RVO with a standard layout struct without any constructors - c++

Question

I have a struct representing a binary message. I want to write a function to get the next such record from a buffer (whether a file or a socket, doesn't matter):
template <typename Record>
Record getNext();
Now, I could write this like:
template <typename Record>
Record getNext() {
Record r;
populateNext(reinterpret_cast<char*>(&r), // maybe ::read()
sizeof(r)); // or equivalent
return r;
}
which is nice and gives me the benefits of RVO. However, it will invoke the default constructor of Record, which may be composed of types with non-trival default constructors which do work that I would like to avoid - these are not necessarily POD types, but they are standard layout.
Is there a way to write getNext() such that we avoid any constructors (default or copy/move) on Record? Ideally, when the user calls:
auto record = getNext<Record>();
The buffer is read directly into the memory of record. Is this possible?

Answer 1

no_init is a constant of type no_init_t.
If you construct a pod from a no_init_t, you get an uninitialized pod, and (assuming elision) there is nothing to be done.
If you construct a non-pod from a no_init_t, you have to override a constructor, and make it not initialize the data. Usually class_name(no_init_t):field1(no_init), field2(no_init){} will do it, and sometimes class_name(no_init_t){} will do it (assuming all contents are pod).
Constructing from no_init on each member can act as a sanity check that the members are indeed pod, however. A non-pod class constructed from no_init will fail to compile until you write the no_init_t constructor.
This (having to no_init each member constructor) does generate some annoying DRY failure, but we don't got reflection, so you are gonna repeat yourself and like it.
namespace {
struct no_init_t {
template<class T, class=std::enable_if_t<std::is_pod<T>{}>>
operator T()const{
T tmp;
return tmp;
}
static no_init_t instance() { return {}; }
no_init_t(no_init_t const&) = default;
private:
no_init_t() = default;
};
static const no_init = no_init_t::instance();
}
struct Foo {
char buff[1000];
size_t hash;
Foo():Foo(""){}
template<size_t N, class=std::enable_if_t< (N<=sizeof(buff)) >>
Foo( char const(&in)[N] ) {
// some "expensive" copy and hash
}
Foo(no_init_t) {} // no initialization!
};
struct Record {
int x;
Foo foo;
Record()=default;
Record(no_init_t):
x(no_init), foo(no_init)
{}
};
Now we can construct Record with no_init and it won't be initialized.
Every POD class is not initialized. Every non-POD class must provide a no_init_t constructor (and presumably implement non-initialization, as best it can).
You then memcpy right over it.
This requires modifying your type, and the types it contains, to support non-initialization.

Answer 2

Something like this?
EDIT:
Addresses comment on alignment. Now uses anonymous union to ensure correct alignment.
TestRecord now incorporates another standard layout type egg
Added proof that even though egg has a default constructor, the class is not constructed prior to being filled by populateNextRecord()
I think this is about as fast as it can be isn't it?
#include <iostream>
#include <array>
#include <algorithm>
struct egg {
egg(int i) : _val(i) {}
egg() {}
int _val = 6;
friend std::ostream& operator<<(std::ostream& os, const egg& e) {
return os << e._val;
}
};
struct TestRecord {
egg x;
double y;
};
void populateNext(uint8_t* first, size_t length)
{
// do work here
TestRecord data_source { 10, 100.2 };
auto source = reinterpret_cast<uint8_t*>(&data_source);
std::copy(source, source + length, first);
}
template<class Record>
struct RecordProxy
{
RecordProxy() {}
uint8_t* data() {
return _data;
}
static constexpr size_t size() {
return sizeof(Record);
}
Record& as_record() {
return _record;
}
union {
Record _record;
uint8_t _data[sizeof(Record)];
};
};
template <typename Record>
RecordProxy<Record> getNext() {
RecordProxy<Record> r;
populateNext(r.data(), // maybe ::read()
r.size()); // or equivalent
return r;
}
using namespace std;
int main()
{
RecordProxy<TestRecord> prove_not_initialised;
auto& r1 = prove_not_initialised.as_record();
cout << "x = " << r1.x << ", y = " << r1.y << endl;
auto buffer = getNext<TestRecord>();
auto& actual_record = buffer.as_record();
cout << "x = " << actual_record.x << ", y = " << actual_record.y << endl;
return 0;
}