Output overloading

This is a discussion on Output overloading within the C++ Programming forums, part of the General Programming Boards category; I came up with a contradiction between the rules for overloading the output operator and what my code allowed me ...

  1. #1
    (?<!re)tired Mario F.'s Avatar
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    Output overloading

    I came up with a contradiction between the rules for overloading the output operator and what my code allowed me to do when I tried to provoque the error.

    The class declares the operator unfriendly:
    Code:
    class Opers {
    public:
    /* ... */
        std::ostream& operator<<(std::ostream&, const Opers&);
    private:
    /*... */
    };
    The cpp file for the class goes on defining it:
    Code:
    #include "oper.h"
    std::ostream& operator<<(std::ostream &os, const Opers &obj) {
        os << obj.val() << " - " << obj.text();
        return os;
    }
    No errors, no warnings and the test in main() outputs correctly. This shouldn't happen from what I've been learning. The operator, being declared a member of Oper, should have the implicit This as the first parameter...

    What am I missing?
    Originally Posted by brewbuck:
    Reimplementing a large system in another language to get a 25% performance boost is nonsense. It would be cheaper to just get a computer which is 25% faster.

  2. #2
    C++ Witch laserlight's Avatar
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    The operator, being declared a member of Oper, should have the implicit This as the first parameter...
    Since operator<< is dyadic, an implicit this cannot be.

    Have you actually tried to use the function?

    My guess is that the operator<< member function is simply an error, but since it was never used, it never was a problem.

    The actual operator<< free function works because it doesnt need to be a friend function. However, since it is declared in the source file, you would not be able to use it since a file that includes oper.h would not have its prototype.
    Quote Originally Posted by Bjarne Stroustrup (2000-10-14)
    I get maybe two dozen requests for help with some sort of programming or design problem every day. Most have more sense than to send me hundreds of lines of code. If they do, I ask them to find the smallest example that exhibits the problem and send me that. Mostly, they then find the error themselves. "Finding the smallest program that demonstrates the error" is a powerful debugging tool.
    Look up a C++ Reference and learn How To Ask Questions The Smart Way

  3. #3
    (?<!re)tired Mario F.'s Avatar
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    I'm not sure if I understand Laserlight.

    I have it prototyped in the class, much like I would have any other function member that is later defined on the cpp file.

    The text in main is called with a simple std::cout << obj; (where obj is an object of Opers type). And it gave me the correct output.
    Originally Posted by brewbuck:
    Reimplementing a large system in another language to get a 25% performance boost is nonsense. It would be cheaper to just get a computer which is 25% faster.

  4. #4
    Just Lurking Dave_Sinkula's Avatar
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    Code:
    class Opers {
    public:
    /* ... */
        std::ostream& operator<<(std::ostream&, const Opers&);
    /*
     Error E2080: 'Opers::operator <<(std::ostream &,const Opers &)' 
     must be declared with one parameter
    */
    private:
    /*... */
    };
    7. It is easier to write an incorrect program than understand a correct one.
    40. There are two ways to write error-free programs; only the third one works.*

  5. #5
    (?<!re)tired Mario F.'s Avatar
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    Wait... I can see...

    My function definition is also a declaration.
    I didn't use the scope operator. I can understand what you mean by free function. The following does give me an error now.

    Code:
    #include "oper.h"
    std::ostream& Opers::operator<<(std::ostream &os, const Opers &obj) {
        os << obj.val() << " - " << obj.text();
        return os;
    }
    Only question now is why did I have access to that function before?
    Originally Posted by brewbuck:
    Reimplementing a large system in another language to get a 25% performance boost is nonsense. It would be cheaper to just get a computer which is 25% faster.

  6. #6
    C++ Witch laserlight's Avatar
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    I have it prototyped in the class, much like I would have any other function member that is later defined on the cpp file.
    But there is no operator<< that takes three arguments. operator<< is dyadic.

    The text in main is called with a simple std::cout << obj; (where obj is an object of Opers type). And it gave me the correct output.
    Amazing.

