“CopyConstructible” requirement for C++ stl container element

Question

Regarding to the requirement for C++ stl container element, the standard says: the element type should be CopyConstructible, and there is a table for CopyConstructible requi

Answer 1

In general, STL containers may copy your elements around at some stage, during some kinds of operations or algorithms, so the litmus test is:

Element original(....);  // construct this awesome object
original.this_and_that();  // do stuff to it until the value is perfect...

Element another(original);

Could you use another happily instead of original?

That's effectively what the CopyConstructible requirement's saying: you better be able to have this copied into another object and still be happy with the result. It's not a draconian restriction - you just need to think it through and write your copy constructor correspondingly.

But, it's significant in that some operations like find() may use == to compare elements (for other containers, it may be '<'), so if a side-effect of being copied is that you can't compare elements meaningfully, then your finds et al may stop working - think that through too! (The Standard says for containers, "== is an equivalence relation" (23.1-5).)