【连载】 C++14学习笔记 —— C++编程新时代.

class A {

public:

        A(): name(“plmm”),id(520) {init(); ...}

        A(int i): name(“plmm”),id(520) {init(); ...}

        A(char c): name(“plmm”),id(520) {init(); ...}

private:

       void init();

       char name;

       int id;

}

class A {

public:

        A(): A("plmm",520) {...}

        A(int i):A("plmm",520) {...}

        A(char c): A("plmm",520) {...}

private:

       A（char c, int i）:name(c),id(i) {init();}

       void init();

       char name;

       int id;

}

class A {

          template<typename T1 ,typename T2> A(T1 c ,T2 i):name(c),id(i) {init();}

public:

         A():A(''plmm", 520) {...}

         A(int i):A(''plmm", 520) {...}

         A(char c):A(''plmm", 520) {...}

private:

       void init();

       char name;

       int id;

}

#include<iostream>

using namespace std;

int main()

{

    int arry[5] = {1,2,3,4,5};

    for(auto e: arry)   //在数组arry中用迭代器e进行遍历.

       cout<<e<<'\t';

}

void foo(std::vector<std::string> & v)

{

     std::vector<std::string>:: iterator i = v.begin();

     //此句可以简化为  auto i = v.begin();

     //因为foo函数的参数已经定义了向量v, 编译器完全可以推导出v.begin()是一个向量成员函数。

     ... 

}

int main()

{

    int i =1;

    float j = 1.1;

    auto k = i+j; //如果i和j在别的比较隐蔽的地方，这样做就很省心了。

}

#include <iostream>

using namespace std;

template<typename T1, typename T2>

double Sum(T1 &t1, T2 &t2)

{

    auto s = t1 + t2;//s的类型会在模板实例化的时候被推导出来

    return s;

}

int main()

{

    int a=3;

    long b=5;

    //float c=1.0f, d=2.3f;

    auto e = Sum<int,long>(a,b);   //e的类型被推导为long

    //auto f = Sum<float,float>(c,d); //f的类型被推导为float

    e = Sum(a,b);

    std::cout << e << std::endl;

}

#include<vector>

using namespace std;

void foo(auto i = 1) {}         //1、auto不能作为函数参数

struct str{

     auto j = 2;                   //2、对于结构体来说，非静态成员变量不能用auto推导。

}

int main()

{

   auto k[3]=x;                  //3、不能用auto声明数组

   vector<auto> v = {1};    //4、在模板实例化时，不能用auto作为参数。

}

using typename = decltype(sizeof(0));

#include <iostream>

using namespace std;

template <typename T1, typename T2>

auto add(T1 t1, T2 t2) -> decltype(t1 + t2)

{

    static_assert(std::is_integral<T1>::value, "Type T1 must be integral");

    static_assert(std::is_integral<T2>::value, "Type T2 must be integral");

    return t1 + t2;

}



int main()

{

    //std::cout << add(1, 3.14) << std::endl;

    std::cout << add(111, 2) << std::endl;

    return 0;

}

template <typename T1, typename T2>

auto add(T1 t1, T2 t2) -> decltype(t1 + t2)

{

    static_assert(std::is_integral<T1>::value, "Type T1 must be integral");

    static_assert(std::is_integral<T2>::value, "Type T2 must be integral");

    return t1 + t2;

}

#include <iostream>

template <typename T>

void Output(const T & value)

{std::cout << value << std::endl;}

int main()

{

     Output("Hello, world!"); //将value匹配成字符串.

}

// 主模板

template<int N>

struct Fib

{

    enum { Result = (5N+1); };

};

// 完全特化版

template <>

struct Fib<1>

{

    enum { Result = 1 };

};

// 完全特化版

template <>

struct Fib<0>

{

    enum { Result = 0 };

};



// 示例

int main()

{

    int i = Fib<10>::Result;

    // std::cout << i << std::endl;

}

// 主模板

template<bool condition, typename T1, typename T2>

struct IfThenElse

{

    typedef T1 ResultType;

};

// 局部特化

template<typename T1, typename T2>

struct IfThenElse<false, T1, T2>

{

    typedef T2 ResultType;

