C++11 array

系统管理员 2022-06-01 10:08 129阅读 0赞

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：

![array提供的方法][array]

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

[array]: /images/20220601/d8e6a9089cff4a978fa3fbc8d1e4af6c.png