C++11 error: double free or corruption (fasttop) [duplicate]

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I have a problem in this code. If I write StringIO::Copy(test2, test1); without delete str2; then this code gave me this output:

old test2: abc
new test2: test
Finished
*** Error in `./out': double free or corruption (fasttop): 0x0000000001180030 ***
Aborted (core dumped)

I really could not find the relation between str2 and other objects. If I add delete str2; then I don't see that error.

#include <iostream>

class String{ 
    public:
        char *Buffer = new char;
        unsigned long Size;

    public:
        friend struct StringIO;
        ~String(void){
                delete Buffer;
        }

        void inline operator=(const char *X);
};



struct StringIO{
    static inline unsigned long Length(const char *X){
        unsigned long Count = 0;
        while(X[Count] != '\0'){
            Count++;
        }return Count;
    }

    static inline void Copy(char *X, const char *Y){ 
        unsigned long Count = 0; if(Length(X) > Length(Y)){
            while(Count <= (Length(X) - Length(Y))){
                X[(Count + Length(Y))] = '\0';  
                Count++;
            }Count = 0; 
        }
        while(Y[Count] != '\0'){
            X[Count] = Y[Count];
            Count++;
        }
    }

    static inline void Copy(String &X, const String &Y) {
        if(X.Buffer != nullptr)
            delete X.Buffer;        
        if(X.Buffer == nullptr)
            X.Buffer = new char; 
        X.Size = Y.Size;
        Copy(X.Buffer, Y.Buffer);
    }

};

void String::operator=(const char *X){
    this->Size = StringIO::Length(X);
    StringIO::Copy(this->Buffer, X);
}

int main(void){ 
    String test1, test2;
    char *str1 = new char;
    char *str2 = new char;
    StringIO::Copy(str1, "test");
    StringIO::Copy(str2, "abc");
    test1 = str1;
    test2 = str2;
    std::cout << "old test2: " << test2.Buffer << std::endl;

    StringIO::Copy(test2, test1); //here is the problem

    std::cout << "new test2: " << test2.Buffer << std::endl;

    //delete str2;

    std::cout << "Finished" << std::endl;
    return 0;
}