1. Problem It Solves
Once the object type is deduced, a member can constrain that type just like any other template parameter. This produces one generic member with a readable participation rule. 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 4: concepts and constrained templates.
Days 11–12: explicit and deduced object parameters.
3. Core Idea
The explicit object parameter opens a gate; the concept is the guard checking whether the arriving object has the operations the body needs. 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>
requires HasValue<Self>
int read(this Self const& self);5. How It Works
The sample builds the smallest expression or object needed for Deducing
thiswith Templates and Concepts.It calls a constrained explicit-object member on a type that satisfies
HasValue. The compiler and library apply the relevant rule before the program observes the result.The program prints or checks the contained value, with invalid object shapes rejected before the body is instantiated, making the important behavior easy to verify.
6. Common Mistakes
Constraining the wrong form of
Selfcan accidentally rejectconstor reference-qualified calls; a syntactic concept can still miss semantic requirements.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 generic mixins whose members should exist only for compatible derived object shapes.
Avoid it when the task involves adding a concept that merely repeats errors already obvious in a non-generic member.
8. Simple Example
A serialization mixin enables save() only when the final object exposes the required fields. The downloadable program keeps the data fixed so the output can be compared without entering input.
Complete sample code
Source file
cpp23/13_deducing_this_templates_concepts/main.cpp
#include <concepts>
#include <iostream>
#if defined(__cpp_explicit_this_parameter)
template <class T>
concept HasValue = requires(const T& object) {
{ object.value } -> std::convertible_to<int>;
};
struct Box {
int value{42};
template <class Self>
int read(this const Self& self) requires HasValue<Self> {
return self.value;
}
};
#endif
int main() {
#if defined(__cpp_explicit_this_parameter)
const Box box;
std::cout << box.read() << '\n';
#else
std::cout << "constrained 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 — Should
HasValue<Self>inspectSelf,remove_reference_t<Self>, or an expression onself, and how can that choice change const-reference calls?