Hello,

Given a container C of value_type T, the expected behavior when applying an action on the container is to see that action distributed over its elements. 

-Copying the container->copies each elem.

-Destroying the container->destroys each elem.

-Moving the container-> moves each elem.

(Moving here in the sense moving from one memory location to another one).

By extrapolation:

-Applying an action-> applys it on each elem.

...

Yet there is one fundamental action that doesn't comply to this mental model:

Assignment !

When you assign one container to another, it is not guaranteed that all elements use Assignment operation.

C<T> v1;

v1 = v2; // or = std::move(v2);

Usually the implementation distributes copy-construction, or move-construction operation on each element instead of copy/move assignment operator. 

And sometimes you see a mixture of both (construction and assignment)

Let say you have:

#include <vector>
#include <iostream>
struct A{
  A(){ std::puts(__PRETTY_FUNCTION__);}
  A(const A&)
  { std::puts(__PRETTY_FUNCTION__);}
  A(A&&)
  { std::puts(__PRETTY_FUNCTION__);}
  A& operator=(const A&) = default;//elete;
  A& operator=(A&&)= default;//elete;
};

int main()
{
  std::vector<A> v1(5);
  std::vector<A> v2;
  std::puts("-----");
  v2 = v1;
  return 0; 
 }

######

Output:

A::A()
A::A()
A::A()
A::A()
A::A()
-----
A::A(const A&)
A::A(const A&)
A::A(const A&)
A::A(const A&)
A::A(const A&)

As you can see...

The mental model of distributing actions over each element is not respected for the copy assignment operation, instead of no log after the "-----" above,  we see "A::A(const A&)".

The proposal:

To keep the mental model of "action distribution over container elements",  i suggest adding the capability of "construction by assignment"

That is, we can construct an object directly by copy assignment or move assignment.

And the following snippet should not be UB.

///

T* p = (T*)malloc(sizeof(T));

T val;

*p = val; // UB here

///

for all T's either trivial or non trivial constructible.

One suggestion is to add 2 new special functions

T& operator = (std::construct_tag, const T&);

T& operator = (std::construct_tag, T&&);

Where the implementation just delegates the construction to the appropriate copy/move constructor. Expl

T& operator = (std::construct_tag, const T& t)

  : T(t)

{ }

T& operator = (std::construct_tag,  T&& t)

  : T(std::move(t))

{ }

The only problemt is : how to detect that the object is being constructed by assignment i.e how to choose one of the above new special functions?

I mean:

///

 T t1, t2;

 T* p;

 T* q = &t1;

 *p = t2; // must pick construct by copy assign

 *q = t2; // must pick regular copy assign

///

Over to you.

Thanks

Sent from my Galaxy

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