The performance cost of C++ abstractions

This chapter is vol3's performance mirror. vol3 covers "why components like vector/string/function are designed the way they are" (design motivation); this ch06 chapter covers "after you use them, what happens on hardware that makes things fast or slow". The thesis is Carruth's There Are No Zero-Cost Abstractions: no zero-cost abstractions, every C++ abstraction maps to a hardware cost.

But the chapter has a consistent contrarian spirit: "has a cost" does not equal "happens every time". The compiler often eliminates the cost for you: devirtualization turns virtual calls into direct calls, the zero-cost model keeps the normal path of exceptions free, and RVO makes return-by-value zero-copy. So the advice that runs through this chapter is repeatedly "measure first, don't hand-write around it prematurely".

Five articles:

• 06-01 Virtual functions and devirtualization: a virtual call through a pointer is 0.55ns (2.5x CRTP), but the compiler often devirtualizes. Don't CRTP-ify prematurely.
• 06-02 The zero-cost model of exceptions: the normal path is 0.25ns (zero cost nailed down), throwing is 857ns (3400x). Reserve exceptions for genuinely exceptional cases.
• 06-03 std::function's SBO: the call is 6x slower, construction takes the SBO path when small and heap-allocates when big (8.5x). Watch out for repeated construction plus a big capture on the hot path.
• 06-04 The cost cheat sheet: a roll-up plus variable storage types, bitfields, the zero cost of enum class, and a sizeof table.
• 06-05 RVO, NRVO, and move: return by value is zero-copy and zero-move (verified with the copy/move counter method); return std::move(local) is an anti-pattern.

Boundary: the design mechanism of components (vector's three pointers, SSO implementation, EBO) belongs to vol3/vol4; vol6 only covers "the cost of running them on hardware". High-frequency Zhihu questions (are virtual functions slow / are exceptions slow / function heap allocation / the move counter-example / return std::move) feed into each article's entry point, none get a standalone article.