1. Problem It Solves
A toolchain turns source text into an executable, while compiler flags select the language rules, diagnostics, and output file. It makes an important assumption visible and checkable.
2. Prerequisites
A source file with a
mainfunction.You should be able to compile a short program and read its output.
3. Core Idea
Picture a pipeline: source is translated, checked under C++20 rules, linked, and then executed. A flag is an instruction attached to that pipeline. Read -std=c++20 as a precise promise; runtime preconditions still belong to the programmer.
4. Minimal Syntax
// g++ -std=c++20 -Wall -Wextra -Wpedantic main.cpp -o main
#if __cplusplus >= 202002L
// C++20 mode is active.
#endif5. How It Works
The program introduces the smallest relevant form of
-std=c++20.It applies the feature to fixed data while required owners remain in scope.
It prints one result that can be checked against the source.
6. Common Mistakes
Compiling without
-std=c++20can reject valid C++20 syntax or silently select an older language mode; ignoring warnings can hide real defects.Also check the required header, C++20 library support, lifetime, and deduced types.
7. When to Use It
Use it when you compile a lesson, test a feature, or want reproducible diagnostics.
Avoid it when you are merely running an executable that has already been built.
8. Simple Example
The program prints __cplusplus and confirms whether the compiler selected C++20 or later. The companion .cpp uses no input, so its result is easy to reproduce.
Complete sample code
Source file
cpp20/01_toolchain_compiler_flags_cpp20/main.cpp
// Day 1: Toolchain, Compiler Flags, and C++20 Mode
#include <iostream>
int main() {
std::cout << "__cplusplus = " << __cplusplus << '\n';
#if __cplusplus >= 202002L
std::cout << "C++20 mode active\n";
#else
std::cout << "C++20 mode not active\n";
#endif
}
9. Key Takeaways
-std=c++20expresses the central C++20 idea of this day.The example isolates one behavior with fixed data.
Compiler checks do not replace lifetime and runtime reasoning.
Prefer the smallest interface that states the real requirement.
10. Self-Check Questions
Easy — What is the main job of
-std=c++20in the minimal example?Medium — If
__cplusplusprints a value below202002, which part of the build command should you inspect?Hard — Why can the same source compile differently when only
-std=c++20is removed, even though the compiler executable is unchanged?