C++ format macro / inline ostringstream

Question

I\'m trying to write a macro that would allow me to do something like: FORMAT(a << \"b\" << c << d), and the result would be a string -- the s

Answer 1

You've all pretty much nailed this already. But it's a little challenging to follow. So let me take a stab at summarizing what you've said...

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That difficulties here are that:

• We are playing with a temporary ostringstream object, so taking addresses is contra-indicated.

• Because it's a temporary, we cannot trivially convert to an ostream object through casting.

• Both the constructor [obviously] and str() are class ostringstream methods. (Yes, we need to use .str(). Using the ostringstream object directly would wind up invoking ios::operator void*(), returning a pointer-like good/bad value and not a string object.)

• operator<<(...) exists as both inherited ostream methods and global functions. In all cases it returns an ostream& reference.

• The choices here for ostringstream()<<"foo" are the inherited method ostream::operator<<(void* ) and the global function operator<<(ostream&,const char* ). The inherited ostream::operator<<(void* ) wins out because we can't convert to an ostream object reference to invoke the global function. [Kudos to coppro!]

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So, to pull this off, we need to:

• Allocate a temporary ostringstream.
• Convert it to an ostream.
• Append data.
• Convert it back to an ostringstream.
• And invoke str().

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Allocating: ostringstream().

Converting: There are several choices. Others have suggested:

• ostringstream() << std::string() // Kudos to *David Norman*
• ostringstream() << std::dec // Kudos to *cadabra*

Or we could use:

• ostringstream() . seekp( 0, ios_base::cur )
• ostringstream() . write( "", 0 )
• ostringstream() . flush()
• ostringstream() << flush
• ostringstream() << nounitbuf
• ostringstream() << unitbuf
• ostringstream() << noshowpos
• Or any other standard manipulator. [#include ] Reference: See "Insert data with format" 1/3 of the way down on this webpage.

We cannot use:

• operator<<( ostringstream(), "" )
• (ostream &) ostringstream()

Appending: Straightforward now.

Converting back: We could just use (ostringstream&). But a dynamic_cast would be safer. In the unlikely event dynamic_cast returned NULL (it shouldn't), the following .str() will trigger a coredump.

Invoking str(): Guess.

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Putting it all together.

#define FORMAT(ITEMS)                                             \
  ( ( dynamic_cast (                             \
         ostringstream() . seekp( 0, ios_base::cur ) << ITEMS )   \
    ) . str() )

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References:

• IOstream Library
• ostringstream
• ostream::operator<<()
  
  
• Type Casting Tutorial
• Wiki: Type Casting

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