work

#include <iostream>

int strlenM(const char* str)
{
    int len{ 0 };
    //for (; *str++; ++len);
    while (*str++)
    {
        ++len;
    }
    return len;
}

void strdupM(const char* src, char* dest)
{
    while (*src)
    {
        *dest++ = *src++;
    }
    *dest = '\0';
};

char* strndupM(const char* src, char* dest, int size)
{
    for (int cnt{ 0 }; *dest++ = (cnt < size - 1 ? *src++ : 0); cnt++);
    return dest;
}

char* strcpyM(const char* src, char* dest)
{
    for (dest += strlenM(dest); *dest++ = *src++;);
    return dest;
}

char* strncpyM(const char* src, int size, char* dest)
{
    //int cnt{ strlenM(dest) };
    //for (dest += cnt; *dest++ = (cnt < size - 1 ? *src++ : '\0'); cnt++);
    int cnt{ 0 };
    for (; *dest++; ++cnt);
    --dest;
    for (; *dest++ = (cnt < size - 1 ? *src++ : '\0'); cnt++);
    return dest;
}

int main()
{
    //char str[20]{ 'H', 'e', 'l','l','o',' ',',','w', 'o', 'r','l','d','!' };
    char str[20]{ "Hello, world!" };
    char* strDyn{ new char[30]{ "Hello" } };

    char* str1{ new char[10]{", world!"} };

    strncpyM(str1, 30, strDyn);

    //strndupM("Test str copy",5, str1);
    std::cout << strDyn << '\n';

    //std::cout << strlenM(strDyn) << '\n';
    //char* strOut{ strDyn };
    //while (*strOut)
    //{
    //  std::cout << *strOut++;
    //}
    //std::cout << '\n';
    delete[] strDyn;
    delete[] str1;
}