inheritance and constructors

This is a discussion on inheritance and constructors within the C++ Programming forums, part of the General Programming Boards category; Hi, Why can't I do the following: Code: #include <stdio.h> #include <stdlib.h> class A { public: A(); A(char *spam); }; ...

  1. #1
    Spam is Good
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    inheritance and constructors

    Hi,

    Why can't I do the following:

    Code:
    #include <stdio.h>
    #include <stdlib.h>
    
    class A {
    
    public:
      A();
      A(char *spam);
    
    };
    
    A::A() {
      fprintf(stderr, "spam is good\n");
    }
    
    A::A(char *spam) {
      fprintf(stderr, "spam is %s\n", spam);
    }
    
    class B : public A {
    
    };
    
    int main() {
    
      // works                                                                      
      A *a_name;
      a_name = new A("awesome");
    
      // doesn't work - how do I make it work?                                                             
      B *a_name;
      a_name = new B("great");
    
      return 0;
    }
    As I get the compile error:
    Code:
    navi:~/projects/0_sandbox/cpp> make test_inheriance
    g++ test_inheriance.cpp  -Wall -O -g -I.  -o test_inheriance
    test_inheriance.cpp: In function ‘int main()’:
    test_inheriance.cpp:31: error: conflicting declaration ‘B* a_name’
    test_inheriance.cpp:27: error: ‘a_name’ has a previous declaration as ‘A* a_name’
    test_inheriance.cpp:32: error: no matching function for call to ‘B::B(const char [6])’
    test_inheriance.cpp:20: note: candidates are: B::B()
    test_inheriance.cpp:20: note:                 B::B(const B&)
    make: *** [test_inheriance] Error 1
    Thx.
    "What comes around, goes around"

  2. #2
    Captain Crash brewbuck's Avatar
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    Just because class A has a particular constructor doesn't mean class B does. For B to support the same constructor as A, you must implement it explicitly and pass the parameter through:

    Code:
    class B : public A
    {
    public:
        B( char *x )
            : A( x )
        {
        }
    };
    Code:
    //try
    //{
    	if (a) do { f( b); } while(1);
    	else   do { f(!b); } while(1);
    //}

  3. #3
    30 Helens Agree neandrake's Avatar
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    Also the first error being reported is because you're re-declaring the same variable, a_name. Call it something else.
    Environment: OS X, GCC / G++
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  4. #4
    Student legit's Avatar
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    It's also a good idea to implement a virtual destructor in the base class when using inheritance.
    MSDN <- Programmers Haven!

  5. #5
    C++まいる!Cをこわせ!
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    It's also a good idea to use std::string rather than char*.
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

  6. #6
    Registered User whiteflags's Avatar
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    Well, not really, since stdio.h is being used.

  7. #7
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    Quote Originally Posted by whiteflags View Post
    Well, not really, since stdio.h is being used.
    That's irrelevant to considerations of using std::string versus char *.

    More generally, there is no particular reason that method/API used to output data determines what data structures are used to hold that data.
    Right 98% of the time, and don't care about the other 3%.

    If I seem grumpy in reply to you, it is likely you deserve it. Suck it up, sunshine, and read this, this, and this before posting again.

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