When me and my friend were preparing for exam, my friend said that x ; is the same as x =3;

It is not true but is x ; same as x =1; or is (x ) ;? Could I generalize it? I.e. x ; or ((((((x ) ) ) ) ) ) ; is equivalent to x =7;

Maybe it's completely wrong and it is true for x; or ( ( x)); equivalent to x =3;

Also it should generalize to --x; and x--;

CodePudding user response：

The behavior of your program can be understood using the following rules from the standard.

From lex.pptoken#3.3:

Otherwise, the next preprocessing token is the longest sequence of characters that could constitute a preprocessing token, even if that would cause further lexical analysis to fail, except that a header-name is only formed within a #include directive.

And from lex.pptoken#5:

[ Example: The program fragment x y is parsed as x y, which, if x and y have integral types, violates a constraint on increment operators, even though the parse x y might yield a correct expression.  — end example ]

is x ; same as x =1;

Using the statement quoted above, x will be parsed as x .

But note that from increment/decrement operator's documentation:

The operand expr of a built-in postfix increment or decrement operator must be a modifiable (non-const) lvalue of non-boolean (since C 17) arithmetic type or pointer to completely-defined object type. The result is prvalue copy of the original value of the operand.

That means the result of x will a prvalue. Thus the next postfix increment cannot be applied on that prvalue since it requires an lvalue. Hence, x will not compile.

Similarly, you can use the above quoted statements to understand the behavior of other examples in your snippet.