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<!doctype html>
<html>
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0, maximum-scale=1.0, user-scalable=no">
<title>Compile-time programming in C++20 and beyond</title>
<meta name="author" content="Louis Dionne">
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<div class="reveal">
<div class="slides">
<section data-markdown=""
data-separator="^====+$"
data-separator-vertical="^----+$"
data-notes="^Note:">
<script type="text/template">
## Compile-time programming in C++20 and beyond
<b>Louis Dionne <[@LouisDionne](https://twitter.com/LouisDionne)></b>
C++ Standard Library Engineer @ Apple
<!-- TODO:
- At all times, make it clear what is expected for C++20 and what isn't
- Mention compilation speed improvements over TMP
- Talk about Immediate functions / `constexpr!`
-->
====
<!-- .slide: class="slide-hidden" -->
### Font test
```c++
class ThisIsAClass {
};
int main() {
// this is a comment
std::vector<Foobar> foobar;
foobar.push_back(Foo{...});
foobar.push_back(Bar{...});
foobar.push_back(Baz{...});
for (auto& x : foobar) {
x.do_something();
}
return 0;
}
```
==============================================================================
### Why do we care about compile-time programming?
====
### Constant initialization
```c++
// Lookup table for e^i with i in [0, 1000]
constexpr std::array<double, 1001> exps = compute_exp(0, 1000);
int main() {
int i = read_from_user();
std::cout << exps[i] << '\n';
return 0;
}
```
Note:
- No static initialization
- Hardcoded in the data segment
====
<!-- .slide: class="slide-hidden" -->
### Defining flexible APIs
```c++
template <typename T>
std::string to_json(T const& t) {
std::string result = "{";
auto members = members_of(t); // not valid today!
for (auto member = begin(members); member != end(members); ++member){
result += member->name();
result += ":";
result += to_json(member->value());
if (std::next(member) != end(members))
result += ",";
}
result += "}";
return result;
}
struct Foo { int x; float y; double z; };
std::string json = to_json(Foo{1, 2.2f, 3.3});
```
Note:
This is indeed compile-time programming since the members are known at
compile-time. This wouldn't be a "real" for-loop.
====
### Boilerplate reduction
```c++
enum class Color { RED, GREEN, BLUE, YELLOW, PURPLE };
std::ostream& operator<<(std::ostream& out, Color color) {
switch (color) {
case Color::RED: out << "RED"; break;
case Color::GREEN: out << "GREEN"; break;
case Color::BLUE: out << "BLUE"; break;
case Color::YELLOW: out << "YELLOW"; break;
case Color::PURPLE: out << "PURPLE"; break;
}
return out;
}
int main() {
Color color = read_from_user();
std::cout << color << '\n';
return 0;
}
```
==============================================================================
### Solving these problems used to be a nerdy topic
====
### Now all the cool kids are on it
- The Reflection TS: [wg21.link/n4746](http://wg21.link/n4746)
- Standard containers and `constexpr`: [p0784](http://wg21.link/p0784)
- Making `std::vector` `constexpr`: [p1004](http://wg21.link/p1004)
- Making `<algorithm>` `constexpr`: [p0202](http://wg21.link/p0202) & al
- `std::is_constant_evaluated()`: [p0595](http://wg21.link/p0595)
- `constexpr!` functions: [p1073](http://wg21.link/p1073)
- Metaclasses: [p0707](http://wg21.link/p0707)
- Value-based reflection: [p0993](http://wg21.link/p0993)
- Calling virtual functions in `constexpr`: [p1064](http://wg21.link/p1064)
- `try-catch` in `constexpr`: [p1002](http://wg21.link/p1002)
<!-- - Mental model for compile-time programming: [p0992](http://wg21.link/p0992) -->
<!-- - Class Types in Non-Type Template Parameters: [p0732](http://wg21.link/p0732) -->
Note:
- Say something good about the direction group.
- Also, mention wg21.link for all paper numbers
====
### This can be difficult to follow
Even committee members are confused
====
### So, what's the plan?
<ol>
<li class="fragment">Make (almost) all of C++ available in `constexpr`</li>
<li class="fragment">Provide a `constexpr` API to _query_ the compiler</li>
<li class="fragment">Provide a means for modifying the AST</li>
</ol>
Note:
Even though this is C++, we do have a plan!
