17.cpp

/**
 @file
 @brief C++17
 @defgroup CPP17 C++17 examples

 https://en.wikipedia.org/wiki/C++17

 https://github.com/AnthonyCalandra/modern-cpp-features/blob/master/CPP17.md

 https://en.cppreference.com/w/cpp/language/history/17

 @{
*/

static_assert(__cplusplus == 201703);

#include <bits/stdc++.h>

using namespace std;

/**
 @defgroup templ17 Template

 https://en.cppreference.com/w/cpp/language/templates

 @{
*/

/**
 @defgroup template_argument_deduction Template argument deduction

 [template_argument_deduction](https://en.cppreference.com/w/cpp/language/template_argument_deduction)

 [class_template_argument_deduction](https://en.cppreference.com/w/cpp/language/class_template_argument_deduction)

 @{
 */

/// @brief pair<int, double> p(1, 2.3);
constexpr pair deducted_pair(1, 2.3);
static_assert(deducted_pair.second == 2.3);

/// @brief auto t = make_tuple(4, 3, 2.5);
constexpr tuple deducted_tuple(4, 2, 2.5);
static_assert(get<2>(deducted_tuple) == 2.5);

/// @brief template with type float by default
template <typename T = float>
struct template_struct {
    T val;
    template_struct()
        : val()
    {
    }
    template_struct(T val)
        : val(val)
    {
    }
};

/// @brief deducted \<int\>
template_struct template_arg_deduction { 1 };

#if __cpp_deduction_guides > 201611

/// @brief deducted \<float\>
template_struct template_default_arg_deduction;

#endif

vector<int> int_vector = { 1, 2, 3, 4 };

/// [deduction_guides](https://en.cppreference.com/w/cpp/container/deque/deduction_guides)

/// @brief deduced deque\<int\>
deque deduction_guide1_queue(int_vector.begin(), int_vector.end());

/// @brief deduced deque\<vector\<int> :: iterator\>
deque deduction_guide2_queue { int_vector.cbegin(), int_vector.cend() };

/// [deduction_guides](https://en.cppreference.com/w/cpp/container/array/deduction_guides)
array deduction_guide_array { 1, 2, 3, 4 };

/// [deduction_guides](https://en.cppreference.com/w/cpp/container/vector/deduction_guides)
/// @brief deduced vector\<int\>
vector deduction_guide1_vector(int_vector.begin(), int_vector.end());
/// @brief deduced vector\<vector\<int\> :: iterator\>
vector deduction_guide2_vector { int_vector.begin(), int_vector.end() };

/**
  @} template_argument_deduction
  */

void deduction_guides_17()
{
    assert(deduction_guide1_queue[0] == 1);
    assert(*deduction_guide2_queue[0] == 1);
    assert(deduction_guide1_vector[0] == 1);
    assert(*deduction_guide2_vector[0] == 1);
}

/**
  @defgroup template_parameters Template parameters

  [template_parameters](https://en.cppreference.com/w/cpp/language/template_parameters)

  @{
 */

template <auto n>
struct B { /* ... */
};

B<5> b1; // OK: non-type template parameter type is int

B<'a'> b2;

// Error:
// B<2.5> b3;

tuple<int, int> foo_tuple()
{
    // return make_tuple(1, -1);
    return { 1, -1 };
}

template <auto... seq>
struct my_integer_sequence {
};

/// @brief auto seq = integer_sequence<int, 0, 1, 2>();
auto seq = my_integer_sequence<0, 1, 2>();

/**

 @} template_parameters
 @} templ17

 @defgroup lambda17 Lambda

 @{
 */

/**
 @defgroup constexpr_lambda Constexpr lambda

 [lambda](https://en.cppreference.com/w/cpp/language/lambda)

 [lambda-expressions-constexpr](https://docs.microsoft.com/en-us/cpp/cpp/lambda-expressions-constexpr)

