TITLE: Declaration or expression - that is the question


PROBLEM: scvicato@unidhp.uni-c.dk (Scanview Software)

The following code shows an apparent confusion on the part of either a 
specific C++ compiler (two were tested) or with C++ in general.

class Bob
{
  int x;

  public:
    Bob( void )  { x = 1; }
    Bob( int y ) : x(y) { }
};

void main( void )
{
  int n = 6;

  Bob(6);       // temporary instance of Bob initialized to 6

  Bob(n);       // This temporary instance fails to compile.
                // The compiler thinks this line is equivalent to: Bob n;

  Bob((int)n);  // However this works by typecasting an int to an int!
}


RESPONSE: clamage@Eng.Sun.COM (Steve Clamage), 29 June 95

[ ... ] The compilers are correctly implementing a C++ rule.

It is possible to write a C++ statement which could be interpreted as
either a declaration-statement or an expression-statement. The C++ rule
is (perhaps arbitrarily) that if a statement CAN BE interpreted as
a declaration, it IS a declaration, and is NOT an expression.

It isn't possible to declare an anonymous temporary variable. That is,
creation of an anonymous temporary variable requires an expression,
and is not a declaration.


"Bob(6);" cannot be interpreted as a declaration, so it must be
a cast-expression. The value 6 is converted to a Bob, and the result
is discarded.

"Bob(n);" can be interpreted as declaring local variable 'n' to be
type Bob. Extra parens are allowed around the declarator (the name
of variable 'n' in this case). Therefore, this is a declaration of 'n'
and not an expression.

"Bob((int)n);" cannot be interpreted as a declaration, so it must
be a cast-expression. (You might think that it could possibly be
interpreted as a declaration of function Bob taking one int parameter,
but the parens around 'int' make that impossible. No type specifier can
begin with a left paren.)

The rule is almost never a problem in real code unless you are doing
something very unusual. I consider creating an anonymous temporary
variable for the sole purpose of its side effects unusual and
ill-advised. It is going to be confusing to anyone who reads the code.
Create a named local variable if you wish, and document that you are
doing so for its side effects.