Anonymous code blocks in c

Question

What does a statement such as this mean ?

int x  = ( { int a; scanf( "%d", &a ); a ; } ) ;

It compiles and runs equivalent to :

int x;
scanf( "%d", &x );

It seems to be like some kind of anonymous function call or something, but I'm not sure. I have not come across statements like ({}) before, and I am unable to find any explanations online. Any help would be very much appreciated, thank you :)

Context:

This is the code that you get when the macros in the following code are expanded :

#define SI ({int a;scanf("%d",&a);a;});
int x = SI;

This is the code used by someone in a programming competition.

Is that programming competition the [IOCCC](http://www.ioccc.org/)? — ObscureRobot, Oct 27 '11 at 06:44
@ObscureRobot: actually, in programming competitions you see lots of such macros, it's simply for reading the input easier and faster (time is often very limited so you don't want to waste it with "nice" wrappers). Doesn't have to be an IOCCC comp necessarily, I've written enough ugly code myself in such comps :p. Here probably some idiot thought macros were fasters than inlines... — KillianDS, Oct 27 '11 at 06:49
possible duplicate of [Weird C++ Syntax](http://stackoverflow.com/questions/6305396/weird-c-syntax) — Luchian Grigore, Oct 27 '11 at 06:54

score 7 · Accepted Answer · edited May 23 '17 at 10:34

7

It is an Statement Expression.
It as an compiler extension supported by GCC and it is not Standard C++,hence it is non portable.
If you compile your code with the -pedantic flag it will tell you so.

This answer of mine talks about it in detail.

edited May 23 '17 at 10:34

Community

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answered Oct 27 '11 at 06:42

Alok Save

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Just noticed that the Q was re-tagged to just `C`,So to be clear `Statement Expressions` are not Standard C or Standard C++. – Alok Save Oct 27 '11 at 06:55
What one maybe can mention is the reason for using it. For "void" macros that require multiple statements you can use the do-while-loop-construct to make the macro "behave" like a function call. This is portable, but when you need to return a value, this is the only possible way to achieve it (afaik). – flolo Oct 27 '11 at 07:09

Anonymous code blocks in c

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