How to Find the Type of an Object in C++

1. Find the Type of Class Objects in C++
2. Use typeid().name() to Find the Type of Class Objects in C++
3. Use typeid().name() With __cxa_demangle to Find the Type of Class Objects in C++
4. Use std::is_same From <type_traits> to Find the Type of an Object in C++
5. Use the dynamic_cast Operator to Find the Type of an Object in C++
6. Use the decltype Operator to Find the Type of an Object in C++
7. Conclusion

In C++, determining the type of an object is a fundamental aspect of programming, enabling developers to make dynamic decisions and perform type-specific operations. Whether it’s checking types at compile time or dynamically identifying object types during runtime, C++ provides several powerful mechanisms to obtain type information.

This article explores various methods and techniques available in C++ to ascertain object types. From utilizing operators like typeid and decltype to leveraging templates from <type_traits> and employing dynamic_cast, each approach offers unique capabilities for type identification.

Let’s delve into these methodologies, accompanied by illustrative code examples and detailed explanations, to unravel the diverse ways C++ enables us to determine object types effectively.

Find the Type of Class Objects in C++

Similar to how simple variables have data types, like int, bool, etc., class objects have a type which is the class they belong to.

#include <iostream>

using namespace std;

class Vehicles {
 public:
  string make;
  string model;
  string year;
};

int main() { Vehicles car; }

In the example shown above, the type of object car defined in main() is the class Vehicles, as the car is an object of Vehicles.

The above example was very simple, and we could easily identify the object type. But, in more complex projects, especially with multiple classes and objects and multiple programmers working together, keeping track of object types can get very confusing.

To prevent this problem, we can use a few methods to determine the type of object, and we will go over them with examples below.

Use typeid().name() to Find the Type of Class Objects in C++

In C++, typeid().name() is a dynamic type identification mechanism that allows programmers to retrieve type information at runtime. This powerful feature assists in determining the exact type of class objects during program execution.

The typeid operator is part of the <typeinfo> header and is commonly used to obtain type information at runtime. When applied to an object, typeid returns a type_info object containing information about the object’s type.

The .name() method associated with typeid returns a human-readable string representing the name of the type. However, it’s crucial to note that the string returned by .name() isn’t standardized across compilers and might not yield easily readable or meaningful results.

It often provides a mangled or decorated type name specific to the compiler implementation.

The typeid().name() syntax follows this structure:

#include <typeinfo>  // Include the <typeinfo> header

// Using typeid().name() for type identification
const char* typeName = typeid(object).name();

• typeid: The operator used to retrieve type information.
• object: The object or expression whose type needs to be identified.
• name(): This is the method returning the type’s name as a const char*.

The use of typeid().name() function is demonstrated below:

#include <iostream>
#include <typeinfo>

using namespace std;

class Vehicles {
 public:
  string make;
  string model;
  string year;
};

class Aircraft {
 public:
  string make;
  string model;
  string year;
};

int main() {
  int x;
  float y;
  bool b;

  Vehicles car;
  Aircraft helicopter;

  cout << "The type of the variable x "
       << " is " << typeid(x).name() << endl;
  cout << "The type of the variable y "
       << " is " << typeid(y).name() << endl;
  cout << "The type of the variable b "
       << " is " << typeid(b).name() << endl;
  cout << "The type of the object car "
       << " is " << typeid(car).name() << endl;
  cout << "The type of the object helicopter "
       << " is " << typeid(helicopter).name() << endl;
}

The above code snippet produces the following output:

The type of the variable x  is i
The type of the variable y  is f
The type of the variable b  is b
The type of the object car  is 8Vehicles
The type of the object helicopter  is 8Aircraft

As seen above, the typeid().name() function works for both simple variables and user-defined class objects. The function returns i, f, and b for int, float, and bool, respectively.

The function returns the names of their respective classes for objects car and helicopter. It is important to note that output class names have a number 8 before them in this example because of how class and variable names are stored in the context of C++.

The typeid().name() function is system and compiler-dependent when used with class objectives and can append any arbitrary character before the class name when returning it. In the system being used, the compiler appends the number of characters in the class name before the class name, i.e., both Vehicles and Aircraft have 8 letters in their names.

Since typeid().name() is implementation dependent, it returns a mangled class name which can, in turn, be demangled using the c++filt command or __cxa_demangle.

Despite the class name being returned in a mangled form, it is still readable and can be used to identify the object’s class. And since the return type is a string, objects can be easily compared to check whether they have the same class.

Use typeid().name() With __cxa_demangle to Find the Type of Class Objects in C++

As seen in the previous example, using typeid().name() returns mangled class names and variable types. To get the demangled result, we can use the __cxa_demangle function from the cxxabi.h library.

