Question

Okay, I searched StackOverflow on how to check if a string is empty or just whitespace. But, it only works with ANSI strings. How can I get it to work with a wstring?

Here is the code:

#include <string>
using namespace std;

//! Checks if a string is empty or is whitespace.
bool IsEmptyOrSpace(const string& str) {
    string::const_iterator it = str.begin();

    do {
        if (it == str.end())
            return true;
    } while (*it >= 0 && *it <= 0x7f && isspace(*(it++)));
    // One of these conditions will be optimized away by the compiler.
    // Which one depends on whether the characters are signed or not.

    return false;
}

My first thought was to change isspace(*(it++)) to iswspace(*(it++)), but the two conditions before that will only work with ASCII, right? Here is what I have so far on attempting to adapt the function to wstring‘s:

bool IsEmptyOrSpaceW(const wstring& str) {
    String::const_iterator it = str.begin();

    do {
        if (it == str.end())
            return true;
    } while (*it >= 0 && *it <= 0x7f && iswspace(*(it++)));
    // One of these conditions will be optimized away by the compiler.
    // Which one depends on whether the characters are signed or not.

        // Do I need to change "*it >= 0 && *it <= 0x7f" to something else?

    return false;
}

Is my approach close to being correct? Either way, how can I implement a Unicode version of this IsEmptyOrSpace() function?

EDIT:
Okay, if you need to know why the *it >= 0 && *it <= 0x7f test is there, I cannot tell you, because I do not know. I got the code for the function from the answer to this question: C++ check if string is space or null
So let me start from scratch, how, in general, may I check if a wstring is EMPTY or just whitespace?

Answer 1

but the two conditions before that will only work with ASCII, right?

That’s right. They make sure the value conforms to the precondition of isspace: the argument “must have the value of an unsigned char or EOF”. Strictly speaking, you only need the *it >= 0 check, which should be optimised out if char is unsigned; alternatively, as mentioned in the comments, you could convert the value to unsigned.

iswspace has no such precondition, so just remove those checks from the wide version:

bool IsEmptyOrSpaceW(const wstring& str) {
    wstring::const_iterator it = str.begin();

    do {
        if (it == str.end())
            return true;
    } while (iswspace(*(it++)));

    return false;
}

As a matter of style, there’s no need to add a strange wart like W to indicate the parameter type, since you can just overload IsEmptyOrSpace with different parameter types.