[C++] Compile-time message initialization check

[szymonwieloch]

In our company we sometimes struggle with extending schemas, particularly with extending messages with new fields. The problem is that if one person extends the schema in one repo and a second person maintains C++ code, somebody may not notice that a fields was added. This results in part of the bytes in the buffer not being initialized.

You have already added the checkEncodingIsComplete() method which solves part of the problem - checks if all data and group elements were added. But doesn't check if all fields were set.

So I want to suggest adding a new method to the encoder which will support compilation-time checks:

msg.sbeSetAllFields(field1, field2, field3);


void sbeSetAllFields(uint16 field1, uint32 field2, uint8 field3)
{
this->field(1).field2(field2).field3(field3);

}

The basic idea is that when you add a new field to the schema, the sbeSetAllFields() method gets extended with new arguments. A code that misses some fields simply won't compile. So every time somebody updates the schema, code stops compiling until you fix all differences. This is a very safe approach. And I believe it's also a zero-cost abstraction on top of the existing API.

There are at least several interesting edge cases which I think are worth mentioning with some rough proposal for handling:

1. Composites.

You can just flatten the list of arguments

void sbeSetAllFields(uint8 field1, uint16 combo_one, uint16 combo_two, uint8 field3)
{
this->field1(field1);
this->combo().one(combo_one).two(combo_two);
this->field3(field3);
}

2. Bit sets

Simply allow a raw value as the argument.

An alternative to the above examples, a lambda could be added as handler for complex types:

template <typename Handler1, typename Handler2>
void sbeSetAllFields(uint8 field1, Handler1 && combo, Handler2 && bitSet, uint8 field4)
{
this->field1(field1);
combo(this->combo());
bitSet(this->bitSet());
this->field4(field4);
}

And example use:

msg.sbeSetAllFields(
1,
[](auto & combo){combo.one(1).two(2);},
[](auto & bitSet){bitSet.clear().someFlag(true);},
8
);

Optional extensions:

1. The method could be extended with <data> objects as those can always be set using a pair of pointer and length.
2. Composites and groups could also get a sbeSetAllFields() method.