C++相关闲碎记录(3)

1、reference wrapper

例如声明如下的模板：

template <typename T>
void foo(T val);

如果调用使用：

int x;
foo(std::ref(x));

T变成int&，而使用调用

int x;
foo(std::cref(x));

T变成const int&。

这个特性被C++标准库用在各个地方，例如：

make_pair()用此特性于是能够创建一个pair<>of reference

make_tuple()用此特性可以创建一个tuple<>of reference

std::vector<MyClass&> coll;       //error
std::vector<std::reference_wrapper<MyClass>> coll;    //ok

2、function type wrapper

#include <iostream>
#include <vector>
#include <functional>
using namespace std;

void func(int x, int y) {
    std::cout << "func" << std::endl;
}

class C {
public:
    void memfunc(int x, int y) const{
        std::cout << "C::memfunc" << std::endl;
    }
};

int main()
{
    std::vector<std::function<void(int,int)>> tasks;
    tasks.push_back(func);
    tasks.push_back([](int x, int y) {
                        std::cout << "lambda" << std::endl;
                    });
    for (std::function<void(int,int)> f : tasks) {
        f(3, 33);
    }

    std::function<void(const C&, int, int)> mf;
    mf = &C::memfunc;
    mf(C(), 2, 3);
    return 0;
}
输入：
func
lambda
C::memfunc

3、挑选最小值和最大值

auto extremes = std::minmax({px, py, pz}, [](int*a, int*b) {
                        return *a < *b;                        
                    });

两值互换：

namespace std {
    template <typename T>
    inline void swap(T& a, T& b) {
        T tmp(std::move(a));
        a = std::move(b);
        b = std::move(tmp);
    }
}

4、class ratio<>

#include <ratio>
#include <iostream>
using namespace std;

int main()
{
   typedef ratio<5,3> FiveThirds;
   cout << FiveThirds::num << "/" << FiveThirds::den << endl;

   typedef ratio<25,15> AlsoFiveThirds;
   cout << AlsoFiveThirds::num << "/" << AlsoFiveThirds::den << endl;

   ratio<42,42> one;
   cout << one.num << "/" << one.den << endl;

   ratio<0> zero;
   cout << zero.num << "/" << zero.den << endl;

   typedef ratio<7,-3> Neg;
   cout << Neg::num << "/" << Neg::den << endl;
}
输出：
5/3
5/3
1/1
0/1
-7/3

5、duration

#include <ratio>
#include <iostream>
#include <chrono>
using namespace std;

int main()
{
   std::chrono::duration<int>                         twentySeconds(20);  //以秒为单位
   std::chrono::duration<double, std::ratio<60>>      halfAMinute(0.5);   //以60秒为单位
   std::chrono::duration<long, std::ratio<1, 1000>>   oneMillisecond(1);  //以1/1000秒为单位

   std::chrono::seconds         twentySeconds(20);
   std::chrono::hours           aDay(24);
   std::chrono::milliseconds    oneMillisecond(1);
}

// 将毫秒单位的duration切割为小时，分钟，秒，毫秒

#include <ratio>
#include <iostream>
#include <chrono>
#include <iomanip>
using namespace std;
using namespace std::chrono;

milliseconds ms(7255042);

template <typename V, typename R>
ostream& operator<<(ostream& os, const chrono::duration<V,R>& d) {
    os << "[" << d.count() << " of " << R::num << "/" << R::den << "]";
    return os;
}

// 将毫秒单位的duration切割为小时，分钟，秒，毫秒
int main()
{
    hours hh = duration_cast<hours>(ms);
    minutes mm = duration_cast<minutes>(ms%chrono::hours(1));
    seconds ss = duration_cast<seconds>(ms%chrono::minutes(1));
    milliseconds msec = duration_cast<milliseconds>(ms%chrono::seconds(1));

    cout << "raw: " << hh << "::" << mm << "::"
         << ss << "::" << msec << endl;
    cout << "    " << setfill('0') << setw(2) << hh.count() << "::"
                                   << setw(2) << mm.count() << "::"
                                   << setw(2) << ss.count() << "::"
                                   << setw(2) << msec.count() << endl;
}

#include <ratio>
#include <iostream>
#include <chrono>
#include <iomanip>
using namespace std;
using namespace std::chrono;

template <typename C>
void printClockData ()
{
    using namespace std;

    cout << "- precision: ";
    // if time unit is less than or equal to one millisecond
    typedef typename C::period P;   // type of time unit
    if (ratio_less_equal<P,milli>::value) {
        // convert to and print as milliseconds
        typedef typename ratio_multiply<P,kilo>::type TT;
        cout << fixed << double(TT::num)/TT::den
             << " milliseconds" << endl;
    }
    else {
        // print as seconds
        cout << fixed << double(P::num)/P::den << " seconds" << endl;
    }
    cout << "- is_steady: " << boolalpha << C::is_steady << endl;
}

