operator== for struct

lordofring · 11-17-2005, 11:31 AM

I have 2 struct variables in a cpp file. I cannot use operator== for the struct.

Code:

#include <stdlib.h>
#include <stdio.h>

typedef struct myStruct_t
{
	int i;
	int j;
};

int main()
{
	myStruct_t a, b;
	a.i = b.i = 2;
	a.j = b.j = 3;

	if (a == b)
	{
		printf("a == b\n");
	}
	else
	{
		printf("a != b\n");
	}
	
	return 0;
}

Does the struct have the default behavior for the operator==?

I compiled it useing "g++ t.cpp" and got the following error message
t.cpp: In function `int main()':
t.cpp:16: error: no match for 'operator==' in 'a == b'

"g++ -v" output:
Reading specs from /usr/lib/gcc/i386-redhat-linux/3.4.2/specs
Configured with: ../configure --prefix=/usr --mandir=/usr/share/man --infodir=/usr/share/info --enable-shared --enable-threads=posix --disable-checking --with-system-zlib --enable-__cxa_atexit --disable-libunwind-exceptions --enable-java-awt=gtk --host=i386-redhat-linux
Thread model: posix
gcc version 3.4.2 20041017 (Red Hat 3.4.2-6.fc3)

AdaHacker · 11-17-2005, 04:07 PM

My C++ is pretty rusty, but I think the "default behavior" is that the == operator isn't defined for structs. Of course, you can always write one.

Code:

bool operator ==(myStruct_t a, myStruct_t b) {
	return (a.i == b.i) && (a.j == b.j);
}

xhi · 11-17-2005, 06:52 PM

exactly, no == is defined for structs... thats a good example from AdaHacker, except you should return int not bool..
IMO, you should use a class here, Im not a big fan of overloading at global level... structs are for data, class is for data and operations on that data which this would qualify as.. I would do somthing like this..

Code:

class Foo
{   
    public:
        Foo(){            
            x=0;
            y=0;
        }
        void setX(int xin){ x=xin; }      
        void setY(int yin){ y=yin; }       
        int getX()const{ return x; }
        int getY()const{ return y; }
        
        int operator==(const Foo& right)const{
            return (x == right.getX()) && (y == right.getY());
        }
        
    private:
        int x;
        int y;
};

eddiebaby1023 · 11-19-2005, 04:05 PM

Or use memcmp to compare the two structs - this has the advantage that you don't need to know the fields in the structures.

Code:

if (memcmp(&a, &b, sizeof(a)))
{
    printf("same\n");
}
else
{
    printf("differ\n");
}

(Not tested, but you get the idea.)

lordofring · 11-20-2005, 06:08 PM

Thanks for everyone's reply.

The reason, that I use the struct instead of the class, is for the public member variables and the light weight. If I use classes, my brain will ask me to write the set()/get() for the variables.

So what's the other difference between the struct and the class. I know all members is public for a struct. Anything elese? Thank you. And I guess a struct must has a default constructor and the default behavior for optertor=. So what's that?

lordofring · 11-20-2005, 06:09 PM

Thanks for everyone's reply.

The reason, that I use the struct instead of the class, is for the public member variables and the light weight. If I use classes, my brain will ask me to write the set()/get() for the variables.

So what's the other difference between the struct and the class. I know all members is public for a struct. Anything else? Thank you. And I guess a struct must has a default constructor and the default behavior for operator=. So what's that?

tuxdev · 11-20-2005, 06:17 PM

Operator = can be overloaded? That just seems so wrong that it would not be possible period.

xhi · 11-20-2005, 09:06 PM

>> So what's the other difference between the struct and the class.

Mainly personal preference.. a *C++* struct and class are not different except for the public/private defaults you have mentioned. Other than that there is nothing that I know of.. Honestly I dont know what the difference in overhead, it would seem like a struct is simpler, and would have less... however they can act identical.. so there may not be a difference...

I use classes because the feel more C++-ey to me.. When I think encapsulation I think class, when I think about defining a simple type like coordinates or somthing, I think struct.. The code that follows is some example.. I knew that in theory there was no difference in the two, but I had never experimented. I killed a few minutes this evening to find out for myself if there were any differences.. And the answer I proved was, no.. atleast not at the level I got to in this example... Check it out.. it does compile as is....

