c++17新特性

# C++17

## Overview
Many of these descriptions and examples come from various resources (see [Acknowledgements](#acknowledgements) section), summarized in my own words.

C++17 includes the following new language features:
- [template argument deduction for class templates](#template-argument-deduction-for-class-templates)
- [declaring non-type template parameters with auto](#declaring-non-type-template-parameters-with-auto)
- [folding expressions](#folding-expressions)
- [new rules for auto deduction from braced-init-list](#new-rules-for-auto-deduction-from-braced-init-list)
- [constexpr lambda](#constexpr-lambda)
- [lambda capture this by value](#lambda-capture-this-by-value)
- [inline variables](#inline-variables)
- [nested namespaces](#nested-namespaces)
- [structured bindings](#structured-bindings)
- [selection statements with initializer](#selection-statements-with-initializer)
- [constexpr if](#constexpr-if)
- [utf-8 character literals](#utf-8-character-literals)
- [direct-list-initialization of enums](#direct-list-initialization-of-enums)
- [fallthrough, nodiscard, maybe_unused attributes](#fallthrough-nodiscard-maybe_unused-attributes)

C++17 includes the following new library features:
- [std::variant](#stdvariant)
- [std::optional](#stdoptional)
- [std::any](#stdany)
- [std::string_view](#stdstring_view)
- [std::invoke](#stdinvoke)
- [std::apply](#stdapply)
- [std::filesystem](#stdfilesystem)
- [std::byte](#stdbyte)
- [splicing for maps and sets](#splicing-for-maps-and-sets)
- [parallel algorithms](#parallel-algorithms)

## C++17 Language Features

### Template argument deduction for class templates
Automatic template argument deduction much like how it's done for functions, but now including class constructors.
```c++
template 
struct MyContainer {
  T val;
  MyContainer() : val{} {}
  MyContainer(T val) : val{val} {}
  // ...
};
MyContainer c1 {1}; // OK MyContainer
MyContainer c2; // OK MyContainer
```

### Declaring non-type template parameters with auto
Following the deduction rules of `auto`, while respecting the non-type template parameter list of allowable types[\*], template arguments can be deduced from the types of its arguments:
```c++
template 
struct my_integer_sequence {
  // Implementation here ...
};

// Explicitly pass type `int` as template argument.
auto seq = std::integer_sequence();
// Type is deduced to be `int`.
auto seq2 = my_integer_sequence<0, 1, 2>();
```
\* - For example, you cannot use a `double` as a template parameter type, which also makes this an invalid deduction using `auto`.

### Folding expressions
A fold expression performs a fold of a template parameter pack over a binary operator.
* An expression of the form `(... op e)` or `(e op ...)`, where `op` is a fold-operator and `e` is an unexpanded parameter pack, are called _unary folds_.
* An expression of the form `(e1 op ... op e2)`, where `op` are fold-operators, is called a _binary fold_. Either `e1` or `e2` is an unexpanded parameter pack, but not both.
```c++
template 
bool logicalAnd(Args... args) {
    // Binary folding.
    return (true && ... && args);
}
bool b = true;
bool& b2 = b;
logicalAnd(b, b2, true); // == true
```
```c++
template 
auto sum(Args... args) {
    // Unary folding.
    return (... + args);
}
sum(1.0, 2.0f, 3); // == 6.0
```

### New rules for auto deduction from braced-init-list
Changes to `auto` deduction when used with the uniform initialization syntax. Previously, `auto x {3};` deduces a `std::initializer_list`, which now deduces to `int`.
```c++
auto x1 {1, 2, 3}; // error: not a single element
auto x2 = {1, 2, 3}; // x2 is std::initializer_list
auto x3 {3}; // x3 is int
auto x4 {3.0}; // x4 is double
```

### constexpr lambda
Compile-time lambdas using `constexpr`.
```c++
auto identity = [](int n) constexpr { return n; };
static_assert(identity(123) == 123);
```
```c++
constexpr auto add = [](int x, int y) {
  auto L = [=] { return x; };
  auto R = [=] { return y; };
  return [=] { return L() + R(); };
};

static_assert(add(1, 2)() == 3);
```
```c++
constexpr int addOne(int n) {
  return [n] { return n + 1; }();
}

static_assert(addOne(1) == 2);
```

