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Item 2: Prefer consts, enums, and inlines to #defines

Concept​

Prefer the compiler to the preprocessor. #define doesn't enter the symbol table, making debugging harder. Use:

  • const or constexpr for constants
  • enum hack for class-scope integer constants
  • inline functions (or templates) instead of #define macros

Code Example​

// Bad: preprocessor macro
#define ASPECT_RATIO 1.653

// Good: typed constant
constexpr double AspectRatio = 1.653;

// Class-scope constant
class GamePlayer {
    static constexpr int NumTurns = 5;  // C++17 inline
    int scores[NumTurns];
};

// Bad: macro "function"
#define CALL_WITH_MAX(a, b) f((a) > (b) ? (a) : (b))

// Good: inline template
template<typename T>
inline const T& callWithMax(const T& a, const T& b) {
    return a > b ? a : b;
}

Full source code

Things to Remember​

  • For simple constants, prefer const objects or constexpr to #defines.
  • For function-like macros, prefer inline functions to #defines.
  • Item 3 — Use const whenever possible
  • Item 30 — Understand the ins and outs of inlining
  • Item 48 — Template metaprogramming (compile-time computation)