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https://github.com/stevinz/reflect

Small, flexible, single-header library for runtime reflection and meta data in C++11.
https://github.com/stevinz/reflect

cpp-library cpp11 meta-data properties reflection reflection-library single-header-library

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Small, flexible, single-header library for runtime reflection and meta data in C++11.

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# Reflect



Small, flexible, single-header library for aggregate (struct / class) runtime reflection and meta data using C++11 features. Minimal STL usage, no other dependencies.







## Installation

- Copy 'reflect.h' to project



- In ONE cpp file, define REGISTER_REFLECTION:

```cpp

#define REGISTER_REFLECTION

#include "reflect.h"

#include "my_struct_1.h"

#include "my_struct_2.h"

#include etc...

```



- Classes / Structs should be simple aggregate types (standard layout)

    - No private or protected non-static data members

    - No user-declared / user-provided constructors

    - No virtual member functions

    - No default member initializers (invalid in C++11, okay in C++14 and higher)

    - See (https://en.cppreference.com/w/cpp/types/is_standard_layout) for more info



- BEFORE using reflection functions, make one call to

```cpp

InitializeReflection();

```







## Usage

### Registration in Header File, example: "transform.h"

```cpp

#ifndef TRANSFORM2D_H

#define TRANSFORM2D_H



#include "reflect.h"

struct Transform2D {

    int width;

    int height;

    std::vector position;

    std::string text;

    REFLECT();

}

#ifdef REGISTER_REFLECTION

    REFLECT_CLASS(Transform2D)

    REFLECT_MEMBER(width)

    REFLECT_MEMBER(height)

    REFLECT_MEMBER(position)

    REFLECT_MEMBER(text)

    REFLECT_END(Transform2D)

#endif



#endif // TRANSFORM2D_H

```







## In Code

### Initialize class instance

```cpp

Transform2D t { };

t.width = 100;

t.height = 100;

t.position = std::vector({1.0, 2.0, 3.0});

t.text = "Hello world!";

```



### TypeData Object

```cpp

// Class TypeData

TypeData data = ClassData();           // By class type

TypeData data = ClassData(t);                       // By class instance

TypeData data = ClassData(type_hash);               // By class type hash

TypeData data = ClassData("Transform2D");           // By class name



// Member TypeData

TypeData data = MemberData(0);         // By class type, member index

TypeData data = MemberData("width");   // By class type, member name

TypeData data = MemberData(t, 0);                   // By class instance, member index

TypeData data = MemberData(t, "width");             // By class instance, member name

TypeData data = MemberData(type_hash, 0);           // By class type hash, member index

TypeData data = MemberData(type_hash, "width");     // By class type hash, member name

```



### Get / Set Member Variables

- Use the ClassMember(class_instance, member_data) function to return a reference to a member variable. This function requires the return type, a class instance (can be void* or class type), and a member variable TypeData object. Before calling ClassMember<>(), member variable type can be checked by comparing to types using helper function TypeHashID()

```cpp

// Member Variable by Index

TypeData member = MemberData(t, 0);

if (member.type_hash == TypeHashID()) {

    // Create reference to member

    int& width = ClassMember(&t, member);

    // Can now set member variable directly

    width = 120;

}



// Member Variable by Name

TypeData member = MemberData(t, "position");

if (member.type_hash == TypeHashID>()) {

    // Create reference to member

    std::vector& position = ClassMember>(&t, member);

    // Can now set member variable directly

    position = { 2.0, 4.0, 6.0 };

}

```







## Iterating Members / Properties

```cpp

int member_count = ClassData(t).member_count;

for (int index = 0; index < member_count; ++index) {

    TypeData member = MemberData(t, index);

    std::cout << " Index: " << member.index << ", ";

    std::cout << " Name: "  << member.name  << ", ";

    std::cout << " Title: " << member.title << ", ";

    std::cout << " Value: ";

    if (member.type_hash == TypeHashID()) {

        std::cout << ClassMember(&t, member);

    } else if (member.type_hash == TypeHashID()) {

        std::cout << ClassMember(&t, member);

    } else if (member.type_hash == TypeHashID>()) {

        std::vector& vec = ClassMember>(&t, member);

        for (auto& number : vec) {

            std::cout << number << ", ";

        }

    }

}

```







## Data from Unknown Class Type

- If using with an entity component system, it's possible you may not have access to class type at runtime. Often a collection of components are stored in a container of void pointers. Somewhere in your code when your class is initialized, store the component class TypeHash:

```cpp

TypeHash saved_hash = ClassData(t).type_hash;

void* class_pointer = (void*)(&t);

```

- Later (if your components are stored as void pointers in an array / vector / etc. with other components) you may still access the member variables of the component without casting the component back to the original type. This is done by using the saved_hash from earlier:

```cpp

using vec = std::vector;

TypeData member = MemberData(saved_hash, 3);

if (member.type_hash == TypeHashID()) {

    vec& rotation = ClassMember(class_pointer, member);

    std::cout << "  Rotation X: " << rotation[0];

    std::cout << ", Rotation Y: " << rotation[1];

    std::cout << ", Rotation Z: " << rotation[2];

}

```







## User Meta Data

### Registration in Header File, example: "transform.h"

- Meta data can by stored as std::string within a class or member type. Set user meta data at compile time using CLASS_META_DATA and MEMBER_META_DATA in the class header file with (int, string) or (sting, string) pairs:

```cpp

#ifdef REGISTER_REFLECTION

    REFLECT_CLASS(Transform2D)

        CLASS_META_DATA(META_DATA_DESCRIPTION, "Describes object in 2D space.")

        CLASS_META_DATA("icon", "assets/transform.png")

    REFLECT_MEMBER(width)

        MEMBER_META_DATA(META_DATA_DESCRIPTION, "Width of this object.")

    REFLECT_MEMBER(height)

        MEMBER_META_DATA(META_DATA_DESCRIPTION, "Height of this object.")

    REFLECT_MEMBER(position)

        MEMBER_META_DATA(META_DATA_HIDDEN, "true")

        MEMBER_META_DATA("tooltip", "Pos")

    REFLECT_END(Transform2D)

#endif

```



### Get / Set Meta Data

- BY REFERENCE, pass a TypeData object (class or member, this can be retrieved many different ways as shown earlier) to the meta data functions to get / set meta data at runtime:

```cpp

// TypeData from class

TypeData& type_data = ClassData();

// or from member variable

TypeData& type_data = MemberData("width");



// Get meta data

std::string description = GetMetaData(type_data, META_DATA_DESCRIPTION);

std::string icon_file   = GetMetaData(type_data, "icon");



// Set meta data

SetMetaData(type_data, META_DATA_DESCRIPTION, description);

SetMetaData(type_data, "icon", icon_file);

```







## Portability



Tested and compiled with:

- GCC 4.2.1

- Emscripten 1.38.43