 @{
 */

// explicit constexpr
auto identity = [](int n) constexpr { return n; };
static_assert(identity(1) == 1);

// lambda with auto argument actually is a template

// implicit auto constexpr:

auto can_be_constexpr1 = [](auto a) { return a; };
auto can_be_constexpr2 = [](int (*fp)(int), auto a) { return fp(a); };

static_assert(can_be_constexpr2(can_be_constexpr1, 3) == 3);

// error: non-constant condition for static assertion
// static int i=0;
// static_assert(can_be_constexpr2(can_be_constexpr1, i) == 0);

auto non_const = [](auto a) {
    static int s;
    return a;
};

// error: no specialization can be constexpr because of s
// static_assert(can_be_constexpr(non_const, 3)==3);

constexpr int const_inc(int n)
{
    return [n] { return n + 1; }();
}

constexpr int (*inc)(int) = const_inc;
static_assert(const_inc(1) == 2);

/**
  @} constexpr_lambda

  @defgroup lcbv Lambda capture this by value

  [Lambda
  capture](https://en.cppreference.com/w/cpp/language/lambda#Lambda_capture)

  @{
 */

void capture_this_by_value()
{
    struct capture_value_o {
        int value { 1 };
        auto get_value_copy()
        {
            return [*this] { return value; };
        }
    };

    capture_value_o mo;
    auto val = mo.get_value_copy();
    mo.value = 2;
    assert(val() == 1);
}

void lambda_17()
{
    assert(can_be_constexpr1(2) == 2);
    int i = 3;
    assert(can_be_constexpr2(can_be_constexpr1, 3) == 3);
    non_const(1);
    capture_this_by_value();
}

/**
 @} lcbv

 @} lambda17

 @defgroup threads17 Threads
 [threads](https://en.cppreference.com/w/cpp/thread)
 @{
  */

/**
  [shared_lock](https://en.cppreference.com/w/cpp/thread/shared_lock)

  [shared_mutex](https://en.cppreference.com/w/cpp/thread/shared_mutex)

  [scoped_lock](https://en.cppreference.com/w/cpp/thread/scoped_lock)
  */

void threads_17()
{
    shared_mutex m;
    shared_lock lock(m);

    mutex mt[2];
    thread t2;
    {
        scoped_lock l1(mt[0], mt[1]);
        t2 = thread([&mt] { scoped_lock l2(mt[0], mt[1]); });
    }
    t2.join();
}

/// @} threads17

/**
 @defgroup lang17 Language
 @brief [language](https://en.cppreference.com/w/cpp/language)
 @{
 */

void references_17()
{
    static_assert(is_reference_v<int&>);

    // L-value:
    static_assert(is_lvalue_reference_v<int&>);

    // R-value
    static_assert(is_rvalue_reference_v<int&&>);
}

/**
 @defgroup folding Folding
 @brief [fold](https://en.cppreference.com/w/cpp/language/fold)
 @{
 */

template <typename... Args>
constexpr bool folding_and(Args... args)
{
    return (true && ... && args);
}

template <typename... Args>
constexpr auto folding_sum(Args... args)
{
    return (... + args);
}

void folding_demo()
{
    static_assert(!folding_and(true, false, true));
    static_assert(folding_sum(1.0, 2.0f, 3) == 6.0);
}

/**
 @} folding
 @defgroup nn Qualified nested namespace
 @brief [namespace](https://en.cppreference.com/w/cpp/language/namespace)

 http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n4026.html

 @{
 */

/// Before: namespace outer_namespace { namespace nested_namespace { } }

namespace outer_namespace::qualified_nested_namespace {
int in_qualified_nested_namespace;
}