### Lambda capture `this` by value
Capturing `this` in a lambda's environment was previously reference-only. An example of where this is problematic is asynchronous code using callbacks that require an object to be available, potentially past its lifetime. `*this` (C++17) will now make a copy of the current object, while `this` (C++11) continues to capture by reference.
```c++
struct MyObj {
  int value {123};
  auto getValueCopy() {
    return [*this] { return value; };
  }
  auto getValueRef() {
    return [this] { return value; };
  }
};
MyObj mo;
auto valueCopy = mo.getValueCopy();
auto valueRef = mo.getValueRef();
mo.value = 321;
valueCopy(); // 123
valueRef(); // 321
```

### Inline variables
The inline specifier can be applied to variables as well as to functions. A variable declared inline has the same semantics as a function declared inline.
```c++
// Disassembly example using compiler explorer.
struct S { int x; };
inline S x1 = S{321}; // mov esi, dword ptr [x1]
                      // x1: .long 321

S x2 = S{123};        // mov eax, dword ptr [.L_ZZ4mainE2x2]
                      // mov dword ptr [rbp - 8], eax
                      // .L_ZZ4mainE2x2: .long 123
```

It can also be used to declare and define a static member variable, such that it does not need to be initialized in the source file.
```c++
struct S {
  S() : id{count++} {}
  ~S() { count--; }
  int id;
  static inline int count{0}; // declare and initialize count to 0 within the class
};
```

### Nested namespaces
Using the namespace resolution operator to create nested namespace definitions.
```c++
namespace A {
  namespace B {
    namespace C {
      int i;
    }
  }
}
// vs.
namespace A::B::C {
  int i;
}
```

### Structured bindings
A proposal for de-structuring initialization, that would allow writing `auto [ x, y, z ] = expr;` where the type of `expr` was a tuple-like object, whose elements would be bound to the variables `x`, `y`, and `z` (which this construct declares). _Tuple-like objects_ include `std::tuple`, `std::pair`, `std::array`, and aggregate structures.
```c++
using Coordinate = std::pair;
Coordinate origin() {
  return Coordinate{0, 0};
}

const auto [ x, y ] = origin();
x; // == 0
y; // == 0
```
```c++
std::unordered_map mapping {
  {"a", 1},
  {"b", 2},
  {"c", 3}
};

// Destructure by reference.
for (const auto& [key, value] : mapping) {
  // Do something with key and value
}
```

### Selection statements with initializer
New versions of the `if` and `switch` statements which simplify common code patterns and help users keep scopes tight.
```c++
{
  std::lock_guard lk(mx);
  if (v.empty()) v.push_back(val);
}
// vs.
if (std::lock_guard lk(mx); v.empty()) {
  v.push_back(val);
}
```
```c++
Foo gadget(args);
switch (auto s = gadget.status()) {
  case OK: gadget.zip(); break;
  case Bad: throw BadFoo(s.message());
}
// vs.
switch (Foo gadget(args); auto s = gadget.status()) {
  case OK: gadget.zip(); break;
  case Bad: throw BadFoo(s.message());
}
```

### constexpr if
Write code that is instantiated depending on a compile-time condition.
```c++
template 
constexpr bool isIntegral() {
  if constexpr (std::is_integral::value) {
    return true;
  } else {
    return false;
  }
}
static_assert(isIntegral() == true);
static_assert(isIntegral() == true);
static_assert(isIntegral() == false);
struct S {};
static_assert(isIntegral() == false);
```

### UTF-8 character literals
A character literal that begins with `u8` is a character literal of type `char`. The value of a UTF-8 character literal is equal to its ISO 10646 code point value.
```c++
char x = u8'x';
```

### Direct list initialization of enums
Enums can now be initialized using braced syntax.
```c++
enum byte : unsigned char {};
byte b {0}; // OK
byte c {-1}; // ERROR
byte d = byte{1}; // OK
byte e = byte{256}; // ERROR
```

