C/C++ literals: which of the following characters are valid inside a numeric literal token (as part of the literal itself) in standard C++?

Introduction / Context:Numeric literals in C and C++ follow strict lexical rules. Only certain characters are permitted within the token that represents an integer or floating-point literal. Understanding which characters are allowed prevents hard-to-spot compile errors and ensures code portability across compilers and locales.

Given Data / Assumptions:

• We refer to the characters inside a numeric literal, not separators in source formatting.
• Standard C++ permits digits, decimal points, exponent markers (e/E, p/P for hex floats), sign characters in exponents, base prefixes (0x, 0b in C++14/20 compilers), digit separators ('') and type suffixes (u, l, f, etc.).
• We evaluate specific candidates: comma, dollar sign, percent sign, and space.

Concept / Approach:

• Commas and spaces are token separators in the grammar, not part of numeric literals.
• Dollar ($) and percent (%) are ordinary punctuation/operators; they are not valid within numeric literals.
• Therefore, none of the listed characters are valid inside a numeric literal token.

Step-by-Step Solution:

Verification / Alternative check:

Why Other Options Are Wrong:

• a comma / a space: Break tokens into multiple tokens.
• a dollar sign ($) / a percent sign (%): Not part of literal syntax; % is the modulo operator.

Common Pitfalls:

• Copying numbers from documents with thousands separators; remove commas or use the digit separator ''.
• Confusing formatting symbols (%, $) with literal syntax; those belong in strings or comments.

Final Answer: