C++ primary expressions - Is it primary expression or not?

Question

1. Why are they called "primary"? In the order of evaluence they are the first?

2. C++03 standard defines the expression in chapter 5 (Note 1):

   An expression is a sequence of operators and operands that specifies a computation.

   Then the 5.1 "Primary expressions" defines the list of primary expressions:

   (1) primary-expression:

   literal
   
   this
   
   ( expression )
   
   id-expression
   

   My main question is in connection the third point:

   ( expression )
   

   So, according to the standard, every expression with brackets are primary expressions and they are calculated firstly. It looks logical, and gives an exact explanation of the behavior of brackets in C++ expressions (precedence).

   So this means that for example

   (variable + 10)
   

   is a primary expression.

   var = (variable + 10) * 3
   

   and according to my theory, it looks logic, BUT from other sources I know

   (variable + 10)
   

   is NOT a primary expression, but WHY? I don't understand, however the standard defines the (expression) as a primary expression.

Please, help me because I can't. Thank you very much, and sorry for my bad English. Hi.

Answer 1

C++ expressions can be complex, which is to say they can be made up of nested expressions, combined through the use of operators, and those nested expressions may in turn be complex.

If you decompose a complex expression into ever smaller units, at some point you'll be left with units that are atomic in the sense that they cannot be decomposed further. Those are primary expressions; they include identifiers, literals, the keyword this, and lambda expressions.

However, it is true that there is one non-atomic construct that the C++ Standard defines as primary: Expressions enclosed in round brackets (aka parentheses). So the (variable + 10) example you give is a primary expression (and so are the sub-expressions variable (which is an identifier), and 10 (which is a literal).

I believe the Standard lists them as primary expressions because they play the some role as truly atomic expressions when it comes to the order of evaluation: Anything within the brackets must be evaluated before the value of the backeted expressions can enter into evaluations with other expressions: In (5+10)*a, the value of 5+10 must be evaluated before it can enter into the evaluation of *a. [Note that this does not mean 5+10 is evaluated before the expression a is evaluated. It only means that 5+10 must be evaluated before the multiplication itself can be evaluated.]

So, bracketed sub-expressions, in this sense, act as if they were atomic.

And I guess this is why the Standard doesn't use the term "atomic expressions" for this concept. They act as if they were atomic, but at least the bracketed variety is not actually atomic. "Primary" seems, to me, to be a good choice of words.