Type system of struct_pack ​

struct_pack features a non-intrusive and complete type system. It supports structs, nested structs, various basic types and various data structures, as well as serialization/deserialization of user-defined data structures. In the following we will introduce the type system of struct_pack.

Serialization types supported by struct_pack ​

The main types supported by struct_pack are: basic types, constrained types, composite types and compatible types.

Basic Type ​

Basic types are signed and unsigned fixed-length integers, floating-point numbers, characters, boolean types, and null types. The following table lists all the basic types supported by struct_pack.

constrained types ​

A constraint type is a specific data structure that satisfies a certain constraint. As long as the constraints are met, the data structure belongs to the same constraint type whether it is a class provided by the standard library or a class provided by a third-party library struct_pack supports the following constraint types.

We will demonstrate the constraints for each type so that users could define data structures of their own based on such constraints.

container ​

value_type, size(), begin() and end() member functions should exist for this type and meanwhile it should not satisfy constraints of set_container, map_container nor string. value_type must be valid struct_pack type.

template <typename Type>
concept container = requires(Type container) {
  typename std::remove_cvref_t<Type>::value_type;
  container.size();
  container.begin();
  container.end();
} && !set_container && !map_container && string;

If the memory layout of this type is contiguous, struct_pack will enable memcpy optimization.

set_container ​

value_type, key_type, size(),begin() and end() member functions should be provided and value_type must be legal struct_pcak type.

template <typename Type>
concept set_ontainer = requires(Type container) {
  typename std::remove_cvref_t<Type>::value_type;
  typename std::remove_cvref_t<Type>::key_type;
  container.size();
  container.begin();
  container.end();
};

map_container ​

value_type, mapped_type, size(),begin() and end() member functions should be provided and value_type abd mapped_type must be legal struct_pcak type.

template <typename Type>
concept map_container = requires(Type container) {
  typename std::remove_cvref_t<Type>::value_type;
  typename std::remove_cvref_t<Type>::mapped_type;
  container.size();
  container.begin();
  container.end();
};

string ​

value_type of this container must be provided of character type. Also size(),begin(),end(),length() and data() member functions must exist. value_type must be legal struct_pcak type.

template <typename Type>
concept is_char_t = std::is_same_v<Type, signed char> ||
    std::is_same_v<Type, char> || std::is_same_v<Type, unsigned char> ||
    std::is_same_v<Type, wchar_t> || std::is_same_v<Type, char16_t> ||
    std::is_same_v<Type, char32_t> || std::is_same_v<Type, char8_t>;

template <typename Type>
concept string =  requires(Type container) {
  requires is_char_t<typename std::remove_cvref_t<Type>::value_type>;
  container.size();
  container.begin();
  container.end();
  container.length();
  container.data();
};

If the memory layout of the type is contiguous, struct_pack will enable memcpy optimization. When serialized to the string_view type, struct_pack enables zero-copy optimization.

array ​

The type needs to be either a C built-in array type, or it has a size() member function that specializes std::tuple_size. The members of the array must also be of the legal struct_pack type.

template <typename Type>
concept array = std::is_array_v<T> || requires(Type arr) {
  arr.size();
  std::tuple_size<std::remove_cvref_t<Type>>{};
};

If the memory layout of this type is contiguous, struct_pack will enable memcpy optimization.

variant ​

The type can only be of type std::variant, which is a type-safe union. The data type in variant must be a legal struct_pack types

expected ​

value_type, error_type, unexpected_type, has_value(), error(), and value() must be provided for this type. value_type and error_type must be be of the legal struct_pack type.

template <typename Type>
concept expected = requires(Type e) {
  typename std::remove_cvref_t<Type>::value_type;
  typename std::remove_cvref_t<Type>::error_type;
  typename std::remove_cvref_t<Type>::unexpected_type;
  e.has_value();
  e.error();
  requires std::is_same_v<void,
                          typename std::remove_cvref_t<Type>::value_type> ||
      requires(Type e) {
    e.value();
  };
};

The size of serialization result of this type depends on the maximum length of value_type and error_type.

optional ​

value_type, has_value(), value should be provided for this type. operator * should be overloaded. It should not satisfy expected constraints. Also value_type should be of legal struct_pack type.

template <typename Type>
concept optional = !expected<Type> && requires(Type optional) {
  optional.value();
  optional.has_value();
  optional.operator*();
  typename std::remove_cvref_t<Type>::value_type;
};

struct_pack will compress it if it is empty.

unique_ptr ​

The class needs to provide: operator*, with copy assignment disabled, and the element_type member defined

template <typename Type>
concept unique_ptr = requires(Type ptr) {
  ptr.operator*();
  typename std::remove_cvref_t<Type>::element_type;
}
&&!requires(Type ptr, Type ptr2) { ptr = ptr2; };

If the value of this object is a null pointer, struct_pack will compress it.

bitset ​

The class needs to provide: size(),flip(),set(),reset(),count(),and the function size() is constexpr. The layout of this type must be trivial and store 8 bit into one byte.

template <typename Type>
  concept bitset = requires (Type t){
    t.flip();
    t.set();
    t.reset();
    t.count();
  } && (Type{}.size()+7)/8 == sizeof(Type);

Struct ​

struct_pack supports struct type. Up to 255 fields are supported and nested fields are supported too. All members should be of valid struct_pack type.

struct type could be struct/class/std::tuple/tuplet::tuple/std::pair

trivial struct ​

If there is a struct/class/std::pair/tuplet::tuple, and all the field of this type is a trivial type, and the type isn't registed by macro STRUCT_PACK_REFL, then this type is a trivial struct in struct_pack.