};

// 示例

int main()

{

     IfThenElse<(1 + 1 == 2), int, char>::ResultType i; //我们什么也没做，只是定义 i的类型为int

}

// 主模板

template<int N>

struct Fib

{

    enum { Result = Fib<N-1>::Result + Fib<N-2>::Result };

};

// 完全特化版

template <>

struct Fib<1>

{

    enum { Result = 1 };

};

// 完全特化版

template <>

struct Fib<0>

{

    enum { Result = 0 };

};

// 示例

int main()

{

    int i = Fib<10>::Result;

    // std::cout << i << std::endl;

} 

#include <iostream>

using namespace std;

template<int N>

class Sum

{

    public:

        enum {value = N + Sum<N-1>::value };

};

template<>

class Sum<0>

{

    public:

        enum {value = 0};

};



int main()

{

    cout << Sum<100>::value << endl;

}

//第一个主模板是一个递归模板，第二个是个主模板的偏特化模板，用来截止主模板的递归。

/* 按字节查表求CRC */

unsigned int byte_crc(unsigned char *ptr, unsigned char len) 

{

unsigned int crc;

unsigned char da;

/* CRC 余式表 */ 

unsigned int xdata crc_ta[256]=

{        

  0x0000, 0x1021, 0x2042, 0x3063, 0x4084, 0x50a5, 0x60c6, 0x70e7,

  0x8108, 0x9129, 0xa14a, 0xb16b, 0xc18c, 0xd1ad, 0xe1ce, 0xf1ef,

  0x1231, 0x0210, 0x3273, 0x2252, 0x52b5, 0x4294, 0x72f7, 0x62d6,

  0x9339, 0x8318, 0xb37b, 0xa35a, 0xd3bd, 0xc39c, 0xf3ff, 0xe3de,

  0x2462, 0x3443, 0x0420, 0x1401, 0x64e6, 0x74c7, 0x44a4, 0x5485,

  0xa56a, 0xb54b, 0x8528, 0x9509, 0xe5ee, 0xf5cf, 0xc5ac, 0xd58d,

  0x3653, 0x2672, 0x1611, 0x0630, 0x76d7, 0x66f6, 0x5695, 0x46b4,

  0xb75b, 0xa77a, 0x9719, 0x8738, 0xf7df, 0xe7fe, 0xd79d, 0xc7bc,

  0x48c4, 0x58e5, 0x6886, 0x78a7, 0x0840, 0x1861, 0x2802, 0x3823,

  0xc9cc, 0xd9ed, 0xe98e, 0xf9af, 0x8948, 0x9969, 0xa90a, 0xb92b,

  0x5af5, 0x4ad4, 0x7ab7, 0x6a96, 0x1a71, 0x0a50, 0x3a33, 0x2a12,

  0xdbfd, 0xcbdc, 0xfbbf, 0xeb9e, 0x9b79, 0x8b58, 0xbb3b, 0xab1a,

  0x6ca6, 0x7c87, 0x4ce4, 0x5cc5, 0x2c22, 0x3c03, 0x0c60, 0x1c41,

  0xedae, 0xfd8f, 0xcdec, 0xddcd, 0xad2a, 0xbd0b, 0x8d68, 0x9d49,

  0x7e97, 0x6eb6, 0x5ed5, 0x4ef4, 0x3e13, 0x2e32, 0x1e51, 0x0e70,

  0xff9f, 0xefbe, 0xdfdd, 0xcffc, 0xbf1b, 0xaf3a, 0x9f59, 0x8f78,

  0x9188, 0x81a9, 0xb1ca, 0xa1eb, 0xd10c, 0xc12d, 0xf14e, 0xe16f,

  0x1080, 0x00a1, 0x30c2, 0x20e3, 0x5004, 0x4025, 0x7046, 0x6067,

  0x83b9, 0x9398, 0xa3fb, 0xb3da, 0xc33d, 0xd31c, 0xe37f, 0xf35e,

  0x02b1, 0x1290, 0x22f3, 0x32d2, 0x4235, 0x5214, 0x6277, 0x7256,

  0xb5ea, 0xa5cb, 0x95a8, 0x8589, 0xf56e, 0xe54f, 0xd52c, 0xc50d,

  0x34e2, 0x24c3, 0x14a0, 0x0481, 0x7466, 0x6447, 0x5424, 0x4405,

  0xa7db, 0xb7fa, 0x8799, 0x97b8, 0xe75f, 0xf77e, 0xc71d, 0xd73c,

