How to get the size of a Table from a netsted table

[agallego]

Hello

Say i parse a flatbuffers from the wire. 

Now I have an immutable array of bytes. For simplicity, say 

std::vector<char> bytes;

Great.

I can get the top level Table type via 

auto t = GetMutableRoot<T>(bytes.data())

and call appropriate methods like 

t->fireMonster()

However, I want to save a nested table to disk and I'm not sure how to get the byte range from the underlying 'bytes` array. 

For example:

Table Foo {

  x: uint;

  y: uint;

  z: [ ubyte ];

} 

Table Bar {

  key:  uint;

  many_foo:  [ Foo ];

}

// top level 

Table Baz {

  key2:  uint;

  many_bar: [ Bar ];

}

How do i get the size of a Foo()

In this example, i have to manually do  something like:

sizeof( uint32_t ) /* x */ + sizeof ( uint32_t ) /* y */ +  t->z().size() /* flatbuffers::Vector<uint8_t> */;

etc. 

Is there a generic way to get the 'size' of a table ? 

Second question. 

Assuming i have the example above, indeed the underlying ` bytes ` is for this example a std::vector < char >; 

Is there a way to share a subset of the ` bytes ` array so I don't have to memcpy each Foo into a new buffer that I can then pass around to disk ? 

Thanks, I've been looking through the API and couldn't find an answer to either. 

Any guidance is appreciated. 

The real use case is to write transaction fragments ( key=value pairs) to disk, but the example above should suffice.

https://raw.githubusercontent.com/senior7515/smf/62a393b22b4231656cf012b95b762df8a122612f/src/flatbuffers/wal.fbs

THanks again!

[agallego]

So to solve this one can do this 

for each Foo:
   create an fbb builder
   re insert values
   fbb.GetBufferPointer() and fbb.GetSize()

But that would be inneficient. 

I can manage both client and server so I can make sure all clients sort the data correctly. 

I guess what I'm hoping the answer to be is this, or get to an answer like this. 

get the first offset of the underlying byte array and copy the sizeof ( struct ) + dynamic array size * 

Thanks!! 

[Wouter van Oortmerssen]

That is not generally possible, since a table consists of several parts (the table, a vtable, and any child strings/vectors/tables) which are not necessarily contiguous/ordered in the same way and/or may have alignment padding, etc.

Like agallego is saying, if you already know you want to memcpy parts of a large FlatBuffer out later, you can store the sub part as a byte array itself (see nested_flatbuffer).

An alternative is using reflection, there's a function called CopyTable that can lift arbitrary tables (with any children) out of a buffer into a new builder, but that is likely a lot less efficient than using nested_flatbuffer.

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