Question

I feel I got a little confused here distinguishing sub-expression from expression.

Compound expression – An expression involving more than one operator.

E.g. 1 + 1 * 1

Expression – An operator with one or more operands.

E.g 1 + 1 or + 1

is there a level below the expression which is called a sub-expression?

Answer 1

An expression may not have an operator. For example, in the following:

int a = 0;
a;

a is an expression. An expression is, to quote the C++ Standard, “a sequence of operators and operands that specifies a computation” (C++03 5/1). Here, a is an “operand” even though there is no operator.

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A subexpression is any expression that is a piece of a larger expression. So, in

int a = 0, b = 0, c = 0, d = 0;
a * b + c * d;

a * b and c * d are subexpressions.

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The word “compound” is usually used with respect to statements, not expressions. A compound statement is a pair of braces that may contain a sequence of other statements. For example, a function body is almost always a compound statement, as are many loop bodies.

Usually, complex expressions are just referred to simply as “expressions” and their parts are referred to as subexpressions.