==============================================================================
### Step 1: Expanding `constexpr`
Necessary in order to use non-trivial data structures and execute non-trivial
logic at compile-time.
====
#### In C++11, `constexpr` was very limited
```c++
constexpr std::size_t count_lower(char const* s,
std::size_t count = 0)
{
return *s == '\0' ? count
: 'a' <= *s && *s <= 'z'
? count_lower(s+1, count + 1)
: count_lower(s+1, count);
}
static_assert(count_lower("aBCdeF") == 3, "");
```
====
#### In C++14, many restrictions lifted
```c++
constexpr std::size_t count_lower(char const* s) {
std::size_t count = 0;
for (; *s != '\0'; ++s) {
if ('a' <= *s && *s <= 'z') {
++count;
}
}
return count;
}
static_assert(count_lower("aBCdeF") == 3, "");
```
Note:
- Loops
- `if` statements
- variables can be modified
- lambdas can be used
====
#### In C++17, `constexpr` is still limited
- No allocations
- No try-catch
- No virtual calls
- No `reinterpret_cast`
====
This means we can't use variable-size containers
```c++
template <typename Predicate>
constexpr std::vector<int>
keep_if(int const* it, int const* last, Predicate pred) {
std::vector<int> result;
for (; it != last; ++it) {
if (pred(*it)) {
result.push_back(*it);
}
}
return result;
}
constexpr int ints[] = {1, 2, 3, 4, 5, 6};
constexpr std::vector<int> odds =
keep_if(begin(ints), end(ints), is_odd);
// doesn't work!
```
====
We have to use fixed-size arrays, which is painful
```c++
template <std::size_t K, typename Predicate>
constexpr std::array<int, K>
keep_if(int const* it, int const* last, Predicate pred) {
std::array<int, K> result;
int* out = begin(result);
for (; it != last; ++it) {
if (pred(*it)) {
*out++ = *it;
}
}
return result;
}
constexpr int ints[] = {1, 2, 3, 4, 5, 6};
constexpr std::array<int, 3> odds =
keep_if<3>(begin(ints), end(ints), is_odd);
```
Note:
Problem is that we don't know the size of the output sequence in advance.
====
#### `constexpr` algorithms don't compose well
====
### Enter [p0784](http://wg21.link/p0784)
More `constexpr` containers
- Enables _new-expressions_ in `constexpr`
- Makes `std::allocator` usable in `constexpr`
- Promotes some `constexpr` objects to static storage
====
#### The following becomes valid
```c++
constexpr int* square(int* first, int* last) {
std::size_t N = last - first;
int* result = new int[N]; // <== HERE
for (std::size_t i = 0; i != N; ++i, ++first) {
result[i] = *first * *first;
}
return result;
}
constexpr void hello() {
int ints[] = {1, 2, 3, 4, 5};
int* squared = square(std::begin(ints), std::end(ints));
delete[] squared;
}
```
====
### Obvious next step
Make `std::vector` `constexpr`
====
#### But `std::vector` uses try-catch
So make try-catch `constexpr`!
Note:
Try-catch is used to provide the strong exception guarantee in case the
underlying operation fails.
====
### Enter [p1002](http://wg21.link/p1002)
Try-catch blocks in `constexpr` functions
```c++
template <std::size_t N>
constexpr std::array<int, N>
square(std::array<int, N> array, int from, int to) {
try {
for (; from != to; ++from) {
array.at(from) = array.at(from) * array.at(from);
}
} catch (std::out_of_range const& e) {
// ...
}
return array;
}
constexpr auto OK = square(std::array{1, 2, 3, 4}, 0, 4);
constexpr auto NOT_OK = square(std::array{1, 2, 3, 4}, 0, 10);
```
====
### But it's not as cool as you think
`throw` is still not allowed
====
Line 14 works because we never execute a `throw` statement:
```c++
template <std::size_t N>
constexpr std::array<int, N>
square(std::array<int, N> array, int from, int to) {
try {
for (; from != to; ++from) {
array.at(from) = array.at(from) * array.at(from);
}
} catch (std::out_of_range const& e) {
// ...
