Note: The C++11 construct std::to_string( x ) is more efficient than this. See end of article for perfomance comparisons.

Quite frequently, a C++ coder will face the issue of turning some value or object into its string representation (usually for output of some kind). Conveniently, when you want to write to terminal or a file, this can be done quite easily using the commonplace operator«() syntax:

int i = 42;
std::cout << "The value of i is " << i << "\n";

Things get a bit more difficult when you want to get the result as a std::string (for example, as parameter to an exception constructor):

std::ostringstream s;
s << "The value of i is " << i;
std::string output = s.str();

There are several footholes to be aware of, and anyway writing up those lines over and over again can become quite tedious. Thus, I present to thee:

By no means an invention of myself, I first came across this construct in 2002. Since then, it has followed me to whatever C++ code I was working upon. Without further ado:

#include <sstream>
 
#define SSTR( x ) static_cast< std::ostringstream & >( std::ostringstream() << std::dec << x ).str()

• std::ostringstream()

Creating an anonymous ostringstream object with which we will construct the string.

• « std::dec

Setting the integer output format to “decimal” looks like a non-op since decimal is the default anyway. However, it has the effect of returning not the original ostringstream object, but a reference to ostream. This is important if the first data element given to the macro is a pointer (e.g. a C-style string): If we would use the ostringstream object directly, function lookup would use ​ostream::operator<<( void * ) in this case (giving us a boolean value) instead of the global ​operator<<( ostream &, char const * ) we want (giving us a string output).

Other versions of this macro that I came across in the 'net use the non-op construct std::ostringstream().seekp( 0, std::ios_base::cur ) to get at that ostream & return value. However, that construct is known to break on Microsoft Visual C++ 2005, and possibly some others, whereas the std::dec trick worked on any platform I have been working on so far, including Solaris and AIX.

• « x

…passes the macro parameter(s) verbatim. This could be a single data value, or any number of values daisy-chained with more operator«().

• static_cast< std::ostringstream & >( … )

We turned our ostringstream object into a ostream reference, and “piped” all our output to it. Now we want to extract the accumulated string from the object; but the member function str() that would do this for us is not defined for ostream, only for ostringstream. So we cast our ostream reference back to the correct type…

• ().str()

…and extract the string.

​Nikos Athanasiou on StackOverflow benchmarked this construct in comparison to the more traditional Stringify( T const & value ) template function (also using std::stringstream), Boost lexical_cast<>, and C++11 to_string():

lexical_cast<string>() ... 125
to_string() .............. 250
SSTR .................... 1003
Stringify() ............. 1229

int i = 42;
std::string s1 = SSTR( i );
std::string s2 = SSTR( "Value of i is: " << i );
std::string s3 = SSTR( i << " is the answer to the universe, life, and everything." );