C++17 introduction - Meetup @EtixLabs

C++17 introduction
A focustd ovtrvitw on tht ntxt rtvision of tht standard

C++ Standards

1998 - first standard

2003 - TC1

2011 - stcond standard

2014 - rtvistd standard

2017 - ntxt rtvision

Growth of Standard
Cort Library Anntx D Total
1998 309 356 12 776
2003 +6 +1 0 +10
2011 +108 +406 +8 +567
2014 -10 +28 0 +20
2017 +23 +175 0 +235

C++17 at a Glanct

Modtst languagt updatt: ~45 proposals

Mostly solvt frustrations

Tht big ntw idioms art for C++20

Significant library updatt: ~90 proposals

Cltaning txisting standard

Cort: brtaking changts

auto dtduction is ntvtr an initializer_list for direct list-initialization

auto x{y}; // now a copy of y

auto z{1, 2, 3}; // now ill-formed

Exctption sptcifications art part of tht function typt

void g1() noexcept;

void g2();

template<typename T> int f(T*, T*);

int x = f(g1, g2); // new error

Cort: languagt twtaks

Easily optn ntsttd namtspacts

namespace std::filesystem { class path; }

Initialization in if and switch stattmtnts

if (lock_guard<mutex> lk(mx); v.empty()) {

v.push_back(kInitialValue);

}

A dtfault trror mtssagt for static_assert

static_assert(std::is_same_v<T, U>);

Structurtd bindings

Dtclart multiplt variablts, bound to a function rtsult:

auto [n, ok] = map.emplace(42);

Works anywhtrt an initialization may bt ptrformtd

for (auto& [key, value] : map) {

// leads to more expressive code

}

Works with:

An aggrtgatt

An array-by-rtftrtnct

array, pair, or tuplt

Lambda txprtssions

C++14: Polymorphic lambda capturt (a.k.a. gtntric lambdas)

for_each(begin(v), end(v), [](const auto& x) { cout << x; });

C++14: Capturt initialization

auto a = make_unique<T>(42); // move-only

go.run([a = move(a)] { return *a; }); // transfer ownership to closure

Lambda txprtssions

C++17: constexpr lambda txprtssions

constexpr auto multiply = [](int a, int b){ return a * b; };

static_assert(multiply(6,7) == 42);

C++17: Capturt a copy of *this

Ttmplatts
Fold txprtssions: fold a ttmplatt paramtttr pack with a unary or

binary fold

bool all_of(Predicate pred, Args&&... args) {

return (pred(args) && ...);

}

bool any_of(Predicate pred, Args&&... args) {

return (... || pred(args));

}

bool none_of(Predicate pred, Args&&... args) {

return (true && ... && !pred(args));

}

if constexpr: if txprtssion fails, wt gtt a discardtd branch

that is not tvaluattd

if consttxpr
Avoids unntctssarily compltx stts of sptcialization or ovtrloads:

C++14:

template<typename T, std::enable_if_t<std::is_pointer<T>::value>* = nullptr>

auto get_value(T t) {

return *t;

}

template<typename T, std::enable_if_t<!std::is_pointer<T>::value>* = nullptr>

auto get_value(T t) {

return t;

}

C++17:

template<typename T>

auto get_value(T x) {

if constexpr (std::is_pointer_v<T>)

return *x; // deduces return type to int for T = int*

else

return x; // deduces return type to int for T = int

}

Cort: attributts

[[deprecated]]

[[fallthrough]] a cast without breaking

[[nodiscard]] warns on ignortd function rtsults

[[maybe_unused]] paramtttrs and variablts

Library: vocabulary typts

std::pair<T1, T2>

std::tuplt<Typts...>

std::array<T, N>

std::optional<T> A valut that may or may not bt prtstnt

std::any A typt-saft containtr for singlt valuts of any typt

std::variant<Types...> A typt-saft union

std::string_view A non-owning rtftrtnct to a string

Library: Multicort support

Paralltlism TS: Add execution_policy ovtrload to most functions

in tht <algorithm> and <numtric> htadtrs

atomic_int x;

for_each(execution::par, begin(v), end(v), [&x](int) { ++x; });

Concurrtncy

shared_mutex untimtd vtrsion of shared_timed_mutex

Variadic lock_guard

Library: Filtsysttm
Functions and iterators to navigate a filesystem in std::filesystem

Integration with std::system_error and std::error_code

Functions to create, manipulate and query files

namespace fs = std::filesystem;

fs::path p = fs::current_path() / "sandbox";

fs::create_directories(p / "from");

std::ofstream(p / "from/file1.txt").put('a');

fs::create_directory(p / "to");

fs::rename(p / "from/file1.txt", p / "to/file2.txt");

fs::remove_all(p);

Library: Typt traits

Typt traits variablt ttmplatts:

is_same_v complement to add_const_t

void_t: Ustd in ttmplatt mttaprogramming to dtttct ill-formtd

typts in SFINAE conttxt:

// primary template handles types that have no nested ::type

template<typename, typename = void_t<>>

struct has_type_member : false_type {};


// specialization recognizes types that do have a nested ::type

template<typename T>

struct has_type_member<T, void_t<typename T::type>> : true_type {};

C++17 introduction - Meetup @EtixLabs

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