}
return array;
}
constexpr auto OK = square(std::array{1, 2, 3, 4}, 0, 4);
constexpr auto NOT_OK = square(std::array{1, 2, 3, 4}, 0, 10);
```
====
Line 15 fails when we execute the `throw` inside `array::at`:
```c++
template <class T, std::size_t Size>
constexpr typename array<T, Size>::reference
array<T, Size>::at(std::size_t n) {
if (n >= Size)
throw std::out_of_range("array::at"); // compilation error here
return elements_[n];
}
```
====
In the future, we can extend the language
Note:
Implementing full EH at compile-time is tricky -- compiler has to unwind the
stack, etc..
====
### Back to `std::vector`
In C++20, this will just work:
```c++
template <typename Predicate>
constexpr std::vector<int>
keep_if(int const* it, int const* last, Predicate pred) {
std::vector<int> result;
for (; it != last; ++it) {
if (pred(*it)) {
result.push_back(*it);
}
}
return result;
}
constexpr std::vector<int> ints = {1, 2, 3, 4, 5, 6};
constexpr std::vector<int> odds =
keep_if(begin(ints), end(ints), is_odd);
```
====
### Promotion to static storage
Also called non-transient allocations
====
#### What happens with this code?
Where is `table` stored?
```c++
constexpr std::vector<int> table = compute_table();
int main(int argc, char* argv[]) {
int from = std::atoi(argv[1]);
int to = std::atoi(argv[2]);
for (; from != to; ++from) {
std::cout << table[from] << '\n';
}
}
```
====
#### Answer: promoted to static storage
- If a `constexpr` object "leaks" from compile-time
- And its destructor would clean up if it were called
- Then it is promoted to static storage
====
### That's really nothing new
Here, `table` will be in the data segment
```c++
constexpr std::array<int, 4> table = {1, 2, 3, 4};
int main(int argc, char* argv[]) {
int from = std::atoi(argv[1]);
int to = std::atoi(argv[2]);
for (; from != to; ++from) {
std::cout << table[from] << '\n';
}
}
```
====
If executing the destructor would not clean up, then it's not a constant expression
====
#### Corollary
If you can evaluate a function call in a constant expression, then that
function call is leak-free for the provided inputs.
====
The compiler turns into a leak detector 😏
Note:
I'm putting Valgrind out of business
====
### Future `constexpr` additions
<ul>
<li class="fragment">`std::string`</li>
<li class="fragment">`std::map` and `std::unordered_map`</li>
<li class="fragment">`std::optional`/`std::variant`?</li>
<li class="fragment">Math functions?</li>
<li class="fragment"><strike>`std::thread` (multithreaded compilation)</strike> 🤕</li>
<li class="fragment">Almost everything? Join the effort!</li>
</ul>
====
### Challenges
- `reinterpret_cast` (e.g. `std::string` SBO)
- non-`constexpr` builtins (e.g. `__builtin_memcpy`)
- Raw memory allocation (e.g. `malloc`)
- Other annotations (e.g. UBSan in libc++)
====
<!-- .slide: class="slide-hidden" -->
### Aside: why no `reinterpret_cast`?
Note:
- Compiler has to catch UB inside `constexpr`
- Otherwise the compilation itself is UB and you get an arbitrary program
- `reinterpret_cast` makes UB difficult to catch
====
<!-- .slide: class="slide-hidden" -->
### Aside: why no raw allocation?
Note:
- Similar problem as `reinterpret_cast`
- Compiler hands you an arbitrary chunk of memory
- Trusts you to initialize and manage lifetime properly
- Otherwise, UB during compilation
====
### Enter [p0595](http://wg21.link/p0595)
- Adds `std::is_constant_evaluated()`
- Allows detecting whether the current function is evaluated as part
of a constant expression
====
### For example
```c++
template <typename T>
void vector<T>::clear() noexcept {
size_t old_size = size();
// destroy [begin(), end())
__annotate_shrink(old_size); // ASAN annotation
__invalidate_all_iterators(); // Debug mode checks
}
```
====
### Making this `constexpr`
```c++
template <typename T>
constexpr void vector<T>::clear() noexcept {
size_t old_size = size();
// destroy [begin(), end())
if (!std::is_constant_evaluated()) {
__annotate_shrink(old_size); // ASAN annotation
__invalidate_all_iterators(); // Debug mode checks
}
}
```
Note:
Be careful, `if constexpr (std::is_constant_evaluated())` is always `true`.