The __cxa_demangle function is a part of the C++ ABI (Application Binary Interface) and is used to demangle (decode) a mangled C++ symbol name into a human-readable format. It’s particularly useful when you’re dealing with symbols generated by C++ compilers, especially when handling template specializations or complex type names.

The syntax for __cxa_demangle is as follows:

char *abi::__cxa_demangle(const char *mangled_name, char *output_buffer,
                          size_t *length, int *status)

Here’s a breakdown of the function’s syntax and its parameters:

• mangled_name (const char*): This parameter represents the mangled C++ symbol name that you want to demangle. It’s a null-terminated string that typically contains a mangled representation of a C++ type, function, or variable name.
• output_buffer (char*): It’s a pointer to a character array that will hold the demangled name. The demangled name, if successfully generated, will be stored in this buffer. It’s important to ensure that the buffer is large enough to accommodate the demangled name. If this parameter is set to nullptr, the function will allocate memory internally to store the demangled name.
• length (size_t*): This parameter represents the length of the output_buffer. If the function successfully demangles the name and writes it into the output_buffer, it updates the length variable to reflect the actual length of the demangled name.
• status (int*): The status parameter is an optional pointer to an integer that will hold the demangling status upon completion of the function. It can be nullptr if you’re not interested in the status. The status indicates the outcome of the demangling process:
  • If the demangling process succeeds, it sets the status to 0 (zero).
  • If the demangling fails or encounters an error, it sets the status to a non-zero value.

The return value of __cxa_demangle is a pointer to the demangled name if successful; otherwise, it returns nullptr.

The use of __cxa_demangle with typeid().name() is shown in the example below:

#include <cxxabi.h>

#include <iostream>

using namespace std;

class Vehicles {
 public:
  string make;
  string model;
  string year;
};

class Aircraft {
 public:
  string make;
  string model;
  string year;
};

int main() {
  int x;
  float y;
  bool b;

  Vehicles car;
  Aircraft helicopter;

  cout << "The type of the variable x "
       << " is "
       << abi::__cxa_demangle(typeid(x).name(), nullptr, nullptr, nullptr)
       << endl;
  cout << "The type of the variable y "
       << " is "
       << abi::__cxa_demangle(typeid(y).name(), nullptr, nullptr, nullptr)
       << endl;
  cout << "The type of the variable b "
       << " is "
       << abi::__cxa_demangle(typeid(b).name(), nullptr, nullptr, nullptr)
       << endl;
  cout << "The type of the object car "
       << " is "
       << abi::__cxa_demangle(typeid(car).name(), nullptr, nullptr, nullptr)
       << endl;
  cout << "The type of the object helicopter "
       << " is "
       << abi::__cxa_demangle(typeid(helicopter).name(), nullptr, nullptr,
                              nullptr)
       << endl;
}

In the scope of this example, we were only concerned with the first argument for the __cxa_demangle function, i.e., mangled name. The rest of the arguments were not required for this case.

The above code produces the following output:

The type of the variable x  is int
The type of the variable y  is float
The type of the variable b  is bool
The type of the object car  is Vehicles
The type of the object helicopter  is Aircraft

As seen from the output, the variable and type names returned are demangled and appear exactly how they do in the code file.

Use std::is_same From <type_traits> to Find the Type of an Object in C++

In C++, determining whether two types are identical at compile-time is a common requirement. The <type_traits> library provides a suite of templates to work with types, and std::is_same is particularly useful for type comparison.

It aids in compile-time assertions, template specialization, and conditional type selection by verifying whether the two types are exactly the same.

Syntax:

#include <type_traits>

template <class T, class U>
struct is_same;

// Usage:
std::is_same<T, U>::value;

• T, U: Types to be compared.
• std::is_same<T, U>::value: Evaluates to true if T and U are the same types; otherwise, it’s false.

Conditional Compilation and Template Specialization

In this example, processType uses std::is_same to determine the type of the argument value at compile-time and execute specialized logic accordingly.

#include <iostream>
#include <type_traits>

template <typename T>
void processType(T value) {
  if (std::is_same<T, int>::value) {
    std::cout << "Processing integer type" << std::endl;
  } else if (std::is_same<T, double>::value) {
    std::cout << "Processing double type" << std::endl;
  } else {
    std::cout << "Processing other types" << std::endl;
  }
}

int main() {
  processType(5);        // Prints "Processing integer type"
  processType(3.14);     // Prints "Processing double type"
  processType("Hello");  // Prints "Processing other types"

  return 0;
}

Output:

Processing integer type
Processing double type
Processing other types

The code illustrates the usage of std::is_same from the <type_traits> library to identify and handle different types of arguments within a templated function. It demonstrates compile-time type comparison, enabling specialized processing for specific argument types.

