CPP17.md

C++17

Overview

Many of these descriptions and examples are taken from various resources (see Acknowledgements section) and summarized in my own words.

C++17 includes the following new language features:

• template argument deduction for class templates
• declaring non-type template parameters with auto
• folding expressions
• new rules for auto deduction from braced-init-list
• constexpr lambda
• lambda capture this by value
• inline variables
• nested namespaces
• structured bindings
• selection statements with initializer
• constexpr if
• utf-8 character literals
• direct-list-initialization of enums
• [[fallthrough]], [[nodiscard]], [[maybe_unused]] attributes
• __has_include
• class template argument deduction

C++17 includes the following new library features:

• std::variant
• std::optional
• std::any
• std::string_view
• std::invoke
• std::apply
• std::filesystem
• std::byte
• splicing for maps and sets
• parallel algorithms
• std::sample
• std::clamp
• std::reduce
• prefix sum algorithms
• gcd and lcm
• std::not_fn
• string conversion to/from numbers

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.

template <typename T = float>
struct MyContainer {
  T val;
  MyContainer() : val{} {}
  MyContainer(T val) : val{val} {}
  // ...
};
MyContainer c1 {1}; // OK MyContainer<int>
MyContainer c2; // OK MyContainer<float>

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:

template <auto... seq>
struct my_integer_sequence {
  // Implementation here ...
};

// Explicitly pass type `int` as template argument.
auto seq = std::integer_sequence<int, 0, 1, 2>();
// 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.

template <typename... Args>
bool logicalAnd(Args... args) {
    // Binary folding.
    return (true && ... && args);
}
bool b = true;
bool& b2 = b;
logicalAnd(b, b2, true); // == true

template <typename... Args>
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<int>, which now deduces to int.

auto x1 {1, 2, 3}; // error: not a single element
auto x2 = {1, 2, 3}; // x2 is std::initializer_list<int>
auto x3 {3}; // x3 is int
auto x4 {3.0}; // x4 is double

constexpr lambda

Compile-time lambdas using constexpr.

auto identity = [](int n) constexpr { return n; };
static_assert(identity(123) == 123);

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);

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.

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.

// 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.

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.

namespace A {
  namespace B {
    namespace C {
      int i;
    }
  }
}

The code above can be written like this:

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.

using Coordinate = std::pair<int, int>;
Coordinate origin() {
  return Coordinate{0, 0};
}

const auto [ x, y ] = origin();
x; // == 0
y; // == 0

std::unordered_map<std::string, int> 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.

{
  std::lock_guard<std::mutex> lk(mx);
  if (v.empty()) v.push_back(val);
}
// vs.
if (std::lock_guard<std::mutex> lk(mx); v.empty()) {
  v.push_back(val);
}

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.

template <typename T>
constexpr bool isIntegral() {
  if constexpr (std::is_integral<T>::value) {
    return true;
  } else {
    return false;
  }
}
static_assert(isIntegral<int>() == true);
static_assert(isIntegral<char>() == true);
static_assert(isIntegral<double>() == false);
struct S {};
static_assert(isIntegral<S>() == 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.

char x = u8'x';

Direct list initialization of enums

Enums can now be initialized using braced syntax.

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. This attribute may only be used in a switch statement, and must be placed before the next case/default label.

switch (n) {
  case 1: 
    // ...
    [[fallthrough]];
  case 2:
    // ...
    break;
  case 3:
    // ...
    [[fallthrough]];
  default:
    // ...
}

• [[nodiscard]] issues a warning when either a function or class has this attribute and its return value is discarded.

[[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'

// 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.

void my_callback(std::string msg, [[maybe_unused]] bool error) {
  // Don't care if `msg` is an error message, just log it.
  log(msg);
}

__has_include

__has_include (operand) operator may be used in #if and #elif expressions to check whether a header or source file (operand) is available for inclusion or not.

One use case of this would be using two libraries that work the same way, using the backup/experimental one if the preferred one is not found on the system.

#ifdef __has_include
#  if __has_include(<optional>)
#    include <optional>
#    define have_optional 1
#  elif __has_include(<experimental/optional>)
#    include <experimental/optional>
#    define have_optional 1
#    define experimental_optional
#  else
#    define have_optional 0
#  endif
#endif

It can also be used to include headers existing under different names or locations on various platforms, without knowing which platform the program is running on, OpenGL headers are a good example for this which are located in OpenGL\ directory on macOS and GL\ on other platforms.

