I'm asking this question because I'm updating my C and C++ course materials and I've had past students ask about it...
From ISO/IEC 9899:2017 section 6.5 Expressions ¶1 (and similar in the C++ standard):
"An expression is a sequence of operators and operands that specifies computation of a value, or that designates an object or a function, or that generates side effects, or that performs a combination thereof. …"
Because the standards writers obviously choose their words carefully, the use of the phrase "sequence of operators and operands" seems potentially misleading to me. It seems to indicate that to be considered an expression there must be more than one operator and also more than one operand. Thus, literals like 123 or variables like XYZ would not be considered expressions because there is no operator, and they certainly can't be considered operands if there is no operator.
However, if 123 and XYZ actually are expressions, wouldn't replacing the phrase "sequence of operators and operands" with "sequence of one or more characters" or something similar be more accurate?
Please tell me what I am misinterpreting about what the standard is stating.
Yes, the definition of "expression" in the C standard is incomplete -- but not in a way that causes any actual problems (other than to picky people like me).
The word "expression" in the text you quoted is in italics, which means that that is the official definition of the term. It's clear from other parts of the standard that 123, for example, is an expression: it's a decimal-constant, which is an integer-constant, which is a constant, which is a primary-expression`, which is a postfix-expression, which (skipping multiple steps) is an expression.
It is not "a sequence of operators and operands". There is no operator, which implies that 123 is not an operand (this can be demonstrated by referring to the definitions of operator and operand elsewhere in the standard).
In practice, I've never heard of anyone, either a compiler implementer or a C programmer, having any real difficulty because of this incomplete definition. Compiler implementers refer to the language grammar. C programmers probably get a pretty good idea of what an "expression" is before reading the standard.
I'd like to see the definition of expression updated in a new edition of the standard. A definition that refers to the grammar rather than attempting an English description would IMHO be an improvement.
But if it isn't updated, we'll all keep using expressions without any problems.
As for C++, Nicol Bolas's answer correctly points out that the C++ standard doesn't have a formal definition of "expression" like the C standard does. It does have similar wording at the top of Clause 8: "An expression is a sequence of operators and operands that specifies a computation." -- but the word "expression" is not in italics and that sentence is part of a "Note", and is therefore non-normative. In C++, the standard defines expressions syntactically.