C++ wstring - 读取字符串字符时出错 | 发生在第 10 个字符之后

Question

I am asking for information to save onto the database like their username and stuff, you can use up to 10 characters perfectly fine, the wstring holds it and I can use it how I like, but if a user were to type 11 characters it would all of a sudden say "Error reading characters of strings" .

At first I thought I didn't have a big enough space for GetWindowText so I pumped that up, I didn't know if you can change the wstring capacity so that is why I am asking here.

Why is wstring only working with 10 or less characters? Thanks!

case WM_COMMAND: {

    switch (LOWORD(wParam))
    {
        case IDB_REGISTERACCOUNT: {
            std::wstringstream SQLStatementStream;
            std::wstring SQLUsername;
            std::wstring SQLPassword;

            //Get user information than store in wide Strings
            GetWindowText(hUserNameRegister, &SQLUsername[0], 50);
            GetWindowText(hPasswordRegister, &SQLPassword[0], 50);

            std::wstring SQLStatement = SQLStatementStream.str();

            break;
        }
    }
    break;
}

Answer 1

Your code is essentially saying, "I am giving you the address of the first character of this string, start writing to it indiscriminately." This is going to be a bad time. What you need to do is first allocate an actual buffer to use to store the result, then if you wanted in the wstring, you can use the wstring's '=' operator to do the proper assignment.

something like:

WCHAR temp[50];
GetWindowText(hUserNameRegister, temp, 50);
SQLUsername = temp

Answer 2

You write past the end of your string. This causes undefined behaviour.

Only use &x[0] on a standard library string when you are in read-only mode, or if you are writing but not going to change the length of the string.

You should have written:

wchar_t SQLUsername[50] = {};
GetWindowTextW(hUserNameRegister, SQLUsername, 50);

Then you can convert to wstring if you wanted with std::wstring wsSQLUsername = SQLUsername; - however your code never actually uses your wstring s before they go out of scope so perhaps you have some other misconceptions too.