/**
 @} nn
 @defgroup sb Structured bindings

 [structured
 binding](https://en.cppreference.com/w/cpp/language/structured_binding)
 @{
 */

static_assert(__cpp_structured_bindings);

void structured_bindings()
{

    int a[2] = { 1, 2 };

    auto [a0, a1] = a;
    // a0_ref = &a; a0_ref = &a;
    auto& [a0_ref, a1_ref] = a;

    float x {};
    char y {};
    int z {};

    unordered_map<string, int> mapping {
        { "a", 1 },
        { "b", 2 },
        { "c", 3 },
    };

    for (const auto& [key, value] : mapping) {
        // Do something with key and value
    }
    // tuple<float&,char&&,int> tpl(x,move(y),z);
    // const auto& [a,b,c] = tpl;
    //  https://en.cppreference.com/w/cpp/language/structured_binding
    auto [min, max] = minmax({ 3, 2, 1 });
    assert(min == 1);
    assert(max == 3);
}

/// @} sb
/**
  Variable declaration in conditionals

  Specs:
  [if](https://en.cppreference.com/w/cpp/language/if),
  [switch](https://en.cppreference.com/w/cpp/language/switch)

 */

void conditional_with_init()
{
    if (auto a = true) {
    };

    switch (int a = 10) {
    }
}

/**
 @defgroup attr New attributes
 @brief
 [fallthrough](https://en.cppreference.com/w/cpp/language/attributes/fallthrough),
 [nodiscard](https://en.cppreference.com/w/cpp/language/attributes/nodiscard),
 [maybe_unused](https://en.cppreference.com/w/cpp/language/attributes/maybe_unused)

 @{
 */

// Will warn if return of foo() is ignored
[[nodiscard]] int foo() { return 1; }
void test_attr()
{
    int a { 1 };
    switch (a) {
    // Indicates that falling through on case 1 is intentional
    case 1:
        [[fallthrough]];
    case 2:
        // Indicates that b might be unused, such as on production builds
        [[maybe_unused]] int b = foo();
        assert(b > 0);
        break;
    }
}

/// @}

/**
 @brief
 [if](https://en.cppreference.com/w/cpp/language/if)
 */

constexpr int const_if()
{
    if constexpr (true)
        return 1;
}

static_assert(const_if());

/**
 Character literal
 @brief
 [character_literal](https://en.cppreference.com/w/cpp/language/character_literal)
 */

static char char_u8 = u8'x';

/**
  Direct list initialization of
  [enum](https://en.cppreference.com/w/cpp/language/enum)s

  [floating_literal](https://en.cppreference.com/w/cpp/language/floating_literal)

  [invoke](https://en.cppreference.com/w/cpp/utility/functional/invoke)
 */

void types_17()
{
    enum byte_e : unsigned char {
    };
    static byte_e b { 123 };

floating_literal:
    static_assert(is_integral_v<int>);
    static_assert(__cpp_hex_float);
    double hex_double = 0x1.2p3;
    assert(hex_double == 9.0);

    static_assert(is_invocable<decltype(types_17)>::value);
    static_assert(is_invocable<int()>::value);
    static_assert(is_invocable_r<int, int()>::value);
    static_assert(is_invocable_r<void, void(int), int>::value);
    static_assert(negation_v<bool_constant<false>>);

    auto inc = [](int a) -> int { return a + 1; };
    static_assert(is_invocable_r<int, decltype(inc), int>::value);
    static_assert(__cpp_lib_invoke);
    assert(invoke(inc, 2) == 3);
}

/// [language](https://en.cppreference.com/w/cpp/language)

void lang_17()
{
    references_17();
    folding_demo();
    structured_bindings();
    conditional_with_init();
    test_attr();
    types_17();

    /// https://en.cppreference.com/w/cpp/language/range-for
    map<int, int> mymap;
    for (auto&& [first, second] : mymap) {
        // use first and second
    }
}