    Let's try a test:

    1. oper.h
    Code:
    #ifndef OPER_H
    #define OPER_H
    
    #include <iosfwd>
    
    class Opers {
    public:
    	std::ostream& operator<<(std::ostream&, const Opers&);
    	int val() const { return 0; }
    	int text() const { return 0; }
    };
    
    #endif
    2. oper.cpp
    Code:
    #include "oper.h"
    
    #include <ostream>
    
    std::ostream& operator<<(std::ostream& os, const Opers& obj) {
    	os << obj.val() << " - " << obj.text();
    	return os;
    }
    3. main.cpp
    Code:
    #include <iostream>
    
    #include "oper.h"
    
    int main() {
    	Opers test;
    	std::cout << test << std::endl;
    }
    What do you get?
    Quote Originally Posted by Bjarne Stroustrup (2000-10-14)
    I get maybe two dozen requests for help with some sort of programming or design problem every day. Most have more sense than to send me hundreds of lines of code. If they do, I ask them to find the smallest example that exhibits the problem and send me that. Mostly, they then find the error themselves. "Finding the smallest program that demonstrates the error" is a powerful debugging tool.
    Look up a C++ Reference and learn How To Ask Questions The Smart Way

  7. #7
    Just Lurking Dave_Sinkula's Avatar
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    I believe this is just a separate function.
    Code:
    std::ostream& operator<<(std::ostream &os, const Opers &obj) {
        os << obj.val() << " - " << obj.text();
        return os;
    }
    Code:
    #include <iostream>
    
    class Opers {
    };
    
    std::ostream& operator<< (std::ostream &os, const Opers &obj) {
        os << "hello world";
        return os;
    }
    
    int main() {
       Opers op;
       std::cout << op << '\n';
        return 0;
    }
    
    /* my output
    hello world
    */
    7. It is easier to write an incorrect program than understand a correct one.
    40. There are two ways to write error-free programs; only the third one works.*

  8. #8
    C++ Witch laserlight's Avatar
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    Only question now is why did I have access to that function before?
    You never did, since it was in a source file. Perhaps some build quirk? Did you remember to rebuild all to check?
    Quote Originally Posted by Bjarne Stroustrup (2000-10-14)
    I get maybe two dozen requests for help with some sort of programming or design problem every day. Most have more sense than to send me hundreds of lines of code. If they do, I ask them to find the smallest example that exhibits the problem and send me that. Mostly, they then find the error themselves. "Finding the smallest program that demonstrates the error" is a powerful debugging tool.
    Look up a C++ Reference and learn How To Ask Questions The Smart Way

  9. #9
    (?<!re)tired Mario F.'s Avatar
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    Ah! The Rebuild All did it. Thanks.

    Now I get the proper errors. Trying to declare operator<< with more than one argument inside the class and a no match error from trying to access the operator<< from main.

    My first time where this strategy to provoque errors to better understand what I'm learning, backfires on me.


    Thanks both again
    Originally Posted by brewbuck:
    Reimplementing a large system in another language to get a 25% performance boost is nonsense. It would be cheaper to just get a computer which is 25% faster.

  10. #10
    (?<!re)tired Mario F.'s Avatar
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    Trying not to start another thread on a related issue...

    It seems I'm not being able to understand correctly how to use the overload input operator.
    The following isn't working. The error states a no match for operator>> on the line colored red.

    Code:
    ------- Opers.h ------
    class Opers {
    public:
    /* ... */
        friend std::istream& operator>>(std::istream&, const Opers&);
    private:
        int val_;
        std::string str_;
    };
    
    ------- Opers.cpp ------
    /* ... */
    std::istream& operator>>(std::istream &is, const Opers &obj) {
        is >> obj.val_ >> obj.str_;
        if(!is)
            obj = Opers();
        return is;
    }
    The idea is to read two values from the stream. What am I missing?
    Originally Posted by brewbuck:
    Reimplementing a large system in another language to get a 25% performance boost is nonsense. It would be cheaper to just get a computer which is 25% faster.

  11. #11
    (?<!re)tired Mario F.'s Avatar
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    Terribly sorry. The second parameter can't be const. The error message was too cryptic for me to look at the obvious.
    Originally Posted by brewbuck:
    Reimplementing a large system in another language to get a 25% performance boost is nonsense. It would be cheaper to just get a computer which is 25% faster.

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