Predefined macros commonly used in ansi c and Microsoft C ++)

_ File _ the source file name is XXX. cpp.
_ Line _ the first line of code in the source file
_ Date _ source file completion date, for example, EEC 11 2012
_ Time _ source file completion time, for example, 21:35:35
_ Timestamp _ source file completion date and time, for example, Tue Dec 11 21:35:35 2012

For example, if a version of the memory allocation function malloc () is used, the above macro is used.

# Define malloc (s) _ malloc_dbg (S, _ normal_block, _ file __, _ line __)

In addition, you can use _ file _ and _ line _ in the program to determine which file has an error:

Char * pszfilename = (char *) malloc (max_path * sizeof (char ));

If (pszfilename = NULL)

Printf ("error in % S % d \ n", _ file __, _ line __);

Of course, in the actual program, _ file _ and _ line _ are used to quickly locate errors and will not be output directly, instead, you can use another program to send this information to developers by email.

 

The _ file _ type can be easily converted to the wchar_t type. msdn has this example:

# Define widen2 (x) L # x

# Define widen (x) widen2 (X)

# DEFINE _ wfile _ widen (_ file __)

Wchar_t * pwsz = _ wfile __;

# In a macro is used to stringizing the macro parameters following it. Simply put, a double quotation mark is added to the left and right sides of the macro variables it references.

For example, after # define string (x) # X is defined, the following two statements are equivalent.

Char * pchar = "hello ";

Char * pchar = string (Hello );

Another # @ is a single quotation mark (charizing operator)

# Define makechar (x) # @ x

Char CH = makechar (B); equivalent to Char CH = 'B.

 

Note that if you encounter # Or # In a macro, the nested Macro will no longer be expanded. What does that mean? For example, char * pchar = string (_ file _) is used. Although _ file _ is a macro, the compiler does not expand it, therefore, pchar will point to "_ file _" instead of "D: \ XXX. the name of the CPP source file. Therefore, you need to add an intermediate conversion macro, first parse _ file _ into a "D: \ XXX. cpp" string.

Two macros are defined as follows:

# DEFINE _ string (x) # x

# Define string (x) _ string (X)

Then, call the following statement to output the path of the source file "".

Char * pchar = string (_ file __);

Printf ("% S % s \ n", pchar, _ file __);

We can compare the differences between string (_ file _) and _ file _. The front will contain double quotation marks, and the last one will not contain double quotation marks.

 

Next, let's talk about the # function. It can be spliced with a token-pasting operator ).

There is an example on msdn:

# Define Paster (n) printf ("token" # N "= % d \ n", Token # N)

Int token9= 100;

Call Paster (9) Again. After the macro is expanded, the token # N is directly merged into token9. The entire statement becomes

Printf ("token" "9" "= % d", token9 );

In the C language, the two connected double quotation marks in the string will be automatically ignored, so the above sentence is equivalent

Printf ("token9 = % d", token9 );.

That is, the output token9 = 100

 

 

With the above basics, let's look at Example 1.

# Define widen2 (x) L # x

# Define widen (x) widen2 (X)

# DEFINE _ wfile _ widen (_ file __)

Wchar_t * pwsz = _ wfile __;

The L in the first macro converts an ANSI string to a unicode string. For example, wchar_t * pstr = l "hello ";

Let's take a look at wchar_t * pwsz = _ wfile __;

_ Wfile _ is first expanded to widen (_ file _), and then expanded to widen2 ("_ file _ string "), concatenate the string in the format of L "_ file _", that is, l "d: \ XXX. CPP "to obtain the Unicode string and assign the string address to the pwsz pointer.

 

In VC, _ T () and text () are also used.

In the tchar. h header file, you can find:

# DEFINE _ T (x) _ T (X)

# DEFINE _ T (x) L # x

In the WINNT. h header file, you can find

# Define text (quote) _ text (quote) // r_winnt

# DEFINE _ text (quote) L # quote // r_winnt

Therefore, it is not difficult to understand why the third statement has an error: Error c2065: 'lsztext': Undeclared identifier

Wprintf (text ("% S % s \ n"), _ T ("hello"), text ("hello "));

Char sztext [] = "hello ";

Wprintf (text ("% S % s \ n"), _ T (sztext), text (sztext ));

After "hello" is defined as a macro, it can run correctly.

# Define sztext "hello"

Wprintf (text ("% S % s \ n"), _ T (sztext), text (sztext ));

Note: Because vc6.0 uses ANSI encoding by default, you must first set it to unicode encoding, select setting from the project menu, and then select Preprocessor in the C/C ++ label dialog box. In the Preprocessor definitions edit box, remove _ MBCS and add _ Unicode and Unicode.