#ifdef __has_include
#  if __has_include(<OpenGL/gl.h>)
#    include <OpenGL/gl.h>
#    include <OpenGL/glu.h>
#  elif __has_include(<GL/gl.h>)
#    include <GL/gl.h>
#    include <GL/glu.h>
#  else
#    error No suitable OpenGL headers found.
# endif
#endif

Class template argument deduction

Class template argument deduction (CTAD) allows the compiler to deduce template arguments from constructor arguments.

std::vector v{ 1, 2, 3 }; // deduces std::vector<int>

std::mutex mtx;
auto lck = std::lock_guard{ mtx }; // deduces to std::lock_guard<std::mutex>

auto p = new std::pair{ 1.0, 2.0 }; // deduces to std::pair<double, double>

For user-defined types, deduction guides can be used to guide the compiler how to deduce template arguments if applicable:

template <typename T>
struct container {
  container(T t) {}

  template <typename Iter>
  container(Iter beg, Iter end);
};

// deduction guide
template <typename Iter>
container(Iter b, Iter e) -> container<typename std::iterator_traits<Iter>::value_type>;

container a{ 7 }; // OK: deduces container<int>

std::vector<double> v{ 1.0, 2.0, 3.0 };
auto b = container{ v.begin(), v.end() }; // OK: deduces container<double>

container c{ 5, 6 }; // ERROR: std::iterator_traits<int>::value_type is not a type

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).

std::variant<int, double> v{ 12 };
std::get<int>(v); // == 12
std::get<0>(v); // == 12
v = 12.0;
std::get<double>(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.

std::optional<std::string> 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.

std::any x {5};
x.has_value() // == true
std::any_cast<int>(x) // == 5
std::any_cast<int&>(x) = 10;
std::any_cast<int>(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).

// 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));

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 lambdas; objects that can be called similarly to a regular function.

template <typename Callable>
class Proxy {
  Callable c_;

public:
  Proxy(Callable c) : c_{ std::move(c) } {}

  template <typename... Args>
  decltype(auto) operator()(Args&&... args) {
    // ...
    return std::invoke(c_, std::forward<Args>(args)...);
  }
};

const 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.

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:

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.

std::byte a {0};
std::byte b {0xFF};
int i = std::to_integer<int>(b); // 0xFF
std::byte c = a & b;
int j = std::to_integer<int>(c); // 0

Note that std::byte is simply an enum, and braced initialization of enums become possible thanks to 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:

std::map<int, string> src {{1, "one"}, {2, "two"}, {3, "buckle my shoe"}};
std::map<int, string> 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:

std::set<int> src {1, 3, 5};
std::set<int> dst {2, 4, 5};
dst.merge(src);
// src == { 5 }
// dst == { 1, 2, 3, 4, 5 }

Inserting elements which outlive the container:

auto elementFactory() {
  std::set<...> s;
  s.emplace(...);
  return s.extract(s.begin());
}
s2.insert(elementFactory());

Changing the key of a map element:

std::map<int, string> 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".

std::vector<int> 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));

std::sample

Samples n elements in the given sequence (without replacement) where every element has an equal chance of being selected.

const std::string ALLOWED_CHARS = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
std::string guid;
// Sample 5 characters from ALLOWED_CHARS.
std::sample(ALLOWED_CHARS.begin(), ALLOWED_CHARS.end(), std::back_inserter(guid),
  5, std::mt19937{ std::random_device{}() });

std::cout << guid; // e.g. G1fW2

std::clamp

Clamp given value between a lower and upper bound.

std::clamp(42, -1, 1); // == 1
std::clamp(-42, -1, 1); // == -1
std::clamp(0, -1, 1); // == 0

// `std::clamp` also accepts a custom comparator:
std::clamp(0, -1, 1, std::less<>{}); // == 0

std::reduce

Fold over a given range of elements. Conceptually similar to std::accumulate, but std::reduce will perform the fold in parallel. Due to the fold being done in parallel, if you specify a binary operation, it is required to be associative and commutative. A given binary operation also should not change any element or invalidate any iterators within the given range.

