Почему нельзя вызывать конструктор без аргументов?

#include <iostream>
#include <cstring>
 
using namespace std;
 
class CString
{
    char* chars = nullptr;
    size_t length;
public:
    CString() { }
 
    CString(const CString& cs )
    {
        length = cs.length;
        chars = new char[length];
        for (size_t i = 0; i < length; i++)
        chars[i] = cs.chars[i];
    }
    CString(size_t l, char c = ' ')
    {
        length = l;
        chars = new char[length];
        for (size_t i = 0; i < length; i++)
            this->chars[i] = c;
    }
 
    CString(string s) { *this = s; }
    ~CString() { Clear(); }
 
    size_t Length() { return this->length; }
 
    void Clear()
    {
        if (chars)
            delete[]chars;
        chars = nullptr;
        length = 0;
    }
 
    CString& operator= (const string& str)
    {
        Clear();
        length = str.length();
        chars = new char[length];
            memcpy(chars, str.c_str(), length);
        return *this;
    }
 
    CString& operator= (const CString& cs)
    {
        if (this == &cs)
            return *this;
        Clear();
        length = cs.length;
        chars = new char[length];
        for (size_t i = 0; i < length; i++)
            this->chars[i] = cs.chars[i];
        return *this;
    }
    char& operator[] (size_t i)
    {
        if (i < this->length)
            return this->chars[i];
        cout << "Array index out of beyond" << endl;
        exit(-1);
    }
 
CString& operator+= (const string& str)
    {
       *this = *this+ CString(str);
        return *this;
    }
 
    CString& operator+= (const CString& cs)
    {
        return *this = *this + cs;
    }
 
    friend const CString& operator+ (const CString& left, const CString& right);
    friend ostream& operator<< (ostream& out, const CString& cv);
};
 
const CString& operator+ (const CString& left, const CString& right)
{
    size_t res_length = left.length + right.length;
    CString* res = new CString(res_length);
    size_t p = 0;
    for (size_t i = 0; i < left.length; i++, p++)
        res->chars[p] = left.chars[i];
    for (size_t i = 0; i < right.length; i++, p++)
        res->chars[p] = right.chars[i];
    return *res;
}
 
 
std::ostream& operator<< (ostream& out, const CString& cs)
{
    out << '\t' << '\'';
    for (size_t i = 0; i < cs.length; i++)
        out << cs.chars[i];
    out << '\'' << endl;
    return out;
}
 
 
void print_strings(CString& a, CString& b, CString& c)
{
    cout << "A size (" << a.Length() << "):\n" << a;
    cout << "B size (" << b.Length() << "):\n" << b;
    cout << "C size (" << c.Length() << "):\n" << c;
}
 
int main()
{
    cout << "Initialize a with no arguments, initialize b with length and symbol, initialize c with text" << endl;
    CString a = CString();
    CString b(3, '*');
    CString c = CString("Hello!");
    print_strings(a, b, c);
    a = CString(b);
    print_strings(a, b, c);
 
    b[2] = ('t');
    print_strings(a, b, c);
 
    cout << "Demonstrate operator '=' for std::string" << endl;
    a = "QWERTY";
    b = "asdfllkn";
    print_strings(a, b, c);
 
    cout << "Demonstrate operator '=' for CString  and operator '+'" << endl;
    c = a + b;
    print_strings(a, b, c);
 
    cout << "Demonstrate operator '+=' for std::string and CString" << endl;
    a += " + 123";
    b += CString(" + 123");
    print_strings(a, b, c);
 
    return 0;
}

friend CString operator+ (const CString& left, const CString& right);