you could use the function toupper() which is included with the string library instead of using those c style casts

also you could just pass the parameter in your function as a refrence like so:

void conv(string& text)
{
    for (int i = 0; i < text.length(); i++)
        text[i] = toupper(text[i]);
}

then you can call it: conv(text); instead of text=conv(text);