Breakpoint on delete[] when destroying char * - c++

Question

I'm doing an assignment for class in which I can't use the string class. I need to use char* as arrays and doing arithmetic with them.
My code being executed in main is the following: I create 2 myString objects and I add them. Then this is done correctly. Both strings get concatenated. but, there's a breakpoint at delete[] str. Can you tell me where I do wrong exactly? I'd like to understand what happens.
myString& myString :: operator+ (const myString& s)
{
myString tmp; //myString temporal
strcpy_s(tmp.str, strlen(str)+1,str); //STR is copied to tmp.str
Alloc(strlen(s.str)+size+1); //Then memory is allocated for both values
strcpy_s(str, strlen(tmp.str)+1, tmp.str); //WE COPY TMP.STR INTO STR NOW WITH ENOUGH SIZE FOR THE NEXT...
strcat_s(str, strlen(s.str) + size+1, s.str); //..ARGUMENT WE CONCATENATE 2 MYSTRING.STR
return (*this);
}
This is the class myString
class myString
{
public:
//Propietats
int size;
char* str;
//CONSTRUCTORS
myString();
myString(const myString&);
//myString(myString&);
myString(const char*, ...);
//Utilities
int Len(char*);
const void Clear();
const void Alloc(const int);
//Operators
bool operator== (const myString&) const;
bool operator== (const char* s) const;
const myString& operator= (myString&);
const myString& operator= (const char* s);
bool operator!= (const myString&) const;
bool operator!= (const char* s) const;
myString& operator+ (const myString&);
myString& operator+ (const char*);
//Metodes
~myString()
{
delete[] str; // **ERROR** THERE'S A BREAKPOINT HERE
}
};
#endif
My error is that there's a breakpoint in delete[] str; And I don't know what to do. It means there's an overflow? How do I solve it?
I'm quite new to pointer arithmetic so don't be harsh.

Answer 1

myString& myString :: operator+ (const myString& s)
{
myString tmp; //myString temporal
tmp.Alloc(strlen(str)+1); // Add this line
strcpy_s(tmp.str, strlen(str)+1,str); //STR is copied to tmp.str
...
You are not allocating room in tmp string. Once allocated, it should work fine.

Answer 2

You are probably corrupting the heap.
myString tmp;
What is tmp.str after this instruction ? Is tmp.str a NULL pointer ? A pointer to a buffer with some default size ?
strcpy_s(tmp.str, strlen(str)+1,str);
Did you make sure tmp.str has room for strlen(str)+1 char ?
You probably should call tmp.Alloc(strlen(str)+1) before this instruction.