Converting std::string to upper case: major performance difference?

Question

So I was playing around with some code and wanted to see which method of converting a std::string to upper case was most efficient. I figured that the two would be somewhat similar performance-wise, but I was terribly wrong. Now I'd like to find out why.

The first method of converting the string works as follows: for each character in the string (save the length, iterate from 0 to length), if it's between 'a' and 'z', then shift it so that it's between 'A' and 'Z' instead.

The second method works as follows: for each character in the string (start from 0, keep going till we hit a null terminator), apply the build in toupper() function.

Here's the code:

#include <iostream>
#include <string>

inline std::string ToUpper_Reg(std::string str)
{
    for (int pos = 0, sz = str.length(); pos < sz; ++pos)
    {
        if (str[pos] >= 'a' && str[pos] <= 'z') { str[pos] += ('A' - 'a'); }
    }

    return str;
}

inline std::string ToUpper_Alt(std::string str)
{
    for (int pos = 0; str[pos] != '\0'; ++pos) { str[pos] = toupper(str[pos]); }

    return str;
}


int main()
{
    std::string test = " abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789~!@#$%^&*()_+=-`'{}[]\\|\";:<>,./?";

    for (size_t i = 0; i < 100000000; ++i) { ToUpper_Reg(test); /* ToUpper_Alt(test); */ }

    return 0;
}

The first method ToUpper_Reg took about 169 seconds per 100 million iterations.
The second method Toupper_Alt took about 379 seconds per 100 million iterations.

What gives?

---

Edit: I changed the second method so that it iterates the string how the first one does (set the length aside, loop while less than length) and it's a bit faster, but still about twice as slow.

---

Edit 2: Thanks everybody for your submissions! The data I'll be using it on is guaranteed to be ascii, so I think I'll be sticking with the first method for the time being. I'll keep in mind that toupper is locale specific for when/if I need it.

Answer 1

std::toupper uses the current locale to do case conversions, which involves a function call and other abstractions. So naturally, it will be slower. But it will also work on non-ASCII text.

Answer 2

toupper() does more than just shift characters in the range [a-z]. For one thing it's locale dependent and can handle more than just ASCII.