Case-insensitive string comparison in C++


What is the best way of doing case-insensitive string comparison in C++ without transforming a string to all uppercase or all lowercase?

Please indicate whether the methods are Unicode-friendly and how portable they are.

8/23/2017 4:35:26 PM

Accepted Answer

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
6/22/2012 6:28:45 PM

Take advantage of the standard char_traits. Recall that a std::string is in fact a typedef for std::basic_string<char>, or more explicitly, std::basic_string<char, std::char_traits<char> >. The char_traits type describes how characters compare, how they copy, how they cast etc. All you need to do is typedef a new string over basic_string, and provide it with your own custom char_traits that compare case insensitively.

struct ci_char_traits : public char_traits<char> {
    static bool eq(char c1, char c2) { return toupper(c1) == toupper(c2); }
    static bool ne(char c1, char c2) { return toupper(c1) != toupper(c2); }
    static bool lt(char c1, char c2) { return toupper(c1) <  toupper(c2); }
    static int compare(const char* s1, const char* s2, size_t n) {
        while( n-- != 0 ) {
            if( toupper(*s1) < toupper(*s2) ) return -1;
            if( toupper(*s1) > toupper(*s2) ) return 1;
            ++s1; ++s2;
        return 0;
    static const char* find(const char* s, int n, char a) {
        while( n-- > 0 && toupper(*s) != toupper(a) ) {
        return s;

typedef std::basic_string<char, ci_char_traits> ci_string;

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