int main()
{
    std::cout << "system_clock: " << std::endl;
    printClockData<std::chrono::system_clock>();
    std::cout << "\nhigh_resolution_clock: " << std::endl;
    printClockData<std::chrono::high_resolution_clock>();
    std::cout << "\nsteady_clock: " << std::endl;
    printClockData<std::chrono::steady_clock>();
}
输出：
system_clock: 
- precision: 0.000001 milliseconds
- is_steady: false

high_resolution_clock:
- precision: 0.000001 milliseconds
- is_steady: false

steady_clock:
- precision: 0.000001 milliseconds
- is_steady: true

下面的程序将timepoint赋值给tp，并转换为日历表示法，window运行会报错，LInux下运行正常：

#include <chrono>
#include <ctime>
#include <string>
#include <iostream>

std::string asString (const std::chrono::system_clock::time_point& tp)
{
    // convert to system time:
    std::time_t t = std::chrono::system_clock::to_time_t(tp);
    std::string ts = std::ctime(&t);    // convert to calendar time
    ts.resize(ts.size()-1);             // skip trailing newline
    return ts; 
}

int main()
{
    // print the epoch of this system clock:
    std::chrono::system_clock::time_point tp;
    std::cout << "epoch: " << asString(tp) << std::endl;

    // print current time:
    tp = std::chrono::system_clock::now();
    std::cout << "now:   " << asString(tp) << std::endl;

    // print minimum time of this system clock:
    tp = std::chrono::system_clock::time_point::min();
    std::cout << "min:   " << asString(tp) << std::endl;

    // print maximum time of this system clock:
    tp = std::chrono::system_clock::time_point::max();
    std::cout << "max:   " << asString(tp) << std::endl;
}
输出：
epoch: Thu Jan  1 08:00:00 1970
now:   Thu Nov 30 21:29:29 2023
min:   Tue Sep 21 08:18:27 1677
max:   Sat Apr 12 07:47:16 2262

#include <chrono>
#include <ctime>
#include <iostream>
#include <string>
using namespace std;

string asString (const chrono::system_clock::time_point& tp)
{
    time_t t = chrono::system_clock::to_time_t(tp); // convert to system time
    string ts = ctime(&t);                          // convert to calendar time
    ts.resize(ts.size()-1);                         // skip trailing newline
    return ts; 
}

int main()
{
    // define type for durations that represent day(s):
    typedef chrono::duration<int,ratio<3600*24>> Days;

    // process the epoch of this system clock
    chrono::time_point<chrono::system_clock> tp;
    cout << "epoch:     " << asString(tp) << endl;

    // add one day, 23 hours, and 55 minutes
    tp += Days(1) + chrono::hours(23) + chrono::minutes(55);
    cout << "later:     " << asString(tp) << endl;

    // process difference from epoch in minutes and days:
    auto diff = tp - chrono::system_clock::time_point();
    cout << "diff:      "
         << chrono::duration_cast<chrono::minutes>(diff).count()
         << " minute(s)" << endl;
    Days days = chrono::duration_cast<Days>(diff);
    cout << "diff:      " << days.count() << " day(s)" << endl;

    // subtract one year (hoping it is valid and not a leap year)
    tp -= chrono::hours(24*365);
    cout << "-1 year:   " << asString(tp) << endl;

    // subtract 50 years (hoping it is valid and ignoring leap years)
    tp -= chrono::duration<int,ratio<3600*24*365>>(50);
    cout << "-50 years: " << asString(tp) << endl;

    // subtract 50 years (hoping it is valid and ignoring leap years)
    tp -= chrono::duration<int,ratio<3600*24*365>>(50);
    cout << "-50 years: " << asString(tp) << endl;
}
输出：
epoch:     Thu Jan  1 08:00:00 1970
later:     Sat Jan  3 07:55:00 1970
diff:      2875 minute(s)
diff:      1 day(s)
-1 year:   Fri Jan  3 07:55:00 1969
-50 years: Thu Jan 16 07:55:00 1919
-50 years: Wed Jan 27 08:00:43 1869

6、timepoint与日历时间的转换

#include <chrono>
#include <ctime>
#include <string>
#include <iostream>

// convert timepoint of system clock to calendar time string
inline std::string asString(const std::chrono::system_clock::time_point& tp) {
    std::time_t t = std::chrono::system_clock::to_time_t(tp);
    std::string ts = ctime(&t);  // convert to calendar time
    ts.resize(ts.size()-1);      //skip trailing newline
    return ts;
}

// convert calender time to timepoint of system clock
inline std::chrono::system_clock::time_point
makeTimePoint(int year, int mon, int day, int hour, int min, int sec=0) {
    struct std::tm t;
    t.tm_sec = sec;
    t.tm_min = min;
    t.tm_hour = hour;
    t.tm_mday = day;
    t.tm_mon = mon - 1;
    t.tm_year = year-1900;
    t.tm_isdst = -1;
    std::time_t tt = std::mktime(&t);
    if (tt == -1) {
        throw "no valid system time";
    }
    return std::chrono::system_clock::from_time_t(tt);
}

int main() {
    auto tp1 = makeTimePoint(2023, 11, 30, 00, 00);
    std::cout << asString(tp1) << std::endl;

    auto tp2 = makeTimePoint(2023, 03, 23, 12, 33);
    std::cout << asString(tp2) << std::endl;
    return 0;
}
输出：
Thu Nov 30 00:00:00 2023
Thu Mar 23 12:33:00 2023

