I frequently need to implement C++ wrappers for "raw" resource handles, like file handles, Win32 OS handles and similar. When doing this, I also need to implement move operators, since the default compiler-generated ones will not clear the moved-from object, yielding double-delete problems.
When implementing the move assignment operator, I prefer to call the destructor explicitly and in-place recreate the object with placement new. That way, I avoid duplication of the destructor logic. In addition, I often implement copy assignment in terms of copy+move (when relevant). This leads to the following code:
/** Canonical move-assignment operator.
Assumes no const or reference members. */
TYPE& operator = (TYPE && other) noexcept {
if (&other == this)
return *this; // self-assign
static_assert(std::is_final<TYPE>::value, "class must be final");
static_assert(noexcept(this->~TYPE()), "dtor must be noexcept");
this->~TYPE();
static_assert(noexcept(TYPE(std::move(other))), "move-ctor must be noexcept");
new(this) TYPE(std::move(other));
return *this;
}
/** Canonical copy-assignment operator. */
TYPE& operator = (const TYPE& other) {
if (&other == this)
return *this; // self-assign
TYPE copy(other); // may throw
static_assert(noexcept(operator = (std::move(copy))), "move-assignment must be noexcept");
operator = (std::move(copy));
return *this;
}
It strikes me as odd, but I have not seen any recommendations online for implementing the move+copy assignment operators in this "canonical" way. Instead, most websites tend to implement the assignment operators in a type-specific way that must be manually kept in sync with the constructors & destructor when maintaining the class.
Are there any arguments (besides performance) against implementing the move & copy assignment operators in this type-independent "canonical" way?
UPDATE 2019-10-08 based on UB comments:
I've read through http://eel.is/c++draft/basic.life#8 that seem to cover the case in question. Extract:
If, after the lifetime of an object has ended ..., a new object is created at the storage location which the original object occupied, a pointer that pointed to the original object, a reference that referred to the original object, ... will automatically refer to the new object and, ..., can be used to manipulate the new object, if ...
There's some obvious conditions thereafter related to the same type and const/reference members, but they seem to be required for any assignment operator implementation. Please correct me if I'm wrong, but this seems to me like my "canonical" sample is well behaved and not UB(?)
UPDATE 2019-10-10 based on copy-and-swap comments:
The assignment implementations can be merged into a single method that takes a value argument instead of reference. This also seem to eliminate the need for the static_assert and self-assignment checks. My new proposed implementation then becomes:
/** Canonical copy/move-assignment operator.
Assumes no const or reference members. */
TYPE& operator = (TYPE other) noexcept {
static_assert(!std::has_virtual_destructor<TYPE>::value, "dtor cannot be virtual");
this->~TYPE();
new(this) TYPE(std::move(other));
return *this;
}
final. – Jacquezswap(which is already a good idea), just writeT& operator=(T t) {swap(t); return *this;}. It’s efficient just as often as the destructor-based approach. – Bindle