### fallthrough, nodiscard, maybe_unused attributes
C++17 introduces three new attributes: `[[fallthrough]]`, `[[nodiscard]]` and `[[maybe_unused]]`.
* `[[fallthrough]]` indicates to the compiler that falling through in a switch statement is intended behavior.
```c++
switch (n) {
  case 1: [[fallthrough]]
    // ...
  case 2:
    // ...
    break;
}
```

* `[[nodiscard]]` issues a warning when either a function or class has this attribute and its return value is discarded.
```c++
[[nodiscard]] bool do_something() {
  return is_success; // true for success, false for failure
}

do_something(); // warning: ignoring return value of 'bool do_something()',
                // declared with attribute 'nodiscard'
```
```c++
// Only issues a warning when `error_info` is returned by value.
struct [[nodiscard]] error_info {
  // ...
};

error_info do_something() {
  error_info ei;
  // ...
  return ei;
}

do_something(); // warning: ignoring returned value of type 'error_info',
                // declared with attribute 'nodiscard'
```

* `[[maybe_unused]]` indicates to the compiler that a variable or parameter might be unused and is intended.
```c++
void my_callback(std::string msg, [[maybe_unused]] bool error) {
  // Don't care if `msg` is an error message, just log it.
  log(msg);
}
```

## C++17 Library Features

### std::variant
The class template `std::variant` represents a type-safe `union`. An instance of `std::variant` at any given time holds a value of one of its alternative types (it's also possible for it to be valueless).
```c++
std::variant v{ 12 };
std::get(v); // == 12
std::get<0>(v); // == 12
v = 12.0;
std::get(v); // == 12.0
std::get<1>(v); // == 12.0
```

### std::optional
The class template `std::optional` manages an optional contained value, i.e. a value that may or may not be present. A common use case for optional is the return value of a function that may fail.
```c++
std::optional create(bool b) {
  if (b) {
    return "Godzilla";
  } else {
    return {};
  }
}

create(false).value_or("empty"); // == "empty"
create(true).value(); // == "Godzilla"
// optional-returning factory functions are usable as conditions of while and if
if (auto str = create(true)) {
  // ...
}
```

### std::any
A type-safe container for single values of any type.
```c++
std::any x {5};
x.has_value() // == true
std::any_cast(x) // == 5
std::any_cast(x) = 10;
std::any_cast(x) // == 10
```

### std::string_view
A non-owning reference to a string. Useful for providing an abstraction on top of strings (e.g. for parsing).
```c++
// Regular strings.
std::string_view cppstr {"foo"};
// Wide strings.
std::wstring_view wcstr_v {L"baz"};
// Character arrays.
char array[3] = {'b', 'a', 'r'};
std::string_view array_v(array, std::size(array));
```
```c++
std::string str {"   trim me"};
std::string_view v {str};
v.remove_prefix(std::min(v.find_first_not_of(" "), v.size()));
str; //  == "   trim me"
v; // == "trim me"
```

### std::invoke
Invoke a `Callable` object with parameters. Examples of `Callable` objects are `std::function` or `std::bind` where an object can be called similarly to a regular function.
```c++
template 
class Proxy {
  Callable c;
public:
  Proxy(Callable c): c(c) {}
  template 
  decltype(auto) operator()(Args&&... args) {
    // ...
    return std::invoke(c, std::forward(args)...);
  }
};
auto add = [](int x, int y) {
  return x + y;
};
Proxy p {add};
p(1, 2); // == 3
```

### std::apply
Invoke a `Callable` object with a tuple of arguments.
```c++
auto add = [](int x, int y) {
  return x + y;
};
std::apply(add, std::make_tuple(1, 2)); // == 3
```

### std::filesystem
The new `std::filesystem` library provides a standard way to manipulate files, directories, and paths in a filesystem.

