C++ concepts basics

Mircea Baja @ ACCU Oxford - Lightning Talks - 18th July 2017

# C++ concepts basics

C++ concepts (N4647): cover the basics and show how they relate to templates
(the "peel off onion layers" technique).

(v2)

---

# Simple generic function

```cpp
template<typename T>
const T & min(const T & a, const T & b) {
  // implementation here
}
```

--
```cpp
if (a < b) return a; else return b;  // 1
if (a <= b) return a; else return b; // 2
if (a > b) return b; else return a;  // 3
if (b < a) return b; else return a;  // 4
```
---

# Concept definition

```cpp
template<typename T>
concept bool StrictlyTotallyOrdered() {
  return EqualityComparable<T>() &&
    requires (const T a, const T b) {
      { a < b } -> Boolean;
      { a > b } -> Boolean;
      { a <= b } -> Boolean;
      { a >= b } -> Boolean;
    };
}
```

requires above is a requires-expression

---

# Usage

```cpp
const StrictlyTotallyOrdered & min(
    const StrictTotallyOrdered & a,
    const StrictTotallyOrdered & b) {
  if (b < a) return b; else return a;
}
```

```cpp
template<StrictlyTotallyOrdered T>
const T & min(const T & a, const T & b) {
  if (b < a) return b; else return a;
}
```

```cpp
template<typename T>
  requires StrictlyTotallyOrdered<T>()
const T & min(const T & a, const T & b) {
  if (b < a) return b; else return a;
}
```

requires above is a requires-clause

---

# Constraints

```cpp
requires(const B b1, const B b2) {
  bool(b1); // expression constraint
  typename B::type; // type constraint
  { !b1 } -> bool; // implicit conversion
  { b1 && b2 } -> Same<bool>; // argument deduction
}
```

```cpp
void fn(Same<bool> x); // invent
fn(b1 && b2); // check if it compiles
```

```cpp
template<Same<bool> T>
void fn(T x);
```

```cpp
template<typename T>
  requires Same<T, bool>
void fn(T x);
```

---

# References

Concepts overview (Bjarne Stroustrup)

http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0557r0.pdf

Concepts proposal wording (Andrew Sutton)

http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/n4674.pdf

Ranges TS (Eric Niebler, Casey Carter)

http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/n4671.pdf

Elements of Programming (Alexander A. Stepanov and Paul McJones)

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# Questions?

https://bajamircea.github.io/presentations/2017-07-18-concepts-basics.html