Increment operator is not invoked at sizeof(++n) expression [duplicate]

In C++, the expressions inside sizeof is not evaluated, except for C99’s VLA ^{as mentioned in comments, since this was earlier tagged for C too}

The sizeof operator is calculated at compile time.

Only the type of the expression (that is calculated at compile time) is then used by sizeof.

From C++ Standard § 5.3.3 Sizeof

The sizeof operator yields the number of bytes in the object
representation of its operand. The operand is either an expression,
which is an unevaluated operand (Clause 5), or a parenthesized
type-id.

^{Some C++ compilers provide VLAs as an extension as commented below.}