/**
 @} lang17

 @defgroup lib17 Library

 @{
 */

void map_demo()
{
    /// @brief https://en.cppreference.com/w/cpp/container/map/extract
    map<int, string> m { { 1, "mango" }, { 2, "papaya" }, { 3, "guava" } };
    auto nh = m.extract(2);
    nh.key() = 4;
    /// @brief https://en.cppreference.com/w/cpp/container/map/insert
    m.insert(move(nh));
    // m == {{1, "mango"}, {3, "guava"}, {4, "papaya"}}

    /// @brief https://en.cppreference.com/w/cpp/container/map/merge
    set<int> src { 1, 3, 5 };
    set<int> dst { 2, 4, 5 };
    dst.merge(src);
    assert(size(dst) == 5);
}

/**
  [variant](https://en.cppreference.com/w/cpp/utility/variant)

  [visit](https://en.cppreference.com/w/cpp/utility/variant/visit)
  */

void variant_demo()
{
    variant<int, float> v, w;
    v = 12; // v contains int
    int i = get<int>(v);
    w = get<int>(v);
    w = get<0>(v); // same effect as the previous line
    w = v; // same effect as the previous line

    // get<double>(v); // error: no double in [int, float]
    // get<3>(v); // error: valid index values are 0 and 1

    try {
        get<float>(w); // w contains int, not float: will throw
    } catch (const bad_variant_access&) {
    }

    using namespace literals;

    variant<string> x("abc");

    // converting constructors work when unambiguous
    x = "def"; // converting assignment also works when unambiguous

    variant<string, void const*> y("abc");

    // casts to void const * when passed a char const *
    assert(holds_alternative<void const*>(y));
    y = "xyz"s;
    assert(holds_alternative<string>(y));

    auto inc = [](int a) -> int { return a + 1; };
    assert(visit(inc, (variant<int>)1) == 2);
}

void optional_demo()
{
    optional<int> i = make_optional(1);
    assert(i.has_value());
    assert(i.value() == 1);
    i = nullopt;
    assert(i.value_or(2) == 2);
}

void clamp_demo()
{
    static_assert(__cpp_lib_clamp);
    ///  clamp(x, low, high) == x < low ? low : x > high ? high : x;
    assert(clamp(0, 1, 3) == 1);
    assert(clamp(2, 1, 3) == 2);
    assert(clamp(4, 1, 3) == 3);
}

void dynamic_memory_17()
{
    int d[2] = { 10, 11 };
    unique_ptr<int[]> u1(d);
    assert(u1.get()[0] == 10); // C++11
    assert(u1[1] == 11);
    assert((bool)u1);
    u1.release();
    const shared_ptr<int> s1 { 0 };
    // cout << s1.element_type.name() << endl;
    // cout << typeid(s1.weak_type).name() << endl;
    assert(typeid(reinterpret_pointer_cast<shared_ptr<int>>(s1)).name()
        == string("St10shared_ptrIS_IiEE"));
    vector<int> s = { 5, 7, 4, 2, 8, 6, 1, 9, 0, 3 };

    // https://en.cppreference.com/w/cpp/numeric/valarray/deduction_guides
    int a[] = { 1, 2, 3 };
    valarray va(a, 3); // uses explicit deduction guide
    static_assert(is_integral_v<remove_reference<decltype(va[0])>::type>);
}

void string_view_demo()
{
    static_assert(__cpp_lib_string_view);
    string s = "abcd";
    string_view v = s;
    assert(v.data() == s.c_str());
    assert(v.substr(1, 2).data() >= s.c_str());
    assert(v.substr(1, 2).data() <= s.c_str() + s.length());
}

/// @} lib17

/**
 @defgroup other17 Other
 @{

 */

/// [inline](https://en.cppreference.com/w/cpp/language/inline)

inline int inline_var;

/**
  @} other17
  */

int main()
{
    deduction_guides_17();
    lang_17();
    lambda_17();
    threads_17();
    map_demo();
    variant_demo();
    optional_demo();
    clamp_demo();
    dynamic_memory_17();
    string_view_demo();
}

/**
  @} CPP17
  */