c++ 实现python的字符串切片功能

#ifndef STRINGFUNCTIONS_H_INCLUDED
#define STRINGFUNCTIONS_H_INCLUDED

#include <exception>
using namespace std;


int len(char* s);
int convert(int index, int length);
char * sliced(char*s, int start, int end, int step =1);//默认参数写在函数声明中
char * nothing();


int len(char* s)
{
    //计算字符串中字符个数（到'\0'为止，不含'\0'）
    int n_s = 0;
    char *s1 = s;
    while(*(s1++) != '\0') n_s++;
    return n_s;
}


int convert(int index, int length)
{
    //索引转换,将负数或者越界的索引转换到0 ~ length 区间
    if (index <= -length) return 0;
    else if (index < 0) return length + index;
    else if(index < length) return index;
    else return length;
}


char * nothing()
{
    //生成指向空字符串的字符指针
    char* c = new char[1];
    *c  ='\0';
    return c;
}


char * sliced(char*s, int start, int end, int step)
{
    // s 代表原字符串，start和end代表切片的起始和结束位置，step代表步长
    if (step>0)
    {
        int leng, start_, end_, n_sliced, r ;
        leng = len(s);
        start_ = convert(start, leng);
        end_   = convert(end, leng);
        if(start_ >= end_) return nothing();

        r = (end_ - start_) % step;
        if(r==0) n_sliced = (end_ - start_)/ step;
        else n_sliced = (end_ - start_)/ step + 1;
        char* sliced= new char[n_sliced+1];


        int i, index;
        for(i=0; i<n_sliced; i++)
        {
            index = start_ + i * step;
            sliced[i] = s[index];
        }
        sliced[i] = '\0';
        return sliced;
    }


    else if (step<0)
    {
        int leng, start_, end_, n_sliced, r ;
        leng = len(s);
        start_ = convert(start, leng);
        end_   = convert(end, leng);
        if (start_ == leng) start_--;
        if(start_ <= end_) return nothing();

        r = (start_ - end_) % (-step);
        if(r==0) n_sliced = (start_ - end_) / (-step);
        else n_sliced = (start_ - end_) / (-step) + 1;
        char* sliced= new char[n_sliced+1];

        int i, index;
        for(i=0; i<n_sliced; i++)
        {
            index = start_ + i * step;
            sliced[i] = s[index];
            //cout << index<<";"<< sliced<<endl;
        }
        sliced[i] ='\0';
        return sliced;
    }

    else  throw invalid_argument("slice step cannot be zero");
}


#endif // STRINGFUNCTIONS_H_INCLUDED

#include <iostream>
#include "stringFunctions.h"
using namespace std;


int main()
{
    char a[100] ="0123456789ABCDEF";

    char *b = sliced(a,2,13);
    cout << b <<endl;
    delete b;

    b = sliced(a,2,10,2);
    cout << b <<endl;
    delete b;

    b = sliced(a,-20,18,1);
    cout << b <<endl;
    delete b;

    b = sliced(a,8,0,-2);
    cout << b <<endl;
    delete b;

    b = sliced(a,25,-7,-2);
    cout << b <<endl;
    delete b;

    return 0;
}