What This Can't Do It can't guarantee something is "ANSI" — it just tells you "not valid UTF-8", so we assume ANSI. If the string contains only ASCII characters, both UTF-8 and ANSI are valid — you can't distinguish.

ansiToUtf8.cpp

#include <iostream>
#include <string>
#include <locale>
#include <codecvt>

std::string ansiToUtf8(const std::string& ansiStr) {
    // Convert ANSI (assumed to be Latin1/Windows-1252) to UTF-16
    std::wstring_convert<std::codecvt<wchar_t, char, std::mbstate_t>> converter;
    std::wstring wideStr = converter.from_bytes(ansiStr);

    // Convert UTF-16 (wide string) to UTF-8
    std::wstring_convert<std::codecvt_utf8_utf16<wchar_t>> utf8Converter;
    return utf8Converter.to_bytes(wideStr);
}

isValidUtf8.cpp

#include <iostream>
#include <string>
#include <locale>
#include <codecvt>

bool isValidUtf8(const std::string& str) {
    size_t i = 0;
    while (i < str.size()) {
        unsigned char c = static_cast<unsigned char>(str[i]);

        int bytes = 0;
        if ((c & 0x80) == 0) {
            bytes = 1; // ASCII
        } else if ((c & 0xE0) == 0xC0) {
            bytes = 2;
        } else if ((c & 0xF0) == 0xE0) {
            bytes = 3;
        } else if ((c & 0xF8) == 0xF0) {
            bytes = 4;
        } else {
            return false; // invalid leading byte
        }

        if (i + bytes > str.size()) return false;

        for (int j = 1; j < bytes; ++j) {
            if ((static_cast<unsigned char>(str[i + j]) & 0xC0) != 0x80) {
                return false; // invalid continuation byte
            }
        }
        i += bytes;
    }
    return true;
}

usage.cpp

int main() {
    std::string maybeAnsi = "\xE9"; // ANSI 'é'
    std::string converted = ansiToUtf8(maybeAnsi);
    std::cout << "UTF-8: " << converted << std::endl;
}