C++11 array

版权声明：本文为博主原创文章，未经博主允许不得转载。 https://blog.csdn.net/m0_38132420/article/details/79173169

大学期间没有学过C++，大学毕业后开发主要的语言是C++，其实应该说是C++98。看了c++ primer等书籍，有好多感觉都用不着。后来接触了golang，渐渐的发现C++98用起来很不方便。现在学习C++11发现，其实C++11已经和现在很多高级语言使用起来一样方便了，且不失去其高效性。所以，C++没有GC。
后续，将慢慢学习C++11，从array开始记录吧。先不研究内部实现，后续再深入研究。array是一个固定大小的顺序容器，不能动态改变大小。实现的方法如下：

方法实例：
1、begin()、end（）和其他stl容器一样的意思

// array::begin example
#include <iostream>
#include <array>

int main ()
{
  std::array<int,5> myarray = { 2, 16, 77, 34, 50 };

  std::cout << "myarray contains:";
  for ( auto it = myarray.begin(); it != myarray.end(); ++it )
    std::cout << ' ' << *it;
  std::cout << '\n';

  return 0;
}

结果：

myarray contains: 2 16 77 34 50

2、rbegin()、rend() 和 begin()、end()反过来

// array::rbegin/rend
#include <iostream>
#include <array>

int main ()
{
  std::array<int,4> myarray = {4, 26, 80, 14} ;

  std::cout << "myarray contains:";
  for ( auto rit=myarray.rbegin() ; rit < myarray.rend(); ++rit )
    std::cout << ' ' << *rit;

  std::cout << '\n';

  return 0;
}

结果：

myarray contains: 14 80 26 4

4、cbengin()、cend()、crbegin()、crend()和没加c的接口一样，只是不能改变迭代子的值。

5、size() 返回数组的大小，对应函数有用constexpr修饰，编译器会把结果当作一个常量对待。max_size()返回的结果和size()结果一致。

#include <iostream>
#include <array>

int main ()
{
  std::array<int,5> myints;
  std::cout << "size of myints: " << myints.size() << std::endl;
  std::cout << "sizeof(myints): " << sizeof(myints) << std::endl;

  return 0;
}

结果：

size of myints: 5
sizeof(myints): 20

6、empty() 判断数组是否为空，如果长度为0，返回false，非0返回true。

// array::empty
#include <iostream>
#include <array>

int main ()
{
  std::array<int,0> first;
  std::array<int,5> second;
  std::cout << "first " << (first.empty() ? "is empty" : "is not empty") << '\n';
  std::cout << "second " << (second.empty() ? "is empty" : "is not empty") << '\n';
  return 0;
}

结果：

first is empty
second is not empty

7、[]操作符—用于取值和赋值

// array::operator[]
#include <iostream>
#include <array>

int main ()
{
  std::array<int,10> myarray;
  unsigned int i;

  // assign some values:
  for (i=0; i<10; i++) myarray[i]=i;

  // print content
  std::cout << "myarray contains:";
  for (i=0; i<10; i++)
    std::cout << ' ' << myarray[i];
  std::cout << '\n';

  return 0;
}

结果：

myarray contains: 0 1 2 3 4 5 6 7 8 9

8、at（int）–和[]操作运算符实现一样的功能。

// array::at
#include <iostream>
#include <array>

int main ()
{
  std::array<int,10> myarray;

  // assign some values:
  for (int i=0; i<10; i++) myarray.at(i) = i+1;

  // print content:
  std::cout << "myarray contains:";
  for (int i=0; i<10; i++)
    std::cout << ' ' << myarray.at(i);
  std::cout << '\n';

  return 0;
}

结果：

myarray contains: 1 2 3 4 5 6 7 8 9 10

9、front()、back() 获取数组的第一个元素（最后一个元素），也可以对其进行赋值。

// array::front
#include <iostream>
#include <array>

int main ()
{
  std::array<int,3> myarray = {2, 16, 77};

  std::cout << "front is: " << myarray.front() << std::endl;   // 2
  std::cout << "back is: " << myarray.back() << std::endl;     // 77

  myarray.front() = 100;

  std::cout << "myarray now contains:";
  for ( int& x : myarray ) std::cout << ' ' << x;

  std::cout << '\n';

  return 0;
}

结果：

front is: 2
back is: 77
myarray now contains: 100 16 77

10、data()获取数组头指针

// array::data
#include <iostream>
#include <cstring>
#include <array>

int main ()
{
  const char* cstr = "Test string";
  std::array<char,12> charray;

  std::memcpy (charray.data(),cstr,12);

  std::cout << charray.data() << '\n';

  return 0;
}

结果：

Test string

11、fill() 将数组每个元素都固定填一个值。

// array::fill example
#include <iostream>
#include <array>

int main () {
  std::array<int,6> myarray;

  myarray.fill(5);

  std::cout << "myarray contains:";
  for ( int& x : myarray) { std::cout << ' ' << x; }

  std::cout << '\n';

  return 0;
}

结果：

myarray contains: 5 5 5 5 5 5

12、swap() 交换两个数组

// swap arrays
#include <iostream>
#include <array>

int main ()
{
  std::array<int,5> first = {10, 20, 30, 40, 50};
  std::array<int,5> second = {11, 22, 33, 44, 55};

  first.swap (second);

  std::cout << "first:";
  for (int& x : first) std::cout << ' ' << x;
  std::cout << '\n';

  std::cout << "second:";
  for (int& x : second) std::cout << ' ' << x;
  std::cout << '\n';

  return 0;
}

结果：

first: 11 22 33 44 55
second: 10 20 30 40 50

可以被std::get操作

// arrays as tuples
#include <iostream>
#include <array>
#include <tuple>

int main ()
{
  std::array<int,3> myarray = {10, 20, 30};
  std::tuple<int,int,int> mytuple (10, 20, 30);

  std::tuple_element<0,decltype(myarray)>::type myelement;  // int myelement

  myelement = std::get<2>(myarray);
  std::get<2>(myarray) = std::get<0>(myarray);
  std::get<0>(myarray) = myelement;

  std::cout << "first element in myarray: " << std::get<0>(myarray) << "\n";
  std::cout << "first element in mytuple: " << std::get<0>(mytuple) << "\n";

  return 0;
}

结果：

first element in myarray: 30
first element in mytuple: 10

重载运算符，对应每个元素重载运算符结果。

// array comparisons
#include <iostream>
#include <array>

int main ()
{
  std::array<int,5> a = {10, 20, 30, 40, 50};
  std::array<int,5> b = {10, 20, 30, 40, 50};
  std::array<int,5> c = {50, 40, 30, 20, 10};

  if (a==b) std::cout << "a and b are equal\n";
  if (b!=c) std::cout << "b and c are not equal\n";
  if (b<c) std::cout << "b is less than c\n";
  if (c>b) std::cout << "c is greater than b\n";
  if (a<=b) std::cout << "a is less than or equal to b\n";
  if (a>=b) std::cout << "a is greater than or equal to b\n";

  return 0;
}

结果：

a and b are equal
b and c are not equal
b is less than c
c is greater than b
a is less than or equal to b
a is greater than or equal to b