Question

I defined a multi-line macro function using line continuation character “\” like the following:

#define SHOWMSG( msg ) \ 
{ \     
    std::ostringstream os; \     
    os << msg; \     
    throw CMyException( os.str(), __LINE__, __FILE__ ); \ 
}

But it could not pass compilation. BTW, I’m using VS2008 compiler. Would you please tell what’s wrong with my aforesaid macro function?

Answer 1

The usual method for a multi-statement macro is something like:

#define SHOWMSG(msg)                                  \
do {                                                  \
    std::ostringstream os;                            \
    os << msg;                                        \
    throw CMyException(os.str(), __LINE__, __FILE__); \
} while (0)

Without that, the semicolon following the closing brace can cause syntactic problems, such as:

if (x)
    SHOWMSG("This is a message");
else
    // whatever

With your code as it was, this would expand to:

if (x) {
    std::ostringstream os;
    os << "This is a message";
    throw CMyException(os.str(), __LINE__, __FILE__);
}
;    // on separate line to emphasize that this is separate statement following
     // the block for the if statement.
else
    // whatever

In this case, the semicolon would form a null statement following the block in the if statement, and the else wouldn’t have an if to match up with.