1. Problem It Solves
Constraints keep invalid template arguments out of an interface, and abbreviated syntax removes boilerplate for simple constrained parameters. It makes an important assumption visible and checkable.
2. Prerequisites
Standard concepts and ordinary function templates.
You should be able to compile a short program and read its output.
3. Core Idea
A constrained template is a normal template with a gate. Writing Concept auto places that gate directly beside the function parameter. Read Concept auto as a precise promise; runtime preconditions still belong to the programmer.
4. Minimal Syntax
std::integral auto square(std::integral auto value);5. How It Works
The program introduces the smallest relevant form of
Concept auto.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
Different constraint placements can look equivalent yet participate differently in overload ordering when their constraint expressions are not structurally related.
Also check the required header, C++20 library support, lifetime, and deduced types.
7. When to Use It
Use it when a function is generic but each parameter has a clear concept-level contract.
Avoid it when you need to name or relate several template parameters in a more explicit declaration.
8. Simple Example
The program defines one named constrained template and one abbreviated function, both restricted to integers. The companion .cpp uses no input, so its result is easy to reproduce.
Complete sample code
Source file
cpp20/10_constrained_templates_abbreviated_functions/main.cpp
// Day 10: Constrained Templates and Abbreviated Function Templates
#include <concepts>
#include <iostream>
template<std::integral T>
T square(T value) {
return value * value;
}
std::integral auto increment(std::integral auto value) {
return value + 1;
}
int main() {
std::cout << "square = " << square(5) << '\n';
std::cout << "increment = " << increment(5) << '\n';
}
9. Key Takeaways
Concept autoexpresses 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
Concept autoin the minimal example?Medium — What template parameter does the compiler invent for
std::integral auto value?Hard — When two parameters must have exactly the same type, why may two separate
std::integral autoparameters be too weak?