What is the problem with the code?

[laserlight]

I don't want to change the interface of operator+.

Why not? It looks like you are giving operator+ the semantics of operator+=, which is a Bad Thing. Furthermore, your implementation of operator+ does unnecessary work: you should just create a buffer for the result, copy over what is necessary, then destroy a's contents and then make a's pointer point to the start of the new result buffer. (Of course, this would be after you have changed this to operator+=.)

EDIT:

because the way operator+ takes char* as an argument violates intuition. I use operator to enhance the intuition.

Joeman's suggestion will allow expressions such as: "hello" + str. I do not see how that violates intuition. But what I had in mind was changing your current operator+ to take its first argument by const reference. This would also allow expressions such as "hello" + str, except that it will likely be less efficient than providing an overload of operator+ with a const char* first parameter.