A trivial type is :

1. fundamental type.
2. array type, and the value type is also a trivial type.
3. trivial_view<T>, which T is a trivial type.
4. trivial struct.

the trivial struct's type info include its memory alignment size.

If two trivial struct has different memory alignment size, they are different type and struct_pack can check it in deserialization.

For example:

#pragma pack(1)
struct foo {
  int a;
  double b;
};
#pragma()
struct bar {
  int a;
  double b;
};
void test() {
  foo f{};
  auto buffer = struct_pack::serialize(f);
  auto result = struct_pack::deserialize<bar>(buffer);
  // foo and bar are different types.
  assert(result.has_value() == false);
}

Remeber, the type registed by macro STRUCT_PACK_REFL is not trivial struct.
For example:

struct foo {
  int a,b,c;
};
struct bar {
  int a,b,c;
};
STRUCT_PACK_REFL(bar,a,b,c);
static_assert(struct_pack::get_type_code<foo>()!=struct_pack::get_type_code<bar>());

the std::tuple is not trivial struct too.

Compatible Type ​

This refers to struct_pack::compatible<T, version_number>. This is a special data type kind of identical to std::optional in cpp. In struct_pack it is dedicated for the forward/backward compatibility purpose.

All new added fields in struct_pack must be of compatible<T, version_number> type, so that a new type with compatible<T, version_number> could be safely deserialized to the old version of the object.

Meanwhile, data buffer of an old object, could be safely deserialized to a new version object with new compatible<T, version_number> fields defined.

The default version_number is 0.

For example, we updates the definition of person:

struct person_v2 {
  int age;
  std::string name;
  struct_pack::compatible<std::string> nick_name;
};

It works in the following scenarios:

auto buffer = struct_pack::serialize(person_v2{.age=24,.name="Betty",.nick_name="NULL"});
auto res = struct_pack::deserialize<person>(buffer);

assert(res.has_value()==true);
assert(res->age==24);
assert(res->name=="Betty");

auto buffer = struct_pack::serialize(person{.age=24,.name="Betty"});
auto res = struct_pack::deserialize<person_v2>(buffer);

assert(res.has_value()==true);
assert(res->age==24);
assert(res->name=="Betty");
assert(res->nick_name.has_value()==false);

NOTE: All T in struct_pack::compatible<T, version_number> must be legal struct_pack type.

For compatibility, all modified fields between versions should follow the rules:

1. Only add new fields and all new added fields must be of type struct_pack::compatible<T, version_number>. The new compatible fields should have larger version number.
2. Old fields must not be modified or deleted, even if it is type struct_pack::compatible<T, version_number>

If compatibility is broken between versions, struct_pack will return an error_code safely. However, if you delete/modify the old struct_pack::compatible<T, version_number> fields, it will lead to undefined behaviors when serialize/deserialize between versions.

Type information and type hash ​

Type hash checking ​

The type-checking of struct_pack relies on the type information obtained at compile time and performs hash computation at compile time to obtain the type hash code used for checking. The steps for calculating the type hash for struct_pack are as follows:

1. Generate a struct_pack type tree from the types to be serialized by static reflection.
2. Recursively traverses this tree at compile time to generate the type string corresponding to the type tree.
3. Compute the 32-bit MD5 hash of the string at compile time.

As shown in figure:

Take the following structure as an example:

struct dummy {
  int32_t id;
  double number;
  std::string str;
};

The type tree begins with a root node of struct, which includes three members of id, number and str. Since id and number are of basic types so they are just leaf nodes in this tree. For the last member str, since it is a string constrained type, with a nested subtype of char, so it is a subtree with a char leaf.

There is an unique type id of each type. Such data information string could be get at compile time given the type tree. So we could get the 32-bit MD5 hash value of such type at compile time.

The checksum is stored in the header of the serialized data. When deserialization, this values is firstly read out and checked between the type information of the output data. Since the type calculation is done at compile time, such checking becomes a compare between two 32-bit integer, which is very efficient.

struct_pack return struct_pack::errc::invalid_argument when type hash checking fails.

Compatibility ​

User are allowed to add new struct_pack::compatible<T> fields on previously defined data structure. For the sack of compatibility, such fields must be skipped during type checking. In the generated type information string, subtrees whose root node is of type struct_compatible<T> will be ignored, which ensures constant hash codes between versions.

Hash conflicts ​

It is possible that the 32-bit MD5 hash value conflicts in struct_pack. A hash conflict during deserialization will lead to undefined behavior of struct_pack. However, such possibility is extremely low. For a randomly incorrect type, such possibility is about 2^(-31). Considering that the probability of writing the wrong type is already low, hash conflicts should be a very rare behavior.

To mitigate hash conflicts, struct_pack by default adds full type strings and full checksums to serialized data in debug mode. Therefore, as long as the user has tested his code in debug mode, even if he accidentally writes the wrong type and is extremely unlucky to have a hash conflict, it can be detected in debug mode.

In summary, struct_pack has efficient and secure type-checking capabilities.