1. Problem It Solves
Accessors often need four forms: mutable lvalue, const lvalue, mutable rvalue, and const rvalue. A deduced explicit object parameter can preserve all four in one body. 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 3: forwarding references and
std::forward.Day 11: explicit object parameter syntax.
3. Core Idea
The object arrives through Self&&. Forwarding self is like reflecting the caller's const and lvalue/rvalue properties onto the selected member. 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
template<class Self>
decltype(auto) get(this Self&& self);5. How It Works
The sample builds the smallest expression or object needed for Deducing
thiswithconstand Value Categories.It checks the reference types returned from mutable, const, and moved objects. The compiler and library apply the relevant rule before the program observes the result.
The program prints or checks compile-time type assertions proving that cv/ref information is preserved, making the important behavior easy to verify.
6. Common Mistakes
Returning
(self.value)without forwarding can collapse rvalue access into an lvalue reference; returning a reference from a temporary can also dangle after the full expression.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 accessors and adapters that must mirror the caller's object category.
Avoid it when the task involves exposing a reference from an rvalue object when the caller may store it beyond the object's lifetime.
8. Simple Example
A wrapper returns T& for a mutable lvalue, const T& for a const lvalue, and T&& for a temporary. The downloadable program keeps the data fixed so the output can be compared without entering input.
Complete sample code
Source file
cpp23/12_deducing_this_const_value_categories/main.cpp
#include <iostream>
#include <type_traits>
#include <utility>
#if defined(__cpp_explicit_this_parameter)
struct Box {
int value{7};
template <class Self>
decltype(auto) get(this Self&& self) {
return (std::forward<Self>(self).value);
}
};
#endif
int main() {
#if defined(__cpp_explicit_this_parameter)
Box box;
const Box fixed;
static_assert(std::is_same_v<decltype(box.get()), int&>);
static_assert(std::is_same_v<decltype(fixed.get()), const int&>);
static_assert(std::is_same_v<decltype(std::move(box).get()), int&&>);
std::cout << fixed.get() << '\n';
#else
std::cout << "deducing this 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 — For
const Box&&, what doesSelfdeduce to and what reference type should a correctly forwarded data member produce?