How do I overload the == operator for the string class in c++?

Question

I am a new c++ programmer and I only recently learned about operator overloading. While working on an independent project I came across an issue, I wanted to compare user input strings with other strings in order to allow the user to navigate around a simple menu. The only thing is I don't know how to compare two strings while ignoring the case. If there is a much simpler way of doing this rather than overloading the == operator, please let me know but also let me know how to overload the == operator for strings because I am very interested.

What a great community. Thanks a lot guys, you answered my question very quick without making me feel dumb!

I've already read that thread. It did not answer what I was asking. — Anton Yershov, Mar 19 '13 at 07:58
There is related example [here](http://stackoverflow.com/questions/14819696/trouble-with-case-insensitive-partial-matching-two-strings). — juanchopanza, Mar 19 '13 at 07:58
Sweet thanks, but I think my question was answered. Waiting 3 minutes to pick accepted answer. :) — Anton Yershov, Mar 19 '13 at 08:02

Armen Tsirunyan · Accepted Answer · 2013-03-19T08:11:07.670

5

Well, I need to make several points here.

If by string you mean char arrays/pointers, then you cannot overload operator ==, since operator overloading is allowed only for user-defined types
If by strings you mean std::string, then you can't overload operator == either, since it is already overloaded :)
In order to do case-insensitive comparison, the best approach is to have a named function such as case_insensitive_equal. Boost has one - boost::iequals(str1, str2)
You could attempt to write your own char_traits to create a case insensitive string type

As to how to write a function comparing strings in case insensitive manner, I'd do this:

bool case_insensitive_equal(const std::string& s1, const std::string& s2)
{
    if(s1.length() != s2. length())
       return false;
    for(int i = 0; i < s1.length(); ++i)
        if(std::toupper(s1[i]) != std::toupper(s2[i])) //tolower would do as well
            return false;
    return true;
}

Instead of loops you could use std::transform and std::equal,but I think this is more efficient.

edited Mar 19 '13 at 08:11

answered Mar 19 '13 at 07:55

Armen Tsirunyan

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Thanks a lot! This really helps. But why do you need to put the & in front of s1 and s2? You aren't really changing the values of the strings in this case. – Anton Yershov Mar 19 '13 at 08:04
@Anton: Yes, indeed, I'm not, that's why there's a `const` there. I am passing the arguments by reference to avoid unnecessary copying – Armen Tsirunyan Mar 19 '13 at 08:05
Okay so this is just to save memory? – Anton Yershov Mar 19 '13 at 08:05
@AntonYershov: Avoiding copying doesn't save memory. It saves time. If you had two 1MB strings it would take quite some time to copy them into the function whereas there is absolutely no need to – Armen Tsirunyan Mar 19 '13 at 08:08
I think you can use `std::equal` without `std::transform`. There is an overload that takes a binary predicate. – juanchopanza Mar 19 '13 at 08:10
@juanchopanza: Yeah, you're right, I forgot :) – Armen Tsirunyan Mar 19 '13 at 08:11
Your code would only work for ascii based locales... – PlasmaHH Mar 19 '13 at 09:30

score 2 · Answer 2 · answered Mar 19 '13 at 07:56

2

You don't overload operator== for std::string because you would have to put the operator into the std namespace. This is (a) not allowed and (b) has already been done by the standard library implementation.

It is also inadvisable to overload any operator== to mean anything other than a true equality. I.e. if a == b then a and b should behave identically.

What you should prefer to do is write a separate function with a meaningful name, e.g.

bool areEqualIgnoringCase(const std::string&, const std::string&);

answered Mar 19 '13 at 07:56

CB Bailey

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Not sure you would need to modify the `std` namespace... – Alex Chamberlain Mar 19 '13 at 07:56
@AlexChamberlain: How else would you write `operator==` for `std::string`? – CB Bailey Mar 19 '13 at 07:58
`namespace ADC { bool operator==(std::string const&, std::string const&); }` Not sure it would work for `std::string` which already has an `operator==`. – Alex Chamberlain Mar 19 '13 at 08:00
@AlexChamberlain: Sure, if you're willing to call your operator like this: `ADC::operator == (s1, s2);` :D – Armen Tsirunyan Mar 19 '13 at 08:09

How do I overload the == operator for the string class in c++?

2 Answers2