Case-insensitive string comparison in C++

Boost includes a handy algorithm for this:

#include <boost/algorithm/string.hpp>
// Or, for fewer header dependencies:
//#include <boost/algorithm/string/predicate.hpp>

std::string str1 = "hello, world!";
std::string str2 = "HELLO, WORLD!";

if (boost::iequals(str1, str2))
{
    // Strings are identical
}

If you are on a POSIX system, you can use strcasecmp. This function is not part of standard C, though, nor is it available on Windows. This will perform a case-insensitive comparison on 8-bit chars, so long as the locale is POSIX. If the locale is not POSIX, the results are undefined (so it might do a localized compare, or it might not). A wide-character equivalent is not available.

Failing that, a large number of historic C library implementations have the functions stricmp() and strnicmp(). Visual C++ on Windows renamed all of these by prefixing them with an underscore because they aren’t part of the ANSI standard, so on that system they’re called _stricmp or _strnicmp. Some libraries may also have wide-character or multibyte equivalent functions (typically named e.g. wcsicmp, mbcsicmp and so on).

C and C++ are both largely ignorant of internationalization issues, so there's no good solution to this problem, except to use a third-party library. Check out IBM ICU (International Components for Unicode) if you need a robust library for C/C++. ICU is for both Windows and Unix systems.

Are you talking about a dumb case insensitive compare or a full normalized Unicode compare?

A dumb compare will not find strings that might be the same but are not binary equal.

Example:

U212B (ANGSTROM SIGN)
U0041 (LATIN CAPITAL LETTER A) + U030A (COMBINING RING ABOVE)
U00C5 (LATIN CAPITAL LETTER A WITH RING ABOVE).

Are all equivalent but they also have different binary representations.

That said, Unicode Normalization should be a mandatory read especially if you plan on supporting Hangul, Thaï and other asian languages.

Also, IBM pretty much patented most optimized Unicode algorithms and made them publicly available. They also maintain an implementation : IBM ICU

My first thought for a non-unicode version was to do something like this:

bool caseInsensitiveStringCompare(const string& str1, const string& str2) {
    if (str1.size() != str2.size()) {
        return false;
    }
    for (string::const_iterator c1 = str1.begin(), c2 = str2.begin(); c1 != str1.end(); ++c1, ++c2) {
        if (tolower(static_cast<unsigned char>(*c1)) != tolower(static_cast<unsigned char>(*c2))) {
            return false;
        }
    }
    return true;
}

You can use strcasecmp on Unix, or stricmp on Windows.

One thing that hasn't been mentioned so far is that if you are using stl strings with these methods, it's useful to first compare the length of the two strings, since this information is already available to you in the string class. This could prevent doing the costly string comparison if the two strings you are comparing aren't even the same length in the first place.

I'm trying to cobble together a good answer from all the posts, so help me edit this:

Here is a method of doing this, although it does transforming the strings, and is not Unicode friendly, it should be portable which is a plus:

bool caseInsensitiveStringCompare( const std::string& str1, const std::string& str2 ) {
    std::string str1Cpy( str1 );
    std::string str2Cpy( str2 );
    std::transform( str1Cpy.begin(), str1Cpy.end(), str1Cpy.begin(), ::tolower );
    std::transform( str2Cpy.begin(), str2Cpy.end(), str2Cpy.begin(), ::tolower );
    return ( str1Cpy == str2Cpy );
}

From what I have read this is more portable than stricmp() because stricmp() is not in fact part of the std library, but only implemented by most compiler vendors.

To get a truly Unicode friendly implementation it appears you must go outside the std library. One good 3rd party library is the IBM ICU (International Components for Unicode)

Also boost::iequals provides a fairly good utility for doing this sort of comparison.

Short and nice. No other dependencies, than extended std C lib.

strcasecmp(str1.c_str(), str2.c_str()) == 0

returns true if str1 and str2 are equal. strcasecmp may not exist, there could be analogs stricmp, strcmpi, etc.

Example code:

#include <iostream>
#include <string>
#include <string.h> //For strcasecmp(). Also could be found in <mem.h>

using namespace std;

/// Simple wrapper
inline bool str_ignoreCase_cmp(std::string const& s1, std::string const& s2) {
    if(s1.length() != s2.length())
        return false;  // optimization since std::string holds length in variable.
    return strcasecmp(s1.c_str(), s2.c_str()) == 0;
}

/// Function object - comparator
struct StringCaseInsensetiveCompare {
    bool operator()(std::string const& s1, std::string const& s2) {
        if(s1.length() != s2.length())
            return false;  // optimization since std::string holds length in variable.
        return strcasecmp(s1.c_str(), s2.c_str()) == 0;
    }
    bool operator()(const char *s1, const char * s2){ 
        return strcasecmp(s1,s2)==0;
    }
};


/// Convert bool to string
inline char const* bool2str(bool b){ return b?"true":"false"; }

int main()
{
    cout<< bool2str(strcasecmp("asd","AsD")==0) <<endl;
    cout<< bool2str(strcasecmp(string{"aasd"}.c_str(),string{"AasD"}.c_str())==0) <<endl;
    StringCaseInsensetiveCompare cmp;
    cout<< bool2str(cmp("A","a")) <<endl;
    cout<< bool2str(cmp(string{"Aaaa"},string{"aaaA"})) <<endl;
    cout<< bool2str(str_ignoreCase_cmp(string{"Aaaa"},string{"aaaA"})) <<endl;
    return 0;
}

Output:

true
true
true
true
true

Visual C++ string functions supporting unicode: http://msdn.microsoft.com/en-us/library/cc194799.aspx

the one you are probably looking for is _wcsnicmp

FYI, strcmp() and stricmp() are vulnerable to buffer overflow, since they just process until they hit a null terminator. It's safer to use _strncmp() and _strnicmp().

