Question

is it possible to make std::string always hold a lower-case string?
here’s how I would use it:

typedef std::basic_string<...> lowercase_string;

void myfunc()
{
  lowercase_string s = "Hello World"; // notice mixed case
  printf(s.c_str());                  // prints "hello world" in lowercase
  std::string s2 = s;
  printf(s2.c_str());                 // prints "hello world" in lowercase
}

Answer 1

You can write your own char traits and pass it to std::basic_string as second template argument.

Here is a minimal example:

template<typename T>
struct lowercase_char_traits : std::char_traits<T>
{
    static T* copy(T* dest, const T* src, std::size_t count )
    {
         for(size_t i = 0 ; i < count ; ++i)
              dest[i] = std::tolower(src[i]);
         return dest;
    }
    static void assign(T & out, T in)
    {
       out = std::tolower(in);
    }
    //implement other overload of assign yourself

    //note that you may have to implement other functionality 
    //depending on your requirement
};

And then define a typedef as:

typedef std::basic_string<char, lowercase_char_traits<char>> lowercase;

And here is a test program:

int main() 
{
    lowercase s1 = "Hello World";
    std::cout << s1.c_str() << std::endl;

    lowercase s2 = "HELLO WORLD";
    std::cout << std::boolalpha << (s1 == s2) << std::endl;

    lowercase s3 = "HELLO";
    s3 += " WorL";
    s3.append("D");
    std::cout << std::boolalpha << (s1 == s3) << std::endl;

    std::cout << s2.c_str() << std::endl;
    std::cout << s3.c_str() << std::endl;
}

Output:

hello world
true
true
hello world
hello world

Cool, isn’t it?

• Online demo

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Note that to have a fully-working lowercase string class, you may need to define other functionality of lowercase_char_traits also, depending on what behavior you want out of such class.

Have a look at the Herb Sutter brilliant article for details and explanation:

• So you want a case-insensitive string class? Your mission, should you choose to accept it, is to write one.

Hope that helps.