Reflection in C++ is defined as the ability of a program to examine and modify its structure and behavior at runtime. This powerful feature enables dynamic code generation, introspection, and metaprogramming. While C++ does not have built-in reflection capabilities like other languages, it offers techniques and libraries for reflection-like functionality.

In this article, we will learn more about reflection, how to implement reflection in C++, Working, advantages and disadvantages of using reflection in C++.

What is Reflection in C++?

In a programming environment reflection is a language’s ability to inspect, introspect, and modify its own structure and behavior at runtime. In simpler terms, it allows a program to examine its own structure, particularly the structure of classes, interfaces, methods, and fields.

Reflection in C++ is implemented through the Run Time Type Information (RTTI) that allows the users to inspect the dynamic type of an object at run time using the typeid operator and the type_info class. This helps to implement dynamic casts, which can be used to determine the actual type of an object and other type-related operations.

Approach

• Include the <typeinfo.h> and <cxxabi.h> headers for demangling type names.
• To get type information use typeid operator which will return type_info object representing the type of the object.
• Compare the types using typeid equality operator which allows for type hierarchy comparisons.
• Safely cast using dynamic_cast which attempts to cast the object to the target type and returns a null pointer if unsuccessful.

Purpose of Using Reflection in C++ application

Following are some of the main reasons for using reflection in C++:

• Type Introspection: Determining the properties and methods of a class or object.
• Dynamic Invocation: Invoking methods or accessing properties dynamically, without necessarily knowing them at compile-time.
• Metadata Access: Retrieving additional information about types, methods, or properties beyond their basic definition.
• Serialization and Deserialization: Converting objects to a serialized format (like JSON or XML) and back. Useful for data persistence, network communication, and inter-process communication.
• Automated Testing: Generating test cases automatically based on class structures.
• Configuration Management: Dynamically configuring objects based on external configuration files. Allowing for runtime modification of object properties and behavior.

Working of Reflection in C++

Let’s understand how reflection works in C++ with the help of the following diagram:

Workflow of reflection

• Metadata Generation: At the compile-time of the program, metadata about classes, methods, and properties is generated. This is done using macros or external tools that parse the source code.
• Store Runtime Type Information (RTTI): Basic runtime type information is stored using typeid and dynamic_cast.
• Reflection Registry: A global registry is initialized to store the metadata. This registry is populated at program startup.
• Using Reflection: At runtime, the program can query the reflection system to get information about types, create instances, or invoke methods dynamically using the reflection API.

C++ Program to Implement Reflection Using RTTI

The following program demonstrates the implementation of reflection in C++ using RTTI (Run Time Type Information).

// C++ Program to Implement Reflection Using Run Time Type
// Information (RTTI)

#include <cxxabi.h>
#include <iostream>
#include <typeinfo>
using namespace std;

// Base class with a virtual destructor
class Base {
public:
    virtual ~Base() {}
};

// Derived class inheriting from Base
class Derived : public Base {
};

int main()
{
  // Creating a Derived object through Base pointer
    Base* basePtr
        = new Derived(); 
                       

    // Get type information of the object pointed to by
    // basePtr
    const type_info& typeInfo = typeid(*basePtr);
    cout << "Type name: " << typeInfo.name() << endl;

    // Demangle the type name (if needed)
    int status;
    char* demangled = abi::__cxa_demangle(
        typeInfo.name(), nullptr, nullptr, &status);
    string typeName
        = (status == 0) ? demangled : typeInfo.name();
    cout << "Demangled type name: " << typeName << endl;
    free(demangled);

    // Compare types using typeid
    if (typeid(*basePtr) == typeid(Derived)) {
        cout << "Object is of type Derived" << endl;
    }

    // Safe casting using dynamic_cast
    Derived* derivedPtr = dynamic_cast<Derived*>(basePtr);
    if (derivedPtr) {
        cout << "Successfully cast to Derived" << endl;
    }
    else {
        cout << "Casting failed" << endl;
    }
  // Freeing the allocated memory

    delete basePtr; 
    return 0;
}

Type name: 7Derived
Demangled type name: Derived
Object is of type Derived
Successfully cast to Derived

Time Complexity: O(1)
Auxiliary Space: O(1)

Explanation: The above example demonstrates the use of RTTI and dynamic casting for run type introspection and safe type conversions. A base* pointer is created at first and it is pointed to a dynamically allocated derived object. The typeid operator is used to get the type information of the object pointed to the base* pointer. The mangled type name is printed and then demangled using abi::__cxa_demangle for a more readable output. The program then compares the retrieved type information with typeid operator to confirm the object type. Dynamic_cast is used to safely cast the base* pointer to a derived* pointer enabling access to derived-specific functionality if successful.

Advantages of Reflection

Following are some of the benefits of using reflection in C++:

• Reflection allows code to be generated at runtime based on the metadata of types. This can be useful for creating flexible and adaptable systems.
• Reflection enables a program to examine its own structure and behavior, which can be helpful for debugging, testing, and analysis.
• Reflection provides the feature of metaprogramming, where the actual code can manipulates other code. This can lead to more concise and maintainable codebases.
• It makes it easier to implement serialization and deserialization.
• It also enables creation of plugin systems and dynamic loading of modules.

Disadvantages of Reflection

Following are some of the drawbacks of using reflection in C++:

• Reflection operations can be slower than direct method calls.
• Reflection can make code more complex and harder to understand.
• It can potentially break type safety if not used carefully.
• It allows runtime access to private members can pose security risks.
• Reflective code can be harder to maintain and refactor.