Logical Comparison == operator overload

Question

I need to do some logical comparison and return a boolean answer.

Here is the code from the .cpp file:

bool MyString::operator==(const MyString& other)const
{
    if(other.Size == this.Size)
    {
            for(int i = 0; i < this.Size+1; i++)
            {
                    if(this[i] == other[i])

                            return true;
            }
    }
    else
            return false;
}

Here is what is called from main.cpp file:

 if (String1 == String4)
 {
    String3.Print ();
 }
 else
 {
     String4.Print ();
 }

Here are there compiling errors I get:

error: request for member `Size` in `this`, which is of non-class type `const MyString* const`
error: no match for `operator[]` in `other[i]`

if(this[i] == other[i]) return true; This is going to cause you problems later. Think about what you're doing there. — Qaz
– Qaz, Commented Apr 28, 2012 at 22:59
That is actually the only error I am getting now. All I want to do is compare the contents of both the strings. How could I possibly do that without having to overload the [] operators as well? — user1363061
– user1363061, Commented Apr 29, 2012 at 0:01

Griwes · Accepted Answer · 2012-04-29 14:15:10Z

4

this is a pointer, hence you have to dereference it:

this->Size;

Also I think that logic of your operator== is flawed - here, it returns true if any of characters is equal to character on same position in second string. Change your loop to

        for(int i = 0; i < this->Size+1; i++)
        {
                if(this[i] != other[i])

                        return false;
        }

and put return true; instead of last part of your code (else clause) to compare entire strings.

As Seth mentioned, you can't use operator[] on this as above - this way it's treated as array (i.e. this[i] is really *(this + i) - so not what's you are thinking it is). Access your internal storage member instead.

edited Apr 29, 2012 at 14:15

answered Apr 28, 2012 at 22:58

Griwes

9,0892 gold badges47 silver badges71 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Ben Voigt Over a year ago

You told him not to use this.Size, then you used it in your example? Also, (*this)[i] is a perfectly reasonable option (in case operator[] does something more complicated than indexing an array). I don't think the loop termination condition is right either.

juanchopanza Over a year ago

Or even better, use std::equal.

David Hammen · Accepted Answer · 2012-04-29 16:41:04Z

2

Problems with your code:

this[i]: You apparently want to access the ith character of the string here. This isn't doing that. Assuming your class overloads operator[], you want (*this)[i]. Alternatively, you could directly access the internal representation of the string.
if(this[i] == other[i]) return true;: Think about what this means with respect to comparing the strings "A1" and "AB".
for () {...}: What happens when you exit the loop? You need to return something if the comparisons manage to make it through the loop without returning.

edited Apr 29, 2012 at 16:41

answered Apr 28, 2012 at 23:28

David Hammen

33.3k9 gold badges64 silver badges110 bronze badges

5 Comments

user1363061 Over a year ago

the class actually doesn't overload operator [], in this case, what should I do? For this loop I just want to check if the two strings are the same in length and the same in content. If so, return true.

David Hammen Over a year ago

You access the internal representation of your string. You certainly have some data member that stores the contents of the string. So access it.

user1363061 Over a year ago

That's what I'm confused on and need help with.

David Hammen Over a year ago

You didn't show your class MyString. How are we supposed to know which element to access? I suggest you enjoy the weekend and go see a tutor on Monday. You appear to have deeper misunderstandings of objects and of C++ than how to overload operator ==.

Lara Bailey Over a year ago

(*this)[i] and this->[i] are very much not the same!

mloskot · Accepted Answer · 2012-04-28 23:42:22Z

You haven't specified if you can use the C++ standard algorithms or not. Here you have illustrated both versions, using hand-written loop and std::equal algorithm:

//#define USE_STD_ALGORITHM 1 // uncomment to enable std::equal version
#include <cassert>
#include <algorithm>
#include <stdexcept>

// NOTE: partial simplest definition for the test and presentation purposes only.
struct MyString
{
    MyString(char const* s, std::size_t size) : data(s), Size(size) {}
    char const& operator[](std::size_t index) const;
    bool operator==(const MyString& other) const;
private:
    char const* data;
    std::size_t Size;
};

char const& MyString::operator[](std::size_t index) const
{
    if (index < Size)
        return data[index];
    throw std::out_of_range("index invalid");
}

bool MyString::operator==(const MyString& other) const
{
    if (this->Size == other.Size)
    {
#ifdef  USE_STD_ALGORITHM
        return std::equal(data, data+Size, other.data);
#else
    bool equal = true;
    for(std::size_t i = 0; i < this->Size; ++i)
    {
        if((*this)[i] != other[i])
        {
            equal = false;
            break;
        }
    }
    return equal;
#endif
    }

    return false;
}

int main()
{
    char const* a = "abc";
    char const* b = "abc";
    MyString sa(a, 3);
    MyString sb(b, 3);
    assert(sa == sb);

    char const* c = "adc";
    MyString sc(c, 3);
    assert(!(sa == sc));

    char const* d = "ab";
    MyString sd(d, 2);
    assert(!(sa == sd));
}

Good luck!

Collectives™ on Stack Overflow

Logical Comparison == operator overload

3 Answers 3

2 Comments

5 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

2 Comments

5 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related