Why so much allocation?

Question

Hi

I am aware that string concatenation is costly operation. I am just trying to understand what is exactly happening when we perform A = A+B

Refer below code:
https://code.sololearn.com/cwznVtPL3ARH

As soon as we allocated str, it is more than small string optimization and hence it allocates memory on heap. So , First output line is something I got clearly.

Then str1 is small enough and no allocation on heap for this. So, no particular line in output due to this code line.

Now, when str = str + str1; it is compiler dependent and generally most of the compiler double the allocation. so, I can digest output showing allocation of 39 bytes. But what about additional 20 bytes allocated earlier?

I thought that additional 20 is cost of copy constructor. But it is not true I believe. [verify this by just changing initial value of str1 same as str. Now, i was expecting two copy constructor call but there are no two extra allocation before + operation]

Any thought on this?

Answer 1

'str + str1'
concatenates the two strings to create a new one (operator+):

first it copy-contruscts the resulting string with the lhs(str)

and then appends (operator+=) the rhs(str1) to it