libc++ (LLVM) Source-Level Deep Analysis

Source path: references/impl/llvm-project/libcxx/include/

LLVM's C++ standard library implementation, known for its most compact memory layout and latest C++ standard support. Core design philosophy: minimal memory layout + trivially relocatable memcpy.

Table of Contents

Chapter	Content
vector and string	Three-pointer vector, 24-byte SSO string (22 bytes inline), split_buffer, memcpy relocate
function and shared_ptr	24-byte SBO function (vtable), make_shared single allocation
map/set and variant	Red-black tree __tree, sentinel node __end_node_, variant visit function pointer table
Ranges and unique_ptr	Pipe operator CRTP, filter_view cache, compressed_pair unique_ptr

Three-Implementation Comparison

Component	libc++ (LLVM)	libstdc++ (GCC)	MSVC STL
sizeof(string)	24	32	32
string SSO	22	15	15
vector growth	2×	~2×	1.5×
vector relocate	memcpy	move+destroy	move+destroy
unique_ptr	8B	8B	8B
trivial_abi	supported	not supported	not supported
function SBO	24B (vtable)	24B (function pointer)	different
unordered_map	chaining	node-based _Hashtable	chaining

Choose libc++ for: macOS/iOS/Android, most compact memory layout, trivially relocatable memcpy, latest C++ standard.