Subscript operator overloading woes (const related?)

This is a discussion on Subscript operator overloading woes (const related?) within the C++ Programming forums, part of the General Programming Boards category; I have the following code: Code: struct TwoInts { int val[2]; int &operator [] (unsigned int); TwoInts(int, int); }; int ...

  1. #1
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    Subscript operator overloading woes (const related?)

    I have the following code:
    Code:
    struct TwoInts
    {
    	int val[2];
    	int &operator [] (unsigned int);
    	TwoInts(int, int);
    };
    
    int &TwoInts::operator [] (unsigned int i) { return val[i]; }
    
    TwoInts::TwoInts(int a, int b) { val[0] = a; val[1] = b; }
    
    TwoInts operator + (const TwoInts &a, const TwoInts &b)
    {
    	return TwoInts(a[0]+b[0], a[1]+b[1]);
    }
    
    int main(int argc, char *argv[]) { return 0; }
    When I compile it, the only line of the + operator gives the following error four times:
    14 C:\test\main.cpp passing `const TwoInts' as `this' argument of `int& TwoInts::operator[](unsigned int)' discards qualifiers
    This despite the fact that it seems to me I'm doing nothing wrong. Sure, a and b are const, but I'm not modifying them, nor am I treating the float returned by the [] operator as non const. Any way to fix this?
    Typing stuff in Code::Blocks 8.02, compiling stuff with MinGW 3.4.5.

  2. #2
    ZuK
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    You're trying to call a non const member function on a const reference.
    You could try this way
    Code:
    #include <iostream>
    #include <string>
    
    using namespace std;
    
    struct TwoInts
    {
    	int val[2];
    	
    	int operator [] (unsigned int) const;
    	int & operator [] (unsigned int);
    	
    	TwoInts(int, int);
    };
    
    int  TwoInts::operator [] (unsigned int i) const { return val[i]; }
    int & TwoInts::operator [] (unsigned int i) { return val[i]; }
    
    TwoInts::TwoInts(int a, int b) { val[0] = a; val[1] = b; }
    
    TwoInts operator + (const TwoInts &a, const TwoInts &b)
    {
    	return TwoInts(a[0]+b[0], a[1]+b[1]);
    }
    
    int main(int argc, char *argv[]) { 
        return 0; 
    }
    Kurt
    Last edited by ZuK; 05-05-2007 at 11:37 AM.

  3. #3
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    Ah, now it compiles without errors.
    I take it it automatically uses the const function whenever it's called on a constant instance, and always uses the non-const function otherwise?
    Typing stuff in Code::Blocks 8.02, compiling stuff with MinGW 3.4.5.

  4. #4
    C++ Witch laserlight's Avatar
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    Yes. The rule of thumb is that subscript operators often come in pairs.
    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

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