C++ Runtime -

(simplified for a class hierarchy A <- B ):

done: return instance_memory;

:

vtable for B: [0] offset_to_top = 0 [1] typeinfo pointer -> typeinfo for B [2] A::foo() thunk (if overriding) vtable for A: [0] offset_to_top = 0 [1] typeinfo pointer -> typeinfo for A

Report ID: CPP-RT-2024-01 Date: April 14, 2026 Author: Systems Software Research Division Subject: Structure, Execution Flow, and Overhead of the C++ Runtime Environment 1. Executive Summary The C++ runtime is the set of software components that support the execution of a compiled C++ program beyond the raw machine code generated by the compiler. Unlike C, which has a relatively minimal runtime, C++ requires substantial behind‑the‑scenes machinery to implement core language features: dynamic initialization of globals, exception handling, run‑time type information (RTTI), new / delete operators, and stack unwinding. This report dissects the C++ runtime into its constituent parts, traces the execution flow from _start to main and beyond, analyzes the cost of each runtime feature, and examines implementation variations across major compilers (GCC/Clang, MSVC) and operating systems. 2. Components of the C++ Runtime The C++ runtime is not monolithic. It consists of several logical layers: c++ runtime

static Logger& getLogger() static Logger instance; // thread‑safe initialization return instance;

static char instance_memory[sizeof(Logger)]; static uint8_t guard = 0; // 0 = uninitialized, 1 = initializing, 2 = done if (guard == 2) goto done; if (__cxa_guard_acquire(&guard)) new (instance_memory) Logger(); __cxa_guard_release(&guard); __cxa_atexit(destroyer, instance_memory, __dso_handle); (simplified for a class hierarchy A &lt;- B

: The stack is unwound again. For each frame between the throw site and the catch frame, the personality routine calls destructors for local objects ( _UA_CLEANUP_PHASE ). Finally, control jumps to the catch block.