Assuming you are looking for a method and not a magic function that already exists, there is frankly no better way. We could all write code snippets with clever tricks for limited character sets, but at the end of the day at somepoint you have to convert the characters.

The best approach for this conversion is to do so prior to the comparison. This allows you a good deal of flexibility when it comes to encoding schemes, which your actual comparison operator should be ignorant of.

You can of course 'hide' this conversion behind your own string function or class, but you still need to convert the strings prior to comparison.

I wrote a case-insensitive version of char_traits for use with std::basic_string in order to generate a std::string that is not case-sensitive when doing comparisons, searches, etc using the built-in std::basic_string member functions.

So in other words, I wanted to do something like this.

std::string a = "Hello, World!";
std::string b = "hello, world!";

assert( a == b );

...which std::string can't handle. Here's the usage of my new char_traits:

std::istring a = "Hello, World!";
std::istring b = "hello, world!";

assert( a == b );

...and here's the implementation:

/*  ---

        Case-Insensitive char_traits for std::string's

        Use:

            To declare a std::string which preserves case but ignores case in comparisons & search,
            use the following syntax:

                std::basic_string<char, char_traits_nocase<char> > noCaseString;

            A typedef is declared below which simplifies this use for chars:

                typedef std::basic_string<char, char_traits_nocase<char> > istring;

    --- */

    template<class C>
    struct char_traits_nocase : public std::char_traits<C>
    {
        static bool eq( const C& c1, const C& c2 )
        { 
            return ::toupper(c1) == ::toupper(c2); 
        }

        static bool lt( const C& c1, const C& c2 )
        { 
            return ::toupper(c1) < ::toupper(c2);
        }

        static int compare( const C* s1, const C* s2, size_t N )
        {
            return _strnicmp(s1, s2, N);
        }

        static const char* find( const C* s, size_t N, const C& a )
        {
            for( size_t i=0 ; i<N ; ++i )
            {
                if( ::toupper(s[i]) == ::toupper(a) ) 
                    return s+i ;
            }
            return 0 ;
        }

        static bool eq_int_type( const int_type& c1, const int_type& c2 )
        { 
            return ::toupper(c1) == ::toupper(c2) ; 
        }       
    };

    template<>
    struct char_traits_nocase<wchar_t> : public std::char_traits<wchar_t>
    {
        static bool eq( const wchar_t& c1, const wchar_t& c2 )
        { 
            return ::towupper(c1) == ::towupper(c2); 
        }

        static bool lt( const wchar_t& c1, const wchar_t& c2 )
        { 
            return ::towupper(c1) < ::towupper(c2);
        }

        static int compare( const wchar_t* s1, const wchar_t* s2, size_t N )
        {
            return _wcsnicmp(s1, s2, N);
        }

        static const wchar_t* find( const wchar_t* s, size_t N, const wchar_t& a )
        {
            for( size_t i=0 ; i<N ; ++i )
            {
                if( ::towupper(s[i]) == ::towupper(a) ) 
                    return s+i ;
            }
            return 0 ;
        }

        static bool eq_int_type( const int_type& c1, const int_type& c2 )
        { 
            return ::towupper(c1) == ::towupper(c2) ; 
        }       
    };

    typedef std::basic_string<char, char_traits_nocase<char> > istring;
    typedef std::basic_string<wchar_t, char_traits_nocase<wchar_t> > iwstring;

I've had good experience using the International Components for Unicode libraries - they're extremely powerful, and provide methods for conversion, locale support, date and time rendering, case mapping (which you don't seem to want), and collation, which includes case- and accent-insensitive comparison (and more). I've only used the C++ version of the libraries, but they appear to have a Java version as well.

Methods exist to perform normalized compares as referred to by @Coincoin, and can even account for locale - for example (and this a sorting example, not strictly equality), traditionally in Spanish (in Spain), the letter combination "ll" sorts between "l" and "m", so "lz" < "ll" < "ma".

Just a note on whatever method you finally choose, if that method happens to include the use of strcmp that some answers suggest:

strcmp doesn't work with Unicode data in general. In general, it doesn't even work with byte-based Unicode encodings, such as utf-8, since strcmp only makes byte-per-byte comparisons and Unicode code points encoded in utf-8 can take more than 1 byte. The only specific Unicode case strcmp properly handle is when a string encoded with a byte-based encoding contains only code points below U+00FF - then the byte-per-byte comparison is enough.

If you don't want to use Boost library then here is solution to it using only C++ standard io header.

#include <iostream>

struct iequal
{
    bool operator()(int c1, int c2) const
    {
        // case insensitive comparison of two characters.
        return std::toupper(c1) == std::toupper(c2);
    }
};

bool iequals(const std::string& str1, const std::string& str2)
{
    // use std::equal() to compare range of characters using the functor above.
    return std::equal(str1.begin(), str1.end(), str2.begin(), iequal());
}

int main(void)
{
    std::string str_1 = "HELLO";
    std::string str_2 = "hello";

    if(iequals(str_1,str_2))
    {
        std::cout<<"String are equal"<<std::endl;   
    }

    else
    {
        std::cout<<"String are not equal"<<std::endl;
    }


    return 0;
}