What's the purpose of using a union with only one member?
C++C++14UnionsC++ Problem Overview
When I was reading seastar source code, I noticed that there is a union structure called tx_side
which has only one member. Is this some hack to deal with a certain problem?
FYI, I paste the tx_side
structure below:
union tx_side {
tx_side() {}
~tx_side() {}
void init() { new (&a) aa; }
struct aa {
std::deque<work_item*> pending_fifo;
} a;
} _tx;
C++ Solutions
Solution 1 - C++
Because tx_side
is a union, tx_side()
doesn't automatically initialize/construct a
, and ~tx_side()
doesn't automatically destruct it.
This allows a fine-grained control over the lifetime of a
and pending_fifo
, via placement-new and manual destructor calls (a poor man's std::optional
).
Here's an example:
#include <iostream>
struct A
{
A() {std::cout << "A()\n";}
~A() {std::cout << "~A()\n";}
};
union B
{
A a;
B() {}
~B() {}
};
int main()
{
B b;
}
Here, B b;
prints nothing, because a
is not constructed nor destructed.
If B
was a struct
, B()
would call A()
, and ~B()
would call ~A()
, and you wouldn't be able to prevent that.
Solution 2 - C++
In simple words, unless explicitly assigned/initialized a value the single member union does not initialize the allocated memory. This functionality can be achieved with std:: optional
in c++17.