17.cpp

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static_assert(__cplusplus == 201703);
 
#include <bits/stdc++.h>
 
using namespace std;
 
constexpr pair deducted_pair(1, 2.3);
static_assert(deducted_pair.second == 2.3);
 
constexpr tuple deducted_tuple(4, 2, 2.5);
static_assert(get<2>(deducted_tuple) == 2.5);
 
template <typename T = float>
struct template_struct {
    T val;
    template_struct() : val() {}
    template_struct(T val) : val(val) {}
};
 
template_struct template_arg_deduction {1};
 
#if __cpp_deduction_guides > 201611
 
template_struct template_default_arg_deduction;
 
#endif
 
vector<int> int_vector = {1, 2, 3, 4};
 
 
deque deduction_guide1_queue(int_vector.begin(), int_vector.end());
 
deque deduction_guide2_queue {int_vector.cbegin(), int_vector.cend()};
 
array deduction_guide_array {1, 2, 3, 4};
 
vector deduction_guide1_vector(int_vector.begin(), int_vector.end());
vector deduction_guide2_vector {int_vector.begin(), int_vector.end()};
 
 
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);
}
 
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 {
};
 
auto seq = my_integer_sequence<0, 1, 2>();
 
// 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);
 
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();
}
 
 
 
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();
 
}
 
 
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&&>);
}
 
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);
}
 
 
namespace outer_namespace::qualified_nested_namespace
{
int in_qualified_nested_namespace;
}
 
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);
}
 
 
 
void conditional_with_init()
{
    if (auto a = true) {
    };
 
    switch (int a = 10) {
    }
}
 
// Will warn if return of foo() is ignored
[[nodiscard]] int foo() {return 0;}
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;
    }
}
 
 
constexpr int const_if()
{
    if constexpr(true)
        return 1;
}
 
static_assert(const_if());
 
static char char_u8 = u8'x';
 
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);
}
 
 
void map_demo()
{
    map<int, string> m{{1, "mango"}, {2, "papaya"}, {3, "guava"}};
    auto nh = m.extract(2);
    nh.key() = 4;
    m.insert(move(nh));
    // m == {{1, "mango"}, {3, "guava"}, {4, "papaya"}}
 
    set<int> src {1, 3, 5};
    set<int> dst {2, 4, 5};
    dst.merge(src);
    // src == { 5 }
    // dst == { 1, 2, 3, 4, 5 }
}
 
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