What’s this C++ syntax that puts a brace-surrounded block where an expression is expected?

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It assigns user input value to a and prints it out. it is done by using a Statement Expression.

Statement Expressions are a gnu gcc compiler extension and are not supported by the C/C++ standards. Hence, any code which uses statement expression is not standard conforming and non-portable.

The IBM XL C/C++ v7.0 also support Statement Expressions & its documentation explains them aptly:

Statement Expressions:

A compound statement is a sequence of statements enclosed by braces. In GNU C, a compound statement inside parentheses may appear as an expression in what is called a Statement expression.

         .--------------.
         V              |
>>-(--{----statement--;-+--}--)--------------------------------><

The value of a statement expression is the value of the last simple expression to appear in the entire construct. If the last statement is not an expression, then the construct is of type void and has no value.

Always compile your code by selecting a standard in GCC: use one of the options -ansi, -std=c90 or -std=iso9899:1990, -std=c++03, -std=c++0x; to obtain all the diagnostics required by the standard, you should also specify -pedantic (or -pedantic-errors if you want them to be errors rather than warnings).

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