====
### How this works
```c++
constexpr int f() {
std::vector<int> v = {1, 2, 3};
v.clear();
return 0;
}
int main() {
// is_constant_evaluated() true, no annotations
constexpr int X = f();
// is_constant_evaluated() false, normal runtime code
int Y = f();
}
```
====
### Another common problem
`constexpr` does not require compile-time evaluation
====
#### Enter `constexpr!` ([p1073](http://wg21.link/p1073))
```c++
constexpr! int square(int x) {
return x * x;
}
constexpr int x = square(3); // OK
int y = 3;
int z = square(y); // ERROR: square(y) is not a constant expression
```
====
### Summary of `constexpr` changes
- Expand `constexpr` to support more use cases
- Allow persisting data structures to the _data segment_
- Allow writing different code for `constexpr` and runtime when needed
- Require compile-time evaluation with `constexpr!`
==============================================================================
### Step 2: an API to speak to the compiler
====
### Basically, extract information about types
====
### We can already do some of it
type_traits, operators like `sizeof`
```c++
struct Foo {
int x;
int y;
};
constexpr std::size_t size = sizeof(Foo);
constexpr bool is_aggregate = std::is_aggregate_v<Foo>;
```
====
### But we're severely limited
Currently, limited to queries whose answer is a primitive type
```c++
struct Foo {
int x;
int y;
};
constexpr auto members = ???; // list of Foo's members
```
Note:
We lack:
- A way to query many properties
- A way to represent the answer to those queries
====
### Enter the Reflection TS: [n4746](http://wg21.link/n4746)
Note:
Explain what a TS is.
====
### Purpose:
- Figure out what this query API should look like
- Not the specific implementation of this API
- For now, the API uses template metaprogramming
====
### Example: extracting members
```c++
struct Foo {
int x;
long y;
};
using MetaFoo = reflexpr(Foo);
using Members = std::reflect::get_data_members_t<MetaFoo>;
using MetaX = std::reflect::get_element_t<0, Members>; // Not an int!
constexpr bool is_public = std::reflect::is_public_v<MetaX>;
using X = std::reflect::get_reflected_type_t<MetaX>; // This is int!
```
Note:
- `reflexpr(Foo)` is a magic type representing `Foo` in the compiler
- `Members` is a magic type satisfying the `ObjectSequence` concept.
- `X` is a magic type representing `Foo::x`
====
### Another example: printing an `enum`
```c++
enum class Color { RED, GREEN, BLUE, YELLOW, PURPLE };
std::ostream& operator<<(std::ostream& out, Color color) {
using Enumerators = std::reflect::get_enumerators_t<reflexpr(Color)>;
auto helper = [&]<std::size_t ...i>(std::index_sequence<i...>) {
([&] {
using Enumerator = std::reflect::get_element_t<i, Enumerators>;
if (color == std::reflect::get_constant_v<Enumerator>) {
out << std::reflect::get_name_v<Enumerator>;
}
}(), ...);
};
constexpr std::size_t N = std::reflect::get_size_v<Enumerators>;
helper(std::make_index_sequence<N>{});
return out;
}
```
Note:
Live example: https://wandbox.org/permlink/sXTQqw54It7hMGes
====
### Other features:
- Get source line/column of a type definition
- Get the name of an entity as a string
- Get member types/enums/etc of a type
- Get base classes of a type
- Get whether variable is `static`/`constexpr`
- Get properties of base classes: `virtual`/`public`/etc
====
### More features planned in the future
- Reflecting functions: [p0670](http://wg21.link/p0670)
- Plans to reflect on arbitrary expressions too!!!
