RVO with a standard layout struct without any constructors

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:

1. Addresses comment on alignment. Now uses anonymous union to ensure correct alignment.

2. TestRecord now incorporates another standard layout type egg

3. 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;
}