_T和L在C ++中是什么意思，如何传递它们? [英] What do _T and L mean in C++ and how can I pass them?

问题描述

我正在尝试用C ++构建一个键盘记录程序.我的键盘记录程序的一部分是捕获屏幕.
经过大量搜索，我决定尝试通过构建一个实例来立即了解它的工作原理.

I am trying to build a keylogger in C++. Part of my keylogger is to capture the screen.
After a lot of searching, I decided to try to understand how it works instantly by trying to build one.

这是我的屏幕捕获代码:

This is my screen capture code:

    HDC hdc = GetDC(NULL); // get the desktop device context
    HDC hDest = CreateCompatibleDC(hdc); // create a device context to use yourself

    // get the height and width of the screen
    int height = GetSystemMetrics(SM_CYVIRTUALSCREEN);
    int width = GetSystemMetrics(SM_CXVIRTUALSCREEN);

    // create a bitmap
    HBITMAP hbDesktop = CreateCompatibleBitmap(hdc, width, height);

    // use the previously created device context with the bitmap
    SelectObject(hDest, hbDesktop);

    // copy from the desktop device context to the bitmap device context
    // call this once per 'frame'
    BitBlt(hDest, 0, 0, width, height, hdc, 0, 0, SRCCOPY);

    /*CImage image; // from this code i tried to understand how to save the bitmap
    image.Attach(hbDesktop); 
    image.Save(pathname, Gdiplus::ImageFormatBMP);*/

    CImage image;//this code is what I came up with eventually
    image.Attach(hbDesktop);
    CHAR buffer[100] = _T("this is a literal string");  // THIS IS WHERE MY PROBLEM STARTS 
    sprintf(buffer,_T("this is a literal string"), 1);
    image.Save(_T(buffer), Gdiplus::ImageFormatBMP);

我试图使程序每次使用不同的名称保存位图文件.问题是，除非我使用 _T 符号，否则最后一行将不起作用，并且当我放入 _T 时，它不会占用缓冲区并说"identifier"Lbuffer"未定义".这是什么意思?我需要在哪里放置这个L标志，为什么?另外，有没有一种更好的方法可以在不使用此 _T 符号的情况下将位图保存到文件中?

I tried to make the program save the bitmap file each time with a different name. The problem is that the last line doesn't work unless I use _T sign, and when I do put the _T, it won't take the buffer and says "identifier "Lbuffer" is undefined". What does this mean? Where do I need to put this L sign and why? Also, is there a better way to save bitmap to a file without using this _T sign?

我尝试查找此内容，并在 https://msdn上进行了查找.microsoft.com/en-us/library/dybsewaf.aspx 他们说这与Unicode有关.为什么我需要Unicode才能使用此功能?

I've tried to look this up, and on https://msdn.microsoft.com/en-us/library/dybsewaf.aspx they say this is about Unicode. Why do I need Unicode for this function?

推荐答案

_T()及其Win32等价的 TEXT()是预处理器宏，如果分别定义了 _UNICODE 或 UNICODE ，则在输入值前加上 L .

_T(), and its Win32 equivilent TEXT(), are preprocessor macros that prepend the input value with L if _UNICODE or UNICODE are defined, respectively.

这些宏只能与字符/字符串文字一起使用.您不能将它们与变量一起使用.因此 _T(buffer)是无效的代码.这就是为什么您收到未定义的 identifer"Lbuffer" 错误(显然在您的项目中定义了 _UNICODE 的原因)的原因.

These macros are meant to be used with character/string literals only. You cannot use them with variables. So _T(buffer) is not valid code. That is why you are getting an identifer "Lbuffer" not defined error (_UNICODE is obviously defined in your project).

如果定义了 _UNICODE / UNICODE ，则 _T("literal")和 TEXT("literal")被编译器评估为 L"literal" -类型为 wchar_t [] 的宽字符串文字.

If _UNICODE/UNICODE are defined, _T("literal") and TEXT("literal") are evaluated by the compiler as L"literal" - a wide string literal of type wchar_t[].

如果未定义 _UNICODE / UNICODE ，则 _T("literal")和 TEXT("literal")评估为"literal" -一种 char <]类型的窄字符串文字.

If _UNICODE/UNICODE are not defined, _T("literal") and TEXT("literal") are evaluated by the compiler as "literal" - a narrow string literal of type char[].

CHAR 不是宏，它是 char 的 typedef ，并且是无条件的(存在等效的 WCHAR 用于 wchar_t 的typedef).

CHAR is not a macro, it is a typedef for char, and is unconditional (there is an equivalent WCHAR typedef for wchar_t).

因此，这一行:

CHAR buffer[100] = _T("this is a literal string");

被评估为:

// when _UNICODE is defined
char buffer[100] = L"this is a literal string"; // DOES NOT COMPILE!
// You cannot assign/fill a char[] array with wchar_t data!

// when _UNICODE is not defined
char buffer[100] = "this is a literal string"; // OK

如果要使用 _T()/ TEXT()，则必须使用 _TCHAR / TCHAR 而不是 CHAR ，因此它们匹配:

If you want to use _T()/TEXT(), you must use _TCHAR/TCHAR instead of CHAR so they match:

_TCHAR buffer[100] = _T("this is a literal string");

_TCHAR / TCHAR 是 _UNICODE / UNICODE 敏感的预处理器 #define s.定义 _UNICODE / UNICODE 时，它们解析为 wchar_t ，否则它们解析为 char :

_TCHAR/TCHAR are _UNICODE/UNICODE-sensitive preprocessor #defines. When _UNICODE/UNICODE are defined, they resolve to wchar_t, otherwise they resolve to char instead:

// when _UNICODE is defined
wchar_t buffer[100] = L"this is a literal string"; // OK

// when _UNICODE is not defined
char buffer[100] = "this is a literal string"; // OK

sprint() 仅适用与 char 数据一起使用. wchar_t 版本是 swprintf()，而 _TCHAR / TCHAR 版本是 _stprintf():

sprint() only works with char data. The wchar_t version is swprintf(), and the _TCHAR/TCHAR version is _stprintf():

char buffer[100];
sprintf(buffer, "this is a literal string", 1); // OK

wchar_t buffer[100];
swprintf(buffer, L"this is a literal string", 1); // OK

_TCHAR buffer[100];
_stprintf(buffer, _T("this is a literal string"), 1); // OK

CImage :: Save() 接受 LPCTSTR 作为输入. LPCTSTR 是 const TCHAR * 的 typedef ，因此在 UNICODE 时解析为 const wchar_t * .code>已定义，否则解析为 const char * .由于它对 UNICODE 敏感，但是您的 buffer 始终是 char [] ，因此您将无法传递 buffer 到 Save()(如果已定义 UNICODE ).您需要改用 TCHAR :

CImage::Save() accepts a LPCTSTR as input. LPCTSTR is a typedef for const TCHAR*, thus resolves to const wchar_t* when UNICODE is defined, otherwise resolves to const char*. As it is UNICODE-sensitive, but your buffer is always a char[], you will not be able to pass buffer to Save() if UNICODE is defined. You need to use TCHAR instead:

// In this case, you should use the Win32 macros instead of the C macros.
// It is not good practice to mix C's _T()/_TCHAR macros with Win32 APIs,
// and Win32's TEXT()/TCHAR macros with C APIs, though technically it will
// work fine.  Best to keep the two APIs separate...

TCHAR buffer[100];
_stprintf(buffer, TEXT("filename_%d.bmp"), 1);
image.Save(buffer, Gdiplus::ImageFormatBMP);    

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