I agree, having nice containers with automatic memory management allows such problems. But this code still works as intended, but it has suboptimal performance. But I think, that it's still better to use an approach allowing such performance issues, rather then bugs specific for manual memory management (memory leaks, use-after-free errors, spatial access errors).

And it's still possible to improve performance here without returning to manual memory management. Just replace it with something like this:

    cat += *p;
    cat += "+";

Now no temporary string is created and thrown away, only cat performs memory allocations under the hood.