  0x26d3, 0x36f2, 0x0691, 0x16b0, 0x6657, 0x7676, 0x4615, 0x5634,

  0xd94c, 0xc96d, 0xf90e, 0xe92f, 0x99c8, 0x89e9, 0xb98a, 0xa9ab,

  0x5844, 0x4865, 0x7806, 0x6827, 0x18c0, 0x08e1, 0x3882, 0x28a3,

  0xcb7d, 0xdb5c, 0xeb3f, 0xfb1e, 0x8bf9, 0x9bd8, 0xabbb, 0xbb9a,

  0x4a75, 0x5a54, 0x6a37, 0x7a16, 0x0af1, 0x1ad0, 0x2ab3, 0x3a92,

  0xfd2e, 0xed0f, 0xdd6c, 0xcd4d, 0xbdaa, 0xad8b, 0x9de8, 0x8dc9,

  0x7c26, 0x6c07, 0x5c64, 0x4c45, 0x3ca2, 0x2c83, 0x1ce0, 0x0cc1,

  0xef1f, 0xff3e, 0xcf5d, 0xdf7c, 0xaf9b, 0xbfba, 0x8fd9, 0x9ff8,

  0x6e17, 0x7e36, 0x4e55, 0x5e74, 0x2e93, 0x3eb2, 0x0ed1, 0x1ef0

};

crc = 0;

while(len--!=0) 

{

  da=(uchar) (crc/256);  // 以8位二进制数的形式暂存CRC的高8位 

  crc<<=8;     // 左移8位，相当于CRC的低8位乘以2的8次方 

  crc^=crc_ta[da^*ptr];  // 高8位和当前字节相加后再查表求CRC，再加上以前的CRC   

  ptr++;

}

return(crc);

}



int main()

{

unsigned char len;

/* 8字节测试数据 */

unsigned char testdata[8]={0x11,0x22,0x33,0x44,0x55,0x66,0x77,0x88};

crc_byte = byte_crc(testdata,len);   // 调用按字节查表求CRC子函数

}

#include <cstdint>

#include <iostream>

#include <iomanip>

namespace crc32 {

template <uint32_t CRC, uint8_t BIT = 0>

struct byte : byte <(CRC >> 1) ^ (-int32_t (CRC & 1) & 0xedb88320), BIT + 1> { };

template <uint32_t CRC>

struct byte <CRC, 8> { uint32_t value{ CRC }; };

template <uint16_t I = 0>

struct array : byte <I>, array <I + 1> { };

template <>

struct array <256> { };

struct table : private array <> {

  const uint32_t& operator [] (int i) const

  {

    return reinterpret_cast<const uint32_t*>(this);

  }

};

}

const crc32::table data;

int main ()

{

  using namespace std;

  uint_fast16_t n = sizeof (data) / sizeof (data[0]);

  uint_fast16_t i = 0;

  uint_fast8_t l = 8;

  do {

    cout << "0x" << setfill('0') << setw(8) << hex << data << ',';

    if (--l == 0) {

      cout << endl;

      l = 8;

    }

  } while (++i < n);

}

namespace crc32 {

...

}

template <uint32_t CRC, uint8_t BIT = 0>

struct byte : byte <(CRC >> 1) ^ (-int32_t (CRC & 1) & 0xedb88320), BIT + 1> { };

template <uint32_t CRC>

struct byte <CRC, 8> { uint32_t value{ CRC }; };

struct byte <CRC, 8> { uint32_t value{ CRC }; };

template <uint16_t I = 0>

struct array : byte <I>, array <I + 1> { };

template <>

struct array <256> { };

struct table : private array <> {

const uint32_t& operator [] (int i) const

{

return reinterpret_cast<const uint32_t*>(this)[i];

}

};

}

const crc32::table data;

//table 继承了array模板，返回了array模板里的数据成员。