Concepts and Constraints with Separate Files

1// Concepts.h
2#pragma once
3#include <concepts>
4#include <iostream>
5
6template <typename T>
7concept Numeric = std::integral<T>
8  || std::floating_point<T>;
9
10template <typename T>
11concept Printable = requires(T t) {
12  { std::cout << t } -> std::same_as<std::ostream&>;
13};

1// MyTemplate.h
2#pragma once
3#include "Concepts.h"
4
5template <Numeric T, Printable U>
6class MyTemplate {
7 public:
8  void foo();
9  void bar();
10};

1// MyTemplate.cpp
2#include <iostream>
3#include "MyTemplate.h"
4
5template <Numeric T, Printable U>
6void MyTemplate<T, U>::foo() {
7  std::cout << "foo() called\n";
8}
9
10template <Numeric T, Printable U>
11void MyTemplate<T, U>::bar() {
12  std::cout << "bar() called\n";
13}
14
15// Explicit instantiations
16template class MyTemplate<int, std::string>;
17template class MyTemplate<double, char>;

1// main.cpp
2#include <string>
3#include "MyTemplate.h"
4
5int main() {
6  MyTemplate<int, std::string> obj1;
7  obj1.foo(); // Logs "foo() called"
8
9  MyTemplate<double, char> obj2;
10  obj2.bar(); // Logs "bar() called"
11
12  // Compile-time error:
13  MyTemplate<std::string, int> obj3; 
14}

1// Concepts.h
2#include <concepts>
3
4template <typename T>
5concept Drawable = requires(T t) {
6  { t.draw() } -> std::same_as<void>;
7};
8
9// MyTemplate.h
10template <Drawable T>
11class Renderer {
12 public:
13  void render(T& obj);
14};
15
16// MyTemplate.cpp
17template <Drawable T>
18void Renderer<T>::render(T& obj) {
19  obj.draw();
20}
21
22// Explicit instantiation for a Drawable type
23class Circle {
24 public:
25  void draw() { /* ... */
26  }
27};
28
29template class Renderer<Circle>;