modern-cpp-features/README.md

C++11/14/17

Overview

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
• inline variables
• nested namespaces
• structured bindings
• selection statements with initializer
• constexpr if

C++17 includes the following new library features:

• std::variant
• std::optional
• std::any
• std::string_view
• std::invoke
• std::apply
• splicing for maps and sets

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>
struct MyContainer {
  T val;
  MyContainer(T val) : val(val) {}
  // ...
};
MyContainer c1{ 1 }; // OK MyContainer<int>
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:

// 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 op1 ... op2 e2), where op1 and op2 are fold-operators, is called a binary fold. Either e1 or e2 are unexpanded parameter packs, but not both.

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

template<typename... Args>
auto sum(Args... args) {
    // Unary folding.
    return (... + args);
}
assert(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 }; // decltype(x2) is std::initializer_list<int>
auto x3{ 3 }; // decltype(x3) is int
auto x4{ 3.0 }; // decltype(x4) is double

Constexpr lambda

Compile-time lambdas using constexpr.

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

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

Nested namespaces

Using the namespace resolution operator to create nested namespace definitions.

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.

using Coordinate = std::pair<int, int>;
Coordinate origin() {
  return Coordinate{0, 0};
}
const auto [ x, y ] = origin();
assert(x == 0);
assert(y == 0);

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);

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 };
assert(std::get<int>(v) == 12);
assert(std::get<0>(v) == 12);
v = 12.0;
assert(std::get<double>(v) == 12.0);
assert(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 {};
  }
}
assert(create(false).value_or("empty") == "empty");
assert(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 };
assert(x.has_value() == true);
assert(std::any_cast<int>(x) == 5);
std::any_cast<int&>(x) = 10;
assert(std::any_cast<int>(x) == 10);

std::string_view

The class template basic_string_view describes an object that can refer to a constant contiguous sequence of char-like objects with the first element of the sequence at position zero. 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, sizeof array);

std::string str{ "   trim me" };
std::string_view v{ str };
v.remove_prefix(std::min(v.find_first_not_of(" "), v.size()));
assert(str == "   trim me");
assert(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.

template <typename Callable>
class Proxy {
    Callable c;
public:
    Proxy(Callable c): c(c) {}
    template <class... Args>
    decltype(auto) operator()(Args&&... args) {
        // ...
        return std::invoke(c, std::forward<Args>(args)...);
    }
};
auto add = [] (int x, int y) {
  return x + y;
};
Proxy<decltype(add)> p{ add };
assert(p(1, 2) == 3);

std::apply

Invoke a Callable object with a tuple of arguments.

auto add = [] (int x, int y) {
  return x + y;
};
assert(std::apply(add, std::make_tuple( 1, 2 )) == 3);

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" } }

C++14 Language Features

TODO

C++14 Library Features

TODO

C++11 Language Features

TODO

C++11 Library Features

TODO