Adding strings, Core dumped

i'm trying to implement string class in C++,

#ifndef STRING
#   define STRING
class String{
    private:
        char *Buffer = new char; 

        inline unsigned long Length(const char *);
        inline unsigned long Length(char *);

        inline unsigned long Length(String);
        inline unsigned long Length(String *);
    public:
        ~String(void){
           delete Buffer;
        }

        unsigned long Size = 0; 

        void Equal(const char *);
        void Equal(char *);

        void Equal(String *);       
        void Equal(String);

        char *Get(void);

       //Something is wrong in Add method.
       char *Add(const char *); 
       char *Add(char *);

       char *Add(String *);
       char *Add(String);
       //Something is wrong in Add method

       char *Multiply(unsigned long *);     
       char *Multiply(unsigned long);

       char& operator[](const unsigned long Index); 
       const char& operator[](const unsigned long Index) const; 
}; 
#   include "StringIO.hpp"
#endif

StringIO.hpp:

#ifndef STRINGIO
#   define STRINGIO

inline unsigned long String::Length(String S){return S.Size;}
inline unsigned long String::Length(String *S){return S->Size;}
void String::Equal(String S){this->Equal(S.Buffer);}
void String::Equal(String *S){this->Equal(S->Buffer);}

inline unsigned long String::Length(const char *S){
    unsigned long Count = 0; while(S[Count] != '\0'){ Count++;} return Count;
}

inline unsigned long String::Length(char *S){
    unsigned long Count = 0; while(S[Count] != '\0'){ Count++;} return Count;
}

void String::Equal(const char *S){ 
    unsigned long Count = 0; while(Count <= this->Length(S)){
        this->Buffer[Count] = S[Count];
        Count++;
    } this->Size = Count - 1;
}

void String::Equal(char *S){
    unsigned long Count = 0; while(Count <= this->Length(S)){
        this->Buffer[Count] = S[Count];
        Count++;
    } this->Size = Count - 1;
}

char *String::Get(void){ return this->Buffer;}

/*Something is wrong in here*/
char *String::Add(String *S){return Add(S->Buffer);}
char *String::Add(String S){return Add(S.Buffer);}

char *String::Add(const char *S){ 
    char *Copy = this->Get(); for(unsigned long Count = 0; Count <= Length(S); Count++){
        Copy[this->Size + Count] = S[Count];
    } return Copy;
}

char *String::Add(char *S){
    char *Copy = this->Get(); for(unsigned long Count = 0; Count <= Length(S); Count++){
        Copy[this->Size + Count] = S[Count];
    } return Copy;
}
/*Something is wrong in here*/

#endif

I write a method for adding Strings, and a simple program adding Strings with this method and prints it to screen:

int main(void){
    String *X = new String; 
    String *Y = new String;

    X->Equal("Hello world!\n");
    Y->Equal("Hello world!\n");

    std::cout << X->Add(Y);

    delete X; delete Y; //The line giving crash 
    return 0; 

there is not compiling error but when i run it, it gives core dumped error. How i can fix it? And be merciful, i'm a beginner in C++.