CRT detected that the application wrote to memory after end of heap buffer 오류 해결

#pragma warning(disable:4996)
#include <iostream>
#include <cstring>
using namespace std;
 
class String
{
private:
	char* a;
public:
	String(const char *C=" ")
	{
	    a = new char[strlen(C) + 1];
		strcpy(a, C);
	}
	String(const String& copy)
	{
		a = new char[strlen(copy.a) + 1];
		strcpy(a, copy.a);
	}
	~String()
	{
		delete []a;
	}
	friend String operator+(String a,String b);
	void showfun() const;
};
 
 
String operator+(String X,String Y)
{
	String res(strcat(X.a , Y.a));
	return res;
}
 
void String::showfun() const
{
	cout << a << endl;
}
 
int main()
{
	String a("Happy");
	String b(" BirthDay");
	String d;
 
	(a + b).showfun();
	return 0;
}