What does reinterpret_cast do binary-wise?

Question

I'm writing a logger in C++, and I've come to the part where I'd like to take a log record and write in to a file.

I have created a LogRecord struct, and would like to serialize it and write it to a file in binary mode.

I have read some posts about serialization in C++, and one of the answers included this following snippet:

reinterpret_cast<char*>(&logRec)

I've tried reading about reinterpret_cast and what it does, but I couldn't fully understand what's really happening in the background.

From what I understand, it takes a pointer to my struct, and turns it into a pointer to a char, so it thinks that the chunk of memory that holds my struct is actually a string, is that true? How can that work?

Answer 1

Casting a struct into an array of bytes (chars) is a classic low impact method of binary serialization. This is based on the assumption that the content of the struct exists contiguously in memory. The casting allows us write this data to a file or socket using the normal APIs.

This only works though if the data is contiguous. This is true for C style structs or PODs in C++ terminology. It will not work with complex C++ objects or any struct with pointers to storage outside the struct. For text data you will need to use fixed size character arrays.

struct {
    int num;
    char name[50];
};

will serialize correctly.

struct {
    int num;
    char* name;
};

will not serialize correctly since the data for the string is stored outside the struct;

If you are sending data across a nework you will also need to ensure that the struct is packed or at least of known alignment and that integers are converted to a consistent endianness (network byte order is normally big endian)

Answer 2

A memory address is just a memory address. Memory isn't inherently special - it's just a huge array of bytes, for all we care. What gives memory its meaning is what we do with it, and the lenses through which we view it.

A pointer to a struct is just an integer that specifies some offset into memory - surely you can treat one integer in any way you want, in your case, as a pointer to some arbitrary number of bytes (chars).

reinterpret_cast() doesn't do anything special except allow you to convert one view of a memory address into another view of a memory address. It's still up to you to treat that memory address correctly.

For instance, char* is the conventional way to refer to a string of characters in C++ - but the type char* literally means "a pointer to a single char". How does it come to mean a pointer to a null-terminated string of characters? By convention, that's how. We treat the type differently depending on the context, but it's up to us to make sure we do so correctly.

For instance, how do you know how many bytes to read through your char* pointer to your struct? The type itself gives you zero information - it's up to you to know that you've really got a byte-oriented pointer to a struct of fixed length.

Remember, under the hood, the machine has no types. A piece of paper doesn't care if you write an essay on each line, or if you scribble all over the thing. It's how we treat it - and how the tools we use (C++) treat it.

Answer 3

From what I understand, it takes a pointer to my struct, and turns it into a pointer to a char, so it thinks that the chunk of memory that holds my struct is actually a string, is that true?

Yes.

How can that work?

A string is just a sequence of bytes, and your object is just a sequence of bytes, so that's how it works.

But it won't if your object is logically more than just a sequence of bytes. Any indirection, and you're hosed. Furthermore, any implementation-defined padding or representation/endianness and your data is non-portable. This might be acceptable; it really depends on your requirements.

Answer 4

Binary-wise, it does nothing at all. This casting is a higher-level concept that has no bearing in any actual machine instructions.

At a low level, a pointer is just a numeric value that holds a memory address. There is nothing to be done in telling the compiler "although you thought the destination memory contained a struct, now please think that it contains a char". The actual address itself doesn't change in any way.