7、<cstring>中的定义式

//在ptr所指的前len个byte中找出字符c
memchr(const void* ptr, int c, size_t len)

//比较ptr1和ptr2所指的前len个byte
memcmp(const void* ptr1, const void* ptr2, size_t len)

//将fromPtr所指的前len个byte复制到toPtr
memcpy(void* toPtr, const void* fromPtr, size_t len)

//将fromPtr所指的前len个byte复制到toPtr（区域可重叠）
memmove(void* toPtr, const void* fromPtr, size_t len)

//将ptr所指的前len个byte赋值为字符c
memset(void* ptr, int c, size_t len)

8、algorithm

（1）find

#include <algorithm>
#include <list>
#include <iostream>
using namespace std;

int main()
{
    list<int> coll;

    // insert elements from 20 to 40
    for (int i=20; i<=40; ++i) {
        coll.push_back(i);
    }

    // find position of element with value 3
    // - there is none, so pos3 gets coll.end()
    auto pos3 = find (coll.begin(), coll.end(),    // range
                      3);                          // value
    
    // reverse the order of elements between found element and the end
    // - because pos3 is coll.end() it reverses an empty range
    reverse (pos3, coll.end());

    // find positions of values 25 and 35
    list<int>::iterator pos25, pos35;
    pos25 = find (coll.begin(), coll.end(),  // range
                  25);                       // value
    pos35 = find (coll.begin(), coll.end(),  // range
                  35);                       // value

    // print the maximum of the corresponding range
    // - note: including pos25 but excluding pos35
    cout << "max: " << *max_element (pos25, pos35) << endl;

    // process the elements including the last position
    cout << "max: " << *max_element (pos25, ++pos35) << endl;
}

（2）find_if

查找最先出现的25或者35

#include <algorithm>
#include <list>
#include <iostream>
using namespace std;

int main()
{
    list<int> coll;

    // insert elements from 20 to 40
    for (int i=20; i<=40; ++i) {
        coll.push_back(i);
    }

    auto pos = find_if(coll.begin(), coll.end(),
                        [](int i) {
                            return i == 25 || i == 35;
                        });
    if (pos == coll.end()) {
        std::cout << "not found" << std::endl;
        exit(1);
    }
    list<int>::const_iterator pos25, pos35;
    if (*pos == 25) {
        // 先找到25
        pos25 = pos;
        pos35 = find(++pos, coll.end(), 35);
        std::cout << *pos35 << std::endl;
    } else {
        pos35 = pos;
        pos25 = find(++pos, coll.end(), 25);
        std::cout << *pos25 << std::endl;
    }
}

9、insert iterator

#include <algorithm>
#include <list>
#include <vector>
#include <set>
#include <iterator>
#include <deque>
#include <iostream>
using namespace std;

int main()
{
    list<int> coll1 = {1, 2, 3, 4, 5, 6, 7, 8};
    vector<int> coll2;

    copy(coll1.begin(), coll1.end(), back_inserter(coll2));

    deque<int> coll3;
    copy(coll1.begin(), coll1.end(), front_inserter(coll3));

    set<int> coll4;
    copy(coll1.begin(), coll1.end(), inserter(coll4, coll4.begin()));

    for (auto &ele : coll2) {
        std::cout << ele << " ";
    }
    std::cout << std::endl;
    for (auto &ele : coll3) {
        std::cout << ele << " ";
    }
    std::cout << std::endl;
    for(auto & ele : coll4) {
        std::cout << ele << " ";
    }
}
输出：
1 2 3 4 5 6 7 8 
8 7 6 5 4 3 2 1
1 2 3 4 5 6 7 8

inserter的作用是在"初始化时接受的第二个实参"所指的位置的前面插入元素，内部调用成员函数insert()，并以新值和新位置作为实参传入，所有的STL容器都提供insert()成员函数，这是唯一可以用于关联式容器身上的一种预定义inserter。

10、stream iterator

#include <algorithm>
#include <vector>
#include <iterator>
#include <string>
#include <iostream>
using namespace std;

int main()
{
    vector<string> coll;

    // 使用ctrl+z 回车进行终止
    copy(istream_iterator<string>(cin), 
         istream_iterator<string>(),
         back_inserter(coll));

    sort(coll.begin(), coll.end());

    unique_copy(coll.cbegin(), coll.cend(), 
                ostream_iterator<string>(cout, "\n"));
}

11、reverse iterator

#include <algorithm>
#include <vector>
#include <iterator>
#include <string>
#include <iostream>
using namespace std;

int main()
{
    vector<int> coll;

    for (int i = 1; i <= 9; i++) {
        coll.push_back(i);
    }

    copy(coll.crbegin(), coll.crend(), 
         ostream_iterator<int>(cout, " "));
}
输出：
9 8 7 6 5 4 3 2 1