C++14 新特性

目录

C++14 新特性

lambda 初始化捕获

Abstract

Demo

废弃标志[[deprecated]]

Abstract

Demo

读写锁std::shared_timed_mutex和std::shared_lock

Abstract

Demo

模板可变整数序列

Abstract

Demo

改变exchange

Abstract

Demo

字符串带引号

Abstract

---

lambda 初始化捕获

Abstract

可以对捕获列表的捕获变量“赋值”，C++14中的这个新特性允许了在捕获列表中定义前面没有出现过的变量，但必须赋予一个值，并且不使用类型说明符和auto，类型由编译器自动推断。

Demo

#include 
#include 
using namespace std;

int main() {
  int a = 2;
  [a = sin(a)]() {
    cout << a << endl;		// 0.909297
    cout << cos(a) << endl;	// 0.6143
  }();

  cout << a << endl;		// 2
  cout << cos(a) << endl;	// -0.416147
  return 0;
}

废弃标志[[deprecated]]

Abstract

C++14中增加了[[deprecated]]标记，可以修饰类、函数、变量等，当程序中使用了被其修饰的模块时，编译器会产生告警，提示用户该标记标记的内容将来可能会被废弃，尽量不要使用。

Demo

struct [[deprecated]] Tmp {};

读写锁std::shared_timed_mutex和std::shared_lock

Abstract

C++14中通过std::shared_timed_mutex和std::shared_lock实现读写锁，保证多个线程可以同时读，但是写线程必须独立运行，写操作不能和读操作同时进行。

Demo

struct ThreadSafe {
  mutable std::shared_timed_mutex _mutex;
  int _value;

  ThreadSafe() { _value = 0; }

  int get() const {
    std::shared_lock loc(_mutex);
    return _value;
  }

  void increase() {
    std::unique_lock lock(_mutex);
    _value += 1;
  }
};

模板可变整数序列

Abstract

类模板 std::integer_sequence 表示一个编译时的整数序列。在用作函数模板的实参时，能推导参数包 Ints 并将它用于包展开。

Demo

#include 
#include 
#include 
#include 

template 
void print_sequence(std::integer_sequence int_seq) {
  std::cout << "The sequence of size " << int_seq.size() << ": ";
  ((std::cout << ints << ' '), ...);
  std::cout << '\n';
}

// 转换数组为 tuple
template 
auto a2t_impl(const Array &a, std::index_sequence) {
  return std::make_tuple(a[I]...);
}

template >
auto a2t(const std::array &a) {
  return a2t_impl(a, Indices{});
}

// 漂亮地打印 tuple

template 
void print_tuple_impl(std::basic_ostream &os, const Tuple &t, std::index_sequence) {
  ((os << (Is == 0 ? "" : ", ") << std::get(t)), ...);
}

template 
auto &operator<<(std::basic_ostream &os, const std::tuple &t) {
  os << "(";
  print_tuple_impl(os, t, std::index_sequence_for{});
  return os << ")";
}

int main() {
  print_sequence(std::integer_sequence{});
  print_sequence(std::make_integer_sequence{});
  print_sequence(std::make_index_sequence<10>{});
  print_sequence(std::index_sequence_for{});

  std::array array = {1, 2, 3, 4};

  // 转换 array 为 tuple
  auto tuple = a2t(array);
  static_assert(std::is_same>::value, "");

  // 打印到 cout
  std::cout << tuple << '\n';
}

改变exchange

Abstract

相对于swap不对后者赋值

Demo

#include 
#include 
#include 

using namespace std;

int main() {
  vector vec1{1, 2, 3, 4};
  vector vec2{5, 6, 7, 8};

  cout << "exchange before: " << endl;
  cout << "vec1: " << endl;
  copy(vec1.begin(), vec1.end(), ostream_iterator{cout, " "});
  cout << endl << "vec2: " << endl;
  copy(vec2.begin(), vec2.end(), ostream_iterator{cout, " "});

  exchange(vec1, vec2);

  cout << endl << "exchange after: " << endl;
  cout << "vec1: " << endl;
  copy(vec1.begin(), vec1.end(), ostream_iterator{cout, " "});
  cout << endl << "vec2: " << endl;
  copy(vec2.begin(), vec2.end(), ostream_iterator{cout, " "});

  return 0;
}

字符串带引号

Abstract

#include 
#include 
#include 

int main() {
  std::string str{"hello world"};
  std::cout << str << std::endl;				// hello world
  std::cout << std::quoted(str) << std::endl;	// "hello world"

  return 0;
}