1. Problem It Solves
Migration is not a flag flip. Teams must identify removed or deprecated facilities, check implementation support, keep behavior stable, and introduce replacements in reviewable steps. The goal is to make the relevant rule visible in code instead of relying on an assumption about what “C++23 support” means.
2. Prerequisites
Days 1–2: support matrices and feature-test macros.
Days 7–23: the language changes being migrated.
3. Core Idea
Treat migration as crossing a bridge with checkpoints: clean C++20 warnings, enable C++23, inspect feature availability, replace risky APIs, then test each supported compiler. Read the syntax from left to right, identify the value, object, or range being transformed, and then check its resulting type, lifetime, or ownership. Standardization and implementation are separate: a C++23 mode may still lack one library component, so a feature-test macro is part of responsible portable use.
4. Minimal Syntax
[[deprecated("use new_api")]] int old_api();5. How It Works
The sample builds the smallest expression or object needed for Deprecations, Compatibility, and C++20–C++23 Migration.
It keeps a deprecated wrapper visible while the running path calls its replacement. The compiler and library apply the relevant rule before the program observes the result.
The program prints or checks the same result through the current API with a warning-free build, making the important behavior easy to verify.
6. Common Mistakes
Changing language mode and every library idiom at once makes regressions difficult to isolate; suppressing deprecation warnings leaves technical debt hidden.
Enabling C++23 mode without checking the relevant feature macro may select code that the installed compiler or standard library does not implement yet.
7. When to Use It
Use it when the task involves planned upgrades with tests, compiler matrices, and small compatibility layers.
Avoid it when the task involves rewriting stable code solely to use every new feature or removing fallbacks before supported compilers catch up.
8. Simple Example
A project keeps old_total() deprecated for one release while all internal calls move to new_total(). The downloadable program keeps the data fixed so the output can be compared without entering input.
Complete sample code
Source file
cpp23/24_cpp20_cpp23_migration/main.cpp
#include <iostream>
[[deprecated("use new_total")]]
int old_total(int first, int second) {
return first + second;
}
int new_total(int first, int second) {
return first + second;
}
int main() {
// Migrated call: the deprecated wrapper remains for compatibility.
std::cout << "total=" << new_total(20, 23) << '\n';
}
9. Key Takeaways
Separate the language rule from compiler and library availability.
Keep lifetime, ownership, and deduced types visible when they affect correctness.
Prefer the smallest syntax that communicates the intent.
Guard facilities that are not yet uniformly implemented.
10. Self-Check Questions
Easy — What is the smallest syntax in Section 4, and what main job does it perform?
Medium — Read the sample program: which value, type, or branch is observed, and why?
Hard — How can a compatibility wrapper remain source-compatible without silently changing overload resolution or ownership behavior?