Here, a big file is copied to a temporary path if there is available space:
```c++
const auto bigFilePath {"bigFileToCopy"};
if (std::filesystem::exists(bigFilePath)) {
  const auto bigFileSize {std::filesystem::file_size(bigFilePath)};
  std::filesystem::path tmpPath {"/tmp"};
  if (std::filesystem::space(tmpPath).available > bigFileSize) {
    std::filesystem::create_directory(tmpPath.append("example"));
    std::filesystem::copy_file(bigFilePath, tmpPath.append("newFile"));
  }
}
```

### std::byte
The new `std::byte` type provides a standard way of representing data as a byte. Benefits of using `std::byte` over `char` or `unsigned char` is that it is not a character type, and is also not an arithmetic type; while the only operator overloads available are bitwise operations.
```c++
std::byte a {0};
std::byte b {0xFF};
int i = std::to_integer(b); // 0xFF
std::byte c = a & b;
int j = std::to_integer(c); // 0
```
Note that `std::byte` is simply an enum, and braced initialization of enums become possible thanks to [direct-list-initialization of enums](#direct-list-initialization-of-enums).

### Splicing for maps and sets
Moving nodes and merging containers without the overhead of expensive copies, moves, or heap allocations/deallocations.

Moving elements from one map to another:
```c++
std::map src {{1, "one"}, {2, "two"}, {3, "buckle my shoe"}};
std::map dst {{3, "three"}};
dst.insert(src.extract(src.find(1))); // Cheap remove and insert of { 1, "one" } from `src` to `dst`.
dst.insert(src.extract(2)); // Cheap remove and insert of { 2, "two" } from `src` to `dst`.
// dst == { { 1, "one" }, { 2, "two" }, { 3, "three" } };
```

Inserting an entire set:
```c++
std::set src {1, 3, 5};
std::set dst {2, 4, 5};
dst.merge(src);
// src == { 5 }
// dst == { 1, 2, 3, 4, 5 }
```

Inserting elements which outlive the container:
```c++
auto elementFactory() {
  std::set<...> s;
  s.emplace(...);
  return s.extract(s.begin());
}
s2.insert(elementFactory());
```

Changing the key of a map element:
```c++
std::map m {{1, "one"}, {2, "two"}, {3, "three"}};
auto e = m.extract(2);
e.key() = 4;
m.insert(std::move(e));
// m == { { 1, "one" }, { 3, "three" }, { 4, "two" } }
```

### Parallel algorithms
Many of the STL algorithms, such as the `copy`, `find` and `sort` methods, started to support the *parallel execution policies*: `seq`, `par` and `par_unseq` which translate to "sequentially", "parallel" and "parallel unsequenced".

```c++
std::vector longVector;
// Find element using parallel execution policy
auto result1 = std::find(std::execution::par, std::begin(longVector), std::end(longVector), 2);
// Sort elements using sequential execution policy
auto result2 = std::sort(std::execution::seq, std::begin(longVector), std::end(longVector));
```

## Acknowledgements
* [cppreference](http://en.cppreference.com/w/cpp) - especially useful for finding examples and documentation of new library features.
* [C++ Rvalue References Explained](http://thbecker.net/articles/rvalue_references/section_01.html) - a great introduction I used to understand rvalue references, perfect forwarding, and move semantics.
* [clang](http://clang.llvm.org/cxx_status.html) and [gcc](https://gcc.gnu.org/projects/cxx-status.html)'s standards support pages. Also included here are the proposals for language/library features that I used to help find a description of, what it's meant to fix, and some examples.
* [Compiler explorer](https://godbolt.org/)
* [Scott Meyers' Effective Modern C++](https://www.amazon.com/Effective-Modern-Specific-Ways-Improve/dp/1491903996) - highly recommended book!
* [Jason Turner's C++ Weekly](https://www.youtube.com/channel/UCxHAlbZQNFU2LgEtiqd2Maw) - nice collection of C++-related videos.
* [What can I do with a moved-from object?](http://stackoverflow.com/questions/7027523/what-can-i-do-with-a-moved-from-object)
* [What are some uses of decltype(auto)?](http://stackoverflow.com/questions/24109737/what-are-some-uses-of-decltypeauto)
* And many more SO posts I'm forgetting...

## Author
Anthony Calandra

## Content Contributors
See: https://github.com/AnthonyCalandra/modern-cpp-features/graphs/contributors

## License
MIT