructor body.
    I thought objects were constructed in reverse order on Saturdays.....oh well.

    gg

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