新版 C++ 特性

auto a = 1;        // a 是 int 类型
auto b = a + 0.1;  // b 是 double 类型

for (range_declaration : range_expression) loop_statement

auto&& __range = range_expression;
for (auto __begin = begin_expr, __end = end_expr; __begin != __end; ++__begin) {
  range_declaration = *__begin;
  loop_statement
}

for (int i : {1, 1, 4, 5, 1, 4}) std::cout << i;

#include <iostream>

struct C {
  int a, b, c, d;

  C(int a = 0, int b = 0, int c = 0, int d = 0) : a(a), b(b), c(c), d(d) {}
};

int* begin(C& p) { return &p.a; }

int* end(C& p) { return &p.d + 1; }

int main() {
  C n = C(1, 9, 2, 6);
  for (auto i : n) std::cout << i << " ";
  std::cout << std::endl;
  // 下面的循环与上面的循环等价
  auto&& __range = n;
  for (auto __begin = begin(n), __end = end(n); __begin != __end; ++__begin) {
    auto ind = *__begin;
    std::cout << ind << " ";
  }
  std::cout << std::endl;
  return 0;
}

#include <iostream>
#include <vector>

int main() {
  int a = 1926;
  decltype(a) b = a / 2 - 146;         // b 是 int 类型
  std::vector<decltype(b)> vec = {0};  // vec 是 std::vector <int> 类型
  std::cout << a << vec[0] << b << std::endl;
  return 0;
}

int fact(int x) { return x ? x * fact(x - 1) : 1; }

int main() {
  constexpr int a = fact(5);  // ERROR: 函数调用在常量表达式中必须具有常量值
  return 0;
}

#include <iostream>
#include <tuple>
#include <vector>

constexpr auto expr = 1 + 1 * 4 - 5 - 1 + 4;

int main() {
  std::vector<int> vec = {1, 9, 2, 6, 0};
  std::tuple<int, int, std::string, std::vector<int>> tup =
      std::make_tuple(817, 114, "514", vec);
  std::cout << std::tuple_size<decltype(tup)>::value << std::endl;

  for (auto i : std::get<expr>(tup)) std::cout << i << " ";
  // std::get<> 中尖括号里面的必须是整型常量表达式
  // expr 常量的值是 3，注意 std::tuple 的首元素编号为 0，
  // 故我们 std::get 到了一个 std::vector<int>
  return 0;
}

constexpr std::tuple<int, int> tup = {1, 2};
std::tuple<int, int> tupA = {2, 3}, tupB;
tupB = tup;
tupB.swap(tupA);

std::tuple<int, int> tupA = {2, 3}, tupB;
tupB = std::make_tuple(1, 2);
std::swap(tupA, tupB);
std::cout << std::get<1>(tupA) << std::endl;

#include <functional>
#include <iostream>

struct Foo {
  Foo(int num) : num_(num) {}

  void print_add(int i) const { std::cout << num_ + i << '\n'; }

  int num_;
};

void print_num(int i) { std::cout << i << '\n'; }

struct PrintNum {
  void operator()(int i) const { std::cout << i << '\n'; }
};

int main() {
  // 存储自由函数
  std::function<void(int)> f_display = print_num;
  f_display(-9);

  // 存储 Lambda
  std::function<void()> f_display_42 = []() { print_num(42); };
  f_display_42();

  // 存储到成员函数的调用
  std::function<void(const Foo&, int)> f_add_display = &Foo::print_add;
  const Foo foo(314159);
  f_add_display(foo, 1);
  f_add_display(314159, 1);

  // 存储到数据成员访问器的调用
  std::function<int(Foo const&)> f_num = &Foo::num_;
  std::cout << "num_: " << f_num(foo) << '\n';

  // 存储到函数对象的调用
  std::function<void(int)> f_display_obj = PrintNum();
  f_display_obj(18);
}

#define def_name(...) def_body(__VA_ARGS__)

def_name();
def_name(1);
def_name(1, 2, 3);
def_name(1, 0.0, "abc");

template <typename... Values>
class Tuple {};

Tuple<> test0;
Tuple<int> test1;
Tuple<int, int, int> test2;
Tuple<int, std::vector<int>, std::map<std::string, std::vector<int>>> test3;

template <typename First, typename... Rest>
class Tuple {};

template <typename... Values>
void fun(Values... values) {}

fun();
fun(1);
fun(1, 2, 3);
fun(1, 0.0, "abc");

// 递归终止函数，可以是0或多个参数。
template <typename T>
T MAX(T a, T b) {
  return a > b ? a : b;
}

// 展开参数包的递归函数
template <typename First, typename... Rest>
First MAX(First first, Rest... rest) {
  return MAX(first, MAX(rest...));
}

// int a = MAX(1); // 编译不通过，但是对1个参数取最大值本身也没有意义
// int b = MAX(1, "abc"); //
// 编译不通过，但是在整数和字符串间取最大值本身也没有意义
int c = MAX(1, 233);              // 233
int d = MAX(1, 233, 666, 10086);  // 10086

// Author: Backl1ght
#include <iostream>
#include <vector>
using namespace std;

template <typename T>
ostream& operator<<(ostream& os, const vector<T>& V) {
  os << "[ ";
  for (const auto& vv : V) os << vv << ", ";
  os << "]";
  return os;
}

namespace DEBUG {
template <typename T>
void _debug(const char* format, T t) {
  cerr << format << '=' << t << endl;
}

template <class First, class... Rest>
void _debug(const char* format, First first, Rest... rest) {
  while (*format != ',') cerr << *format++;
  cerr << '=' << first << ",";
  _debug(format + 1, rest...);
}

#define debug(...) _debug(#__VA_ARGS__, __VA_ARGS__)
}  // namespace DEBUG

using namespace DEBUG;

int main(int argc, char* argv[]) {
  int a = 666;
  vector<int> b({1, 2, 3});
  string c = "hello world";

  // before
  cout << "a=" << a << ", b=" << b << ", c=" << c
       << endl;  // a=666, b=[ 1, 2, 3, ], c=hello world
  // 如果用printf的话，在只有基本数据类型的时候是比较方便的，然是如果要输出vector等的内容的话，就会比较麻烦

  // after
  debug(a, b, c);  // a=666, b=[ 1, 2, 3, ], c=hello world

  return 0;
}