In this months DDJ Herb Sutter writes some code like:

class AsyncLogFile {
public:
void write( string str ) { a.Send( [=] { log.write( str ); } ); }

My question is what does [=] mean, I thought square brackets were for
indexing arrays unless maybe the operator[] is overloaded, but then a
google codesearch shows other uses of [=], does it have a special or
common meaning?

--
[ comp.std.c++ is moderated.  To submit articles, try just posting with ]
[ your news-reader.  If that fails, use mailto:std-c++@netlab.cs.rpi.edu]
[              --- Please see the FAQ before posting. ---               ]
[ FAQ: http://www.comeaucomputing.com/csc/faq.html                      ]

On Sep 20, 2:13 am, Duncan Smith <dsmith1...@gmail.com> wrote:
> In this months DDJ Herb Sutter writes some code like:
>
> class AsyncLogFile {
> public:
> void write( string str ) { a.Send( [=] { log.write( str ); } ); }
>
> My question is what does [=] mean, I thought square brackets were for
> indexing arrays unless maybe the operator[] is overloaded, but then a
> google codesearch shows other uses of [=], does it have a special or
> common meaning?

The meaning of [=] makes only sense in the whole context of a
so-called lambda-expression, which is a new expression form
in C++0x and defines a lambda-closure, which is a compiler-generated
class-type functor. The whole lambda-expression above is

[=] { log.write( str ); }

and defines a closure type which is - roughly said - constructed like
this:

struct __ {
 std::string s;
 __(const std::string& s) : s(s) {}
 void operator()() {
   log.write( str );
 }
};

assuming that log is a global-scope variable.

Such a closure *captures* local variables, and the = within
the [] defines the default-policy, how this is done. If this
is =, all local variables are captured by *copy* (i.e. the
string s was copied. The other alternative for the default
capture is "by-reference", where you use the token & instead
of = within the [] token.

HTH & Greetings from Bremen,

Daniel Kr   gler

--
[ comp.std.c++ is moderated.  To submit articles, try just posting with ]
[ your news-reader.  If that fails, use mailto:std-c++@netlab.cs.rpi.edu]
[              --- Please see the FAQ before posting. ---               ]
[ FAQ: http://www.comeaucomputing.com/csc/faq.html                      ]