I'm not sure, on the other hand, if it's a good idea to do this with the vector. With vector, we're going to create a new string every time. Plus, we're also going to have to convert between string and vector of string twice (we get a string from the database and we have to write a string to the database). Instead, I'd try to make this append more efficient.

As I looked around I found that:

std::string operator+ allocates a new string and copies the two operand strings every time. repeat many times and it gets expensive, O(n).
std::string append and operator+= on the other hand, bump the capacity by 50% every time the string needs to grow. Which reduces the number of memory allocations and copy operations significantly, O(log n).

So I think we'll be alright if we do:

this->attachmentHolders += attachmentHolder;
this->attachmentHolders += ATTACHMENT_DELIMITER;

But in this one-liner, you're using the + operator. So, having this

this->attachmentHolders += attachmentHolder;
this->attachmentHolders += ATTACHMENT_DELIMITER;

we have 3 strings allocated:

• this->attachmentHolders
• attachmentHolder
• ATTACHMENT_DELIMITER

Having one-liner:

this->attachmentHolders += (attachmentHolder + ATTACHMENT_DELIMITER)

We're going to have 4 strings allocated:

• this->attachmentHolders
• attachmentHolder
• ATTACHMENT_DELIMITER
• result of + operation (attachmentHolder + ATTACHMENT_DELIMITER)

I'm going to go with the tow-liner, if you feel like we should change this, please, let me know but keep in mind what's above.