C++实用工具代码

大家在编码过程中经常会遇到各种繁琐又费事的小功能，比如转换字符串，获取时间戳，保存文件，统计函数运行时间等等。
开一篇帖子把这些工具性的小代码贴出来，供大家快速调用。

很方便地获取当前的日期时间，适合用来给文件命名

#include 

using namespace std;
int main() {
    std::cout << "Hello, World!" << std::endl;
    //获取当前系统时间并转为string
    time_t t = time(0);
    char tmp[64];
    strftime(tmp, sizeof(tmp), "%Y-%m-%d %H:%M:%S", localtime(&t));
    //输出时间
    std::cout << tmp << std::endl;
    //输出时间戳
    std::cout << t << std::endl;
    //t转为string
    std::string s = std::to_string(t);
    std::cout << s << std::endl;
    //tmp转为string
    std::string s1 = std::string(tmp);
    std::cout << s1 << std::endl;

    return 0;
}

如果需要获取毫秒级时间戳怎么办？用chorono库
优化一下上面的例子，如果需要毫秒级时间戳，可以用chrono获取最后的毫秒数，再拼上去

#include 
#include 
#include 
#include 
using namespace std;

string get_cur_time_ms() {
    auto now = chrono::system_clock::now();
    auto in_time_t = chrono::system_clock::to_time_t(now);
    stringstream ss;
    ss << put_time(localtime(&in_time_t), "%Y-%m-%d %X");
    auto now_time_point_int = chrono::duration_cast(now.time_since_epoch()).count();
    ss << ":" << now_time_point_int%1000;
    return ss.str();
}

int main() {

    cout << get_cur_time_ms() << endl;

    return 0;
}

chrono的精度极高，可以达到纳秒级，我们可以用它统计代码的运行时间

#include 
#include 
using namespace std;
int main() {
    // 开始计时
    auto start = chrono::high_resolution_clock::now();
    // 运行你的代码
    for (int i = 0; i < 99999; ++i) {
        int m = i % 10;
        int n = i / 10;
        int k = n % 10;
        int l = n / 10;
        int j = l % 10;
    }
    // 结束计时
    auto finish = chrono::high_resolution_clock::now();
    // 统计时间，单位为微秒，也可替换为毫秒 milliseconds 、纳秒 nanoseconds 等。
    auto duration = chrono::duration_cast(finish - start);
    cout << "Duration: " << duration.count() << " microseconds" << endl;
    //将start和finish转换为int
    int start_int = chrono::time_point_cast(start).time_since_epoch().count();
    int finish_int = chrono::time_point_cast(finish).time_since_epoch().count();
    cout << "start_int: " << start_int << endl;
    cout << "finish_int: " << finish_int << endl;
    
    return 0;
}

解决所有转来转去的问题

//这个是char*,char数组和string互转的库
#include 
#include 
using namespace std;
int main() {
    std::cout << "Hello, World!" << std::endl;
    //将字符串转换为整数
    string str = "123";
    int num = stoi(str);
    cout << num << endl;
    //将整数转换为字符串
    int integer = 123;
    string str_test = to_string(integer);
    cout << str_test << endl;
    //将浮点数转换为字符串
    double d = 123.456;
    string str_test2 = to_string(d);
    cout << str_test2 << endl;
    //将字符串转换为float
    string str_test3 = "123.456";
    float f = stof(str_test3);
    cout << f << endl;
    //将string转换为char*
    string str_test4 = "123";
    char *c = new char[str_test4.length() + 1];
    strcpy(c, str_test4.c_str());
    cout << c << endl;
    //将char*转换为string
    char *c_test = "123";
    string str_test5 = c_test;
    cout << str_test5 << endl;
    //将char*转换为int
    char *c_test2 = "123";
    int i = atoi(c_test2);
    cout << i << endl;
    //将int转换为char*
    int i_test = 123;
    char *c_test3 = new char[10];
    sprintf(c_test3, "%d", i_test);
    cout << c_test3 << endl;
    //char数组转换为string
    char c_test4[] = "123";
    string str_test6 = c_test4;
    cout << str_test6 << endl;
    //string转换为char数组
    string str_test7 = "123";
    char c_test5[str_test7.length() + 1];
    strcpy(c_test5, str_test7.c_str());
    cout << c_test5 << endl;
    //char数组转换为char*
    char c_test6[] = "123";
    char *c_test7 = c_test6;
    cout << c_test7 << endl;
    //char*转换为char数组
    char *c_test8 = "123";
    char c_test9[strlen(c_test8) + 1];
    strcpy(c_test9, c_test8);
    cout << c_test9 << endl;

    return 0;
}

将数据存入文件也是一个经常用得到的操作
这里举个例子，获取当前时间，把内容存入txt文件中

#include 
#include 
#include 
#include 
#include 
#include 
using namespace std;

string get_cur_time_ms() {
    auto now = chrono::system_clock::now();
    auto in_time_t = chrono::system_clock::to_time_t(now);
    stringstream ss;
    ss << put_time(localtime(&in_time_t), "%Y-%m-%d %X");
    auto now_time_point_int = chrono::duration_cast(now.time_since_epoch()).count();
    ss << ":" << now_time_point_int%1000;
    return ss.str();
}

int main() {

    ofstream outfile("time.txt");
    for (int i = 0; i < 9999; i++) {
        string time_str = get_cur_time_ms();
        const char* c_time = time_str.c_str();
        cout << c_time << endl;
        outfile << c_time << endl;
    }
    outfile.close();

    return 0;
}