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]`

Answer 1

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 (ie this[i] is really *(this + i) - so not what's you are thinking it is). Access your internal storage member instead.

Answer 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.

Answer 3

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!