Note:
Consider joking that the Committee is not even aware that we made a step in
that direction.
====
### Final syntax not settled yet: [p0953](http://wg21.link/p0953)
```c++
struct Foo {
int x;
long y;
};
constexpr std::reflect::Record meta_foo = reflexpr(Foo);
constexpr std::vector members = meta_foo.get_data_members();
constexpr std::reflect::RecordMember meta_x = members[0];
constexpr bool is_public = meta_x.is_public();
constexpr std::reflect::Type x = meta_x.get_reflected_type();
using X = unreflexpr(x); // This is int!
```
====
### Another example: print an `enum`
```c++
enum class Color { RED, GREEN, BLUE, YELLOW, PURPLE };
std::ostream& operator<<(std::ostream& out, Color color) {
constexpr std::vector enumerators = reflexpr(Color).get_enumerators();
for... (constexpr std::reflect::Enumerator enumerator : enumerators) {
if (color == enumerator.get_constant()) {
out << enumerator.get_name();
}
}
return out;
}
```
====
### `for...`? [P0589](http://wg21.link/p0589)
```c++
std::ostream& operator<<(std::ostream& out, Color color) {
constexpr std::vector enumerators = reflexpr(Color).get_enumerators();
for... (constexpr std::reflect::Enumerator enumerator : enumerators) {
if (color == enumerator.get_constant()) {
out << enumerator.get_name();
}
}
return out;
}
```
Note:
Explain it can't be a normal for-loop, because we could for example
instantiate templates inside the for-loop. So this needs to be an
unrolled for-loop.
====
### Expands roughly to
```c++
std::ostream& operator<<(std::ostream& out, Color color) {
constexpr std::vector enumerators = reflexpr(Color).get_enumerators();
{
constexpr std::reflect::Enumerator enumerator = enumerators[0];
if (color == enumerator.get_constant()) {
out << enumerator.get_name();
}
}
{
constexpr std::reflect::Enumerator enumerator = enumerators[1];
if (color == enumerator.get_constant()) {
out << enumerator.get_name();
}
}
// ...
return out;
}
```
====
### Status of reflection
1. Reflection TS figures out the compiler query API
2. Will rebase on top of the `constexpr` work
3. Aim is to write normal-looking C++ code
==============================================================================
### Step 3: Code injection
Note:
So far, we can:
1. Write complex compile-time algorithms
2. Query the compiler AST
Goal: modify the compiler's AST
====
### Alternatives considered in [p0633](http://wg21.link/p0633)
<ol>
<li class="fragment strike" data-fragment-index="1">Raw string injection</li>
<li class="fragment strike" data-fragment-index="1">Programmatic API</li>
<li> Token-sequence injection</li>
</ol>
====
### Illustration: creating interfaces
```c++
struct Shape {
int area() const;
void scale_by(double factor);
};
```
Should magically transform to
```c++
struct Shape {
virtual int area() const = 0;
virtual void scale_by(double factor) = 0;
virtual ~Shape() noexcept { }
};
```
====
### Define the transformation
```c++
constexpr! void interface(std::reflect::Type source) {
for (auto f : source.functions()) {
if (!f.has_access())
f.make_public();
f.make_pure_virtual();
-> f;
}
-> class C {
virtual ~C() noexcept { }
};
}
```
Note:
The fragments are queued for injection and will be injected
====
### Apply the transformation
```c++
struct Shape {
int area() const;
void scale_by(double factor);
};
class IShape {
constexpr {
interface(reflexpr(Shape));
}
};
```
====
### What about metaclasses ([p0707](http://wg21.link/p0707))?
They can be built on top of code injection
====
```c++
class(interface) IShape {
int area() const;
void scale_by(double factor);
};
```
Desugars roughly to
```c++
class IShape {
class __IShape {
int area() const;
void scale_by(double factor);
};
constexpr { interface(reflexpr(__IShape)); }
};
```
====
### Status of code injection
- Provide a way to influence the AST
- Exact means not settled yet
- Should interact well with `constexpr` control flow
- Metaclasses can be built on top