Code:

#include <iostream>

using namespace std;

/*
output
----------
FooBar:: outFooBar()
Bar:: outBar()
Foo:: out()
FooTwo:: outTwo()
Bar:: overloaded comparison
FooBar:: overloaded comaprison
*/

class Foo
{
    public:
        void outFoo(){ cout << "Foo:: out()" << endl; }
};

struct FooTwo
{
    void outTwo(){ cout << "FooTwo:: outTwo()" << endl; }
};

struct Bar : public Foo, public FooTwo
{
    void outBar(){
        cout << "Bar:: outBar()" << endl;
        outFoo();
        outTwo();
    }
    int operator==(const Bar& r) const{
        cout << "Bar:: overloaded comparison" << endl;
        return 0;
    }
};

class FooBar : public Bar
{
    public:
        void outFooBar(){
            cout << "FooBar:: outFooBar()" << endl;
            outBar();
        }
    int operator==(const FooBar& r) const{
        cout << "FooBar:: overloaded comaprison" << endl;
        return 0;
    }
};

int main(int c, char** v)
{
    FooBar fbA = FooBar();
    FooBar fbB = FooBar();
    Bar barA = Bar();
    Bar barB = Bar();

    fbA.outFooBar();  // test all of our print messages....
    barA==barB;     // test that comparison op in the struct
    fbA==fbB;       // ..
    return 0;
}

<edit> removed the virtual function example.. it didnt illustrate what i was thinking quite the way i was thinking it.. maybe next time</edit>

Mistro116@yahoo.com · 11-20-2005, 11:08 PM

Well theres only one thing I would add:

structs are super super helpful in C, but when it comes to C++ as it was stated, its a personal
preference between classes and structs. In C, structs let you have arrays that can be
dynamically allocated and all that good stuff, which you can't do with classes. In C++, its a
lot more flexible, kind of like java.

When you wrote your code, comparing a structure with more than one element using an ==
operator immediately sparked a warning, but I would if you could do it if it was a structure
with just one element. To compare structures, you usually need to write a function that
compares each element of the structure to each element of another true, and then return a
boolean in the case of C++ or an integer denoting true or false (1 or 0) in C.

If you get some time, try a structure with one element using the == operator, id like to see
the results and some people's feedback.

Hope this helps.
Mistro116

xhi · 11-20-2005, 11:50 PM

Quote:

To compare structures, you usually need to write a function that
compares each element of the structure to each element of another true, and then return a
boolean in the case of C++ or an integer denoting true or false (1 or 0) in C.

Thats exactly what you are doing with the operator==.. What I found out here is that a *C++* struct and *C* struct are two very different things... While C++ has given the struct the *same abilities* as a class, total encapsulation.. Like you said, in C you are forced to write other functions, as there is no concept of a member function.. I have never tried to put a function ptr into a struct.. maybe you could rig up somthing like that?? that would be some ugly code for sure.. anyhow I made a modification or two on the code...

here is a struct showing the use of the == operator with the multi member value struct.. its like a C struct on steroids

Code:

struct Fooey : public Bar
{
    Fooey():a(5),b(5){}

    // normal compare op you would see for objects
    int operator==(const Fooey& r) const{
        return ((a == r.a) && (b == r.b));
    }

    // illustration only..  very anti intuitive!
    int operator==(int r) const{
        return a==r);
    }
    int a, b;
};

here are a couple extra comparisons for main

Code:

    Fooey fooeyA;
    Fooey fooeyB;

    if(fooeyA==fooeyB) cout << "Fooey A and Fooey B *are* equal" << endl;
    if(fooeyA==5) cout << "The value a in Fooey A is " <<  fooeyA.a << endl;

lordofring · 11-21-2005, 12:09 PM

Quote:

If you get some time, try a structure with one element using the == operator, id like to see
the results and some people's feedback.

I tried this. It's the same as the 2 or more elements.

Summary: In C++, the struct is as same as the class, except that everything is public and no default functions.