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What is the C++ preprocessor? What is the difference between #include with angle brackets and quotes? What is an include guard and why is it needed? What are predefined macros like __LINE__ and __FILE__? What are conditional compilation directives used for? What is the difference between a macro constant and a constexpr/const constant? What are the # (stringize) and ## (token paste) operators? How does #pragma once compare to traditional include guards? What are the dangers of macros affecting names and how do you mitigate them?
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Why are function-like macros discouraged in modern C++?

Intermediate

Answer

Function-like macros do blind text substitution with no type checking, no scope, and pitfalls like multiple argument evaluation and operator-precedence bugs. Modern C++ replaces them with inline functions, templates, and constexpr, which are type-safe, respect scope, and debuggable. Macros remain only for things the language can't do (conditional compilation, stringizing).
#define SQ(x) ((x)*(x))
SQ(i++)   // i incremented twice! use a constexpr function instead
Real-world example A MAX macro double-evaluates its arguments causing a subtle bug; a template function fixes it.

Common follow-ups: What is the double-evaluation problem? | What replaces macros?

Constexpr & Compile-Time Templates Preprocessor & Macros
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