Compilers (GCC, Clang, MSVC)

The three production C++ compilers — GCC, Clang/LLVM, and Microsoft MSVC — each implement the same ISO standard but differ in flags, diagnostics, standard library, and ABI.

Why it matters

Portable C++ is code that compiles clean on all three; CI matrices routinely build every commit against GCC + Clang + MSVC to catch nonconformance early (one compiler accepting an extension another rejects). They also differ in which standard features ship first and how aggressively they optimize, so the compiler is a real variable in both correctness and performance.

How it works

Each pairs a frontend with a standard library and emits native code (see how-c-works-compilation-model):

Compiler	Default stdlib	Platforms	Flag style
GCC (g++)	libstdc++	Linux, MinGW	-std=c++20 -O2 -Wall
Clang (clang++)	libc++ or libstdc++	macOS, Linux, Win	-std=c++20 -O2 -Wall (GCC-compatible)
MSVC (cl.exe)	MS STL	Windows	/std:c++20 /O2 /W4

• Optimization maps -O0 (debug, no opt) → -O1 → -O2 (release default) → -O3 (aggressive) → -Os (size); MSVC uses /Od//O1//O2.
• Warnings are your linter — -Wall -Wextra -Wpedantic (GCC/Clang) or /W4; add -Werror in CI to make warnings fatal.
• ABI: GCC and Clang share the Itanium C++ ABI on Linux (interchangeable objects); MSVC uses its own — you cannot link MSVC and MinGW objects together.
• Clang doubles as a tooling platform: clangd, clang-tidy, and clang-format reuse its frontend.

Example

The same source, three invocations:

g++     -std=c++20 -O2 -Wall -Wextra main.cpp -o app   # Linux
clang++ -std=c++20 -O2 -Wall -Wextra main.cpp -o app   # macOS, libc++
cl      /std:c++20 /O2 /W4 main.cpp                     # Windows MSVC

Clang famously pioneered readable diagnostics with carets and fix-it hints; GCC has since caught up, while MSVC’s wording still differs — a reason to read errors from more than one.

Pitfalls

• Compiler-specific extensions (__builtin_*, MSVC __declspec, VLAs in GCC) compile on one toolchain and break the others — guard with -Wpedantic.
• Mixing standard libraries — objects built against libstdc++ and libc++ have incompatible std::string/std::list layouts; link one consistently.
• -O3 is not free — it can bloat code and occasionally expose latent UB that -O0 hid; -O2 is the usual release default.
• Default standard varies — older GCC defaulted to C++14/17; always pass -std= explicitly.

See also

• how-c-works-compilation-model
• c-standards-c-11-14-17-20-23