1. Problem It Solves
C++23 removes several artificial restrictions from constexpr function bodies. More ordinary-looking code can be declared constexpr, although only a valid constant-evaluation path may run at compile time. 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
Day 7: constant evaluation context and
if consteval.Basic
constexprvariables and functions.
3. Core Idea
A constexpr function is a two-use tool, not a promise that every call is compile-time. C++23 lets the toolbox contain more constructs while the evaluator still checks the chosen path. 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
constexpr int value() { static constexpr int n = 7; return n; }5. How It Works
The sample builds the smallest expression or object needed for C++23
constexprExtensions.It uses a C++23-allowed static constant inside a
constexprfunction when supported. The compiler and library apply the relevant rule before the program observes the result.The program prints or checks a value proven by
static_assertor a clear support message on this toolchain, making the important behavior easy to verify.
6. Common Mistakes
Assuming every statement in a
constexprfunction can execute during constant evaluation confuses declaration rules with evaluation rules.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 algorithms that should work both at compile time and runtime with one readable body.
Avoid it when the task involves forcing large work into compile time when it slows builds without improving correctness.
8. Simple Example
A small table-size calculation is checked at compile time and reused at runtime. The downloadable program keeps the data fixed so the output can be compared without entering input.
Complete sample code
Source file
cpp23/08_constexpr_extensions/main.cpp
#include <iostream>
#if defined(__cpp_constexpr) && __cpp_constexpr >= 202211L
constexpr int cached_value() {
// Static constexpr locals became usable here in C++23.
static constexpr int value = 7;
return value;
}
#endif
int main() {
#if defined(__cpp_constexpr) && __cpp_constexpr >= 202211L
constexpr int value = cached_value();
static_assert(value == 7);
std::cout << "value=" << value << '\n';
#else
std::cout << "C++23 constexpr extension unavailable\n";
#endif
}
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 — Why may a
constexprfunction contain a construct that prevents one path from being a constant expression while another path still succeeds?