The default binary operation is std::plus with an initial value of 0.

const std::array<int, 3> a{ 1, 2, 3 };
std::reduce(std::cbegin(a), std::cend(a)); // == 6
// Using a custom binary op:
std::reduce(std::cbegin(a), std::cend(a), 1, std::multiplies<>{}); // == 6

Additionally you can specify transformations for reducers:

std::transform_reduce(std::cbegin(a), std::cend(a), 0, std::plus<>{}, times_ten); // == 60

const std::array<int, 3> b{ 1, 2, 3 };
const auto product_times_ten = [](const auto a, const auto b) { return a * b * 10; };

std::transform_reduce(std::cbegin(a), std::cend(a), std::cbegin(b), 0, std::plus<>{}, product_times_ten); // == 140

Prefix sum algorithms

Support for prefix sums (both inclusive and exclusive scans) along with transformations.

const std::array<int, 3> a{ 1, 2, 3 };

std::inclusive_scan(std::cbegin(a), std::cend(a),
    std::ostream_iterator<int>{ std::cout, " " }, std::plus<>{}); // 1 3 6

std::exclusive_scan(std::cbegin(a), std::cend(a),
    std::ostream_iterator<int>{ std::cout, " " }, 0, std::plus<>{}); // 0 1 3

const auto times_ten = [](const auto n) { return n * 10; };

std::transform_inclusive_scan(std::cbegin(a), std::cend(a),
    std::ostream_iterator<int>{ std::cout, " " }, std::plus<>{}, times_ten); // 10 30 60

std::transform_exclusive_scan(std::cbegin(a), std::cend(a),
    std::ostream_iterator<int>{ std::cout, " " }, 0, std::plus<>{}, times_ten); // 0 10 30

GCD and LCM

Greatest common divisor (GCD) and least common multiple (LCM).

const int p = 9;
const int q = 3;
std::gcd(p, q); // == 3
std::lcm(p, q); // == 9

std::not_fn

Utility function that returns the negation of the result of the given function.

const std::ostream_iterator<int> ostream_it{ std::cout, " " };
const auto is_even = [](const auto n) { return n % 2 == 0; };
std::vector<int> v{ 0, 1, 2, 3, 4 };

// Print all even numbers.
std::copy_if(std::cbegin(v), std::cend(v), ostream_it, is_even); // 0 2 4
// Print all odd (not even) numbers.
std::copy_if(std::cbegin(v), std::cend(v), ostream_it, std::not_fn(is_even)); // 1 3

String conversion to/from numbers

Convert integrals and floats to a string or vice-versa. Conversions are non-throwing, do not allocate, and are more secure than the equivalents from the C standard library.

Users are responsible for allocating enough storage required for std::to_chars, or the function will fail by setting the error code object in its return value.

These functions allow you to optionally pass a base (defaults to base-10) or a format specifier for floating type input.

• std::to_chars returns a (non-const) char pointer which is one-past-the-end of the string that the function wrote to inside the given buffer, and an error code object.
• std::from_chars returns a const char pointer which on success is equal to the end pointer passed to the function, and an error code object.

Both error code objects returned from these functions are equal to the default-initialized error code object on success.

Convert the number 123 to a std::string:

const int n = 123;

// Can use any container, string, array, etc.
std::string str;
str.resize(3); // hold enough storage for each digit of `n`

const auto [ ptr, ec ] = std::to_chars(str.data(), str.data() + str.size(), n);

if (ec == std::errc{}) { std::cout << str << std::endl; } // 123
else { /* handle failure */ }

Convert from a std::string with value "123" to an integer:

const std::string str{ "123" };
int n;

const auto [ ptr, ec ] = std::from_chars(str.data(), str.data() + str.size(), n);

if (ec == std::errc{}) { std::cout << n << std::endl; } // 123
else { /* handle failure */ }

Acknowledgements

• cppreference - especially useful for finding examples and documentation of new library features.
• C++ Rvalue References Explained - a great introduction I used to understand rvalue references, perfect forwarding, and move semantics.
• clang and gcc'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
• Scott Meyers' Effective Modern C++ - highly recommended book!
• Jason Turner's C++ Weekly - nice collection of C++-related videos.
• What can I do with a moved-from object?
• What are some uses of decltype(auto)?
• 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