By using std::is_same, the code distinguishes between int and double argument types and executes corresponding logic, printing messages based on the detected types. Additionally, the code highlights how other types are handled with a default message.

Template Metaprogramming

This example illustrates how std::is_same can be employed in template metaprogramming to perform compile-time type checks, enabling different code paths based on type characteristics.

#include <iostream>
#include <type_traits>

template <typename T>
void printType() {
  if (std::is_same<T, int>::value) {
    std::cout << "Type is int" << std::endl;
  } else if (std::is_same<T, double>::value) {
    std::cout << "Type is double" << std::endl;
  } else {
    std::cout << "Type is unknown" << std::endl;
  }
}

int main() {
  std::cout << "Type info: ";
  printType<int>();     // Prints "Type is int"
  printType<double>();  // Prints "Type is double"
  printType<char>();    // Prints "Type is unknown"

  return 0;
}

Output:

Type info: Type is int
Type is double
Type is unknown

The code demonstrates a templated function printType() using <type_traits> to identify and display the type of its template argument. It employs std::is_same to check if the provided type matches predefined types like int or double.

The program showcases type identification by invoking printType with various template arguments (int, double, and char), printing corresponding messages for recognized types and an "unknown" message otherwise.

Use the dynamic_cast Operator to Find the Type of an Object in C++

C++ offers various methods to determine object types during runtime, and one prominent approach is through the dynamic_cast operator. This operator allows for the dynamic casting of pointers and references across class hierarchies, confirming and converting types during runtime.

Syntax:

dynamic_cast<new_type>(expression)

• new_type: The desired type to cast to.
• expression: Pointer or reference to an object.

Returns:

• For pointers: Returns a nullptr if the cast fails for a pointer.
• For references: Throws a std::bad_cast exception if the cast fails for a reference.

Example:

#include <iostream>

class Base {
 public:
  virtual ~Base() {}  // Adding a virtual destructor for polymorphism
};

class Derived : public Base {
  // Class definition
};

int main() {
  Base baseObj;
  Derived derivedObj;

  Base* ptrBase = &derivedObj;

  Derived* ptrDerived = dynamic_cast<Derived*>(ptrBase);
  if (ptrDerived) {
    std::cout << "Object is of type Derived" << std::endl;
  } else {
    std::cout << "Object is not of type Derived" << std::endl;
  }

  return 0;
}

Output:

Object is of type Derived

This code creates a base class Base with a virtual destructor to enable polymorphism. It then defines a derived class Derived inheriting from Base.

In the main function, it creates instances of both classes and assigns the address of the derivedObj to a pointer of type Base. It attempts to perform a dynamic_cast from the Base pointer to a Derived pointer.

Since the Base class has a virtual destructor, allowing it to be polymorphic, the cast successfully identifies the object as being of type Derived. This check determines the object type and prints a corresponding message.

Use the decltype Operator to Find the Type of an Object in C++

C++ offers various methods to deduce or determine the type of an object or expression. One powerful tool among them is the decltype operator.

Introduced in C++11, decltype allows the programmer to obtain the type of an expression at compile time. It’s particularly useful in situations where the type is not explicitly known or when handling complex type hierarchies and template meta-programming.

The syntax of decltype is straightforward. It examines the type of an expression and returns the exact type. For example:

int x = 10;
decltype(x) y;  // y will have the same type as x, which is int

Here, decltype(x) deduces the type of x, and y becomes an int since x is an int.

Example:

#include <iostream>

int main() {
  int number = 10;
  decltype(number) result;

  std::cout << "Type of result: " << typeid(result).name() << std::endl;

  return 0;
}

Output:

Type of result: i

This code snippet demonstrates the utilization of the decltype operator to deduce the type of a variable named result.

In this code, an integer variable number is initialized with a value of 10. The line decltype(number) result; employs decltype to deduce the type of number and assigns it to result.

However, the code then uses typeid(result).name() to retrieve and display the type information of the result via the typeid function. When compiled and executed, the code will print the type of result to the console.

It’s important to note that the output from typeid(result).name() might provide a platform-specific or mangled type name rather than a human-readable name, depending on the compiler and platform being used.

Conclusion

The article explores multiple methods in C++ to determine object types. It covers techniques like typeid().name() for runtime type identification, std::is_same for compile-time type comparison, dynamic_cast for dynamic type confirmation, and decltype for deducing types at compile-time.

Each method’s syntax and usage are detailed through code examples. Understanding these techniques equips developers to effectively handle object types in C++ projects, enabling dynamic decisions and type-specific operations.