Skip to content

Latest commit

 

History

History
179 lines (129 loc) · 7.97 KB

any.md

File metadata and controls

179 lines (129 loc) · 7.97 KB

std::any

std::any is a feature introduced in C++17. It provides a type-safe container for single values of any type.

Explanation

  • std::any can store an instance of any type that satisfies CopyConstructible.
  • It's a better, type-safe alternative to void* for storing values of unknown type at compile time.
  • std::any_cast is a function template in C++ that is used to retrieve the value stored in an std::any object. It serves two primary purposes: to safely extract a value of a specific type from an std::any object and to check the type of the stored value at runtime.
  • If the cast to the original type fails (i.e., if you try to cast it to a different type), an exception of type std::bad_any_cast is thrown.

Example

#include <iostream>
#include <any>
#include <string>

int main() {
    std::any a = 1; // Storing an int
    std::cout << std::any_cast<int>(a) << '\n'; // Casting back to int

    a = 3.14; // Now storing a double
    std::cout << std::any_cast<double>(a) << '\n'; // Casting back to double

    a = std::string("Hello, std::any!");
    std::cout << std::any_cast<std::string>(a) << '\n'; // Casting back to std::string

    // Attempting to cast to an incorrect type will throw std::bad_any_cast
    try {
        std::cout << std::any_cast<int>(a) << '\n';
    } catch (const std::bad_any_cast& e) {
        std::cout << e.what() << '\n';
    }
}

In this example:

  • std::any a is first assigned an integer, then a double, and finally a std::string.
  • For each assignment, it's cast back to its original type using std::any_cast.
  • The last block demonstrates the exception handling in case of a wrong cast. When we try to cast the std::string to int, std::bad_any_cast is thrown.

std::any with and without satisfying the CopyConstructible condition

To illustrate a non-CopyConstructible type, we can create a class that explicitly deletes its copy constructor. This class can't be used with std::any directly, as it violates the CopyConstructible requirement.

#include <iostream>
#include <any>
#include <string>

class NonCopyable {
public:
    NonCopyable() = default;
    NonCopyable(const NonCopyable&) = delete; // deleting the copy constructor
    NonCopyable& operator=(const NonCopyable&) = delete;
};

int main() {
    NonCopyable nonCopyableObj;

    // This line will cause a compilation error because NonCopyable is not CopyConstructible
    // std::any a = nonCopyableObj;
    
    // Uncommenting the above line will lead to a compilation error
    // Instead, you can store a pointer to the object
    std::any a = &nonCopyableObj; // Storing a pointer is fine

    // Retrieving the pointer back
    auto storedObj = std::any_cast<NonCopyable*>(a);
    // Use storedObj as a pointer to NonCopyable
}

In the second example, attempting to store an instance of NonCopyable in std::any will result in a compilation error. However, storing a pointer to a NonCopyable object works because pointers are CopyConstructible.

How to find out if a class is CopyConstructible

1. Check the Source Code

A class is CopyConstructible if it has a public copy constructor. Look for something like:

ClassName(const ClassName&);

If this constructor is public and not deleted, the class is CopyConstructible.

2. Use Type Traits

C++ provides type traits in the <type_traits> header that can be used to check certain properties at compile-time. You can use std::is_copy_constructible to check if a type is CopyConstructible. For example:

#include <type_traits>

if constexpr (std::is_copy_constructible<ThirdPartyClass>::value) {
    // The class is CopyConstructible
} else {
    // The class is not CopyConstructible
}

This method is useful when you want to write code that depends on whether a type is CopyConstructible or not.

3. Attempt Compilation

As a quick-and-dirty method, you can write a small piece of code that attempts to copy an instance of the class. If the code fails to compile, it's likely that the class is not CopyConstructible. However, this method is less precise and should be used cautiously.

std::any vs templates

In C++, both std::any and templates serve different purposes and have different use cases. Understanding the differences and when to use each is important for effective C++ programming.

1. std::any:

  • Type Erasure: std::any is a type-erased container that can hold an instance of any type. This means that the type of the object stored in std::any is not known at compile time, only at runtime. It can store any type, but type information is erased, and you need to know the type to retrieve the stored object.

  • Runtime Polymorphism: Since std::any works with any type, it provides a form of runtime polymorphism. You can store different types in the same container or pass around different types using std::any.

  • Performance: Using std::any incurs runtime overhead. This includes type checks, storage management, and potential dynamic memory allocations. This makes it less efficient compared to templates, which are resolved at compile time.

  • Usage Example:

    #include <any>
#include <iostream>

int main() {
    std::any value = 10;  // Storing an int
    value = std::string("Hello");  // Storing a string
    
    try {
        std::cout << std::any_cast<std::string>(value) << std::endl;
    } catch (const std::bad_any_cast& e) {
        std::cout << "Bad cast: " << e.what() << std::endl;
    }
    
    return 0;
}

  • Use Case: std::any is useful when you need to store or pass around objects of different types without knowing the type in advance or when working with APIs that require handling various types generically.

2. Templates:

  • Compile-Time Polymorphism: Templates provide compile-time polymorphism, meaning the code is generated at compile time based on the types provided. This allows for type-safe and efficient code generation.

  • Type Safety: Templates are type-safe, as the types are known at compile time. This allows the compiler to catch type errors before the program runs.

  • Performance: Templates are generally more performant than std::any because the compiler generates specialized code for each type, eliminating the need for runtime type checks and dynamic allocations.

  • Usage Example:

    #include <iostream>

template<typename T>
void print(T value) {
    std::cout << value << std::endl;
}

int main() {
    print(10);          // Prints an integer
    print("Hello");     // Prints a string literal
    print(3.14);        // Prints a double

    return 0;
}

  • Use Case: Templates are ideal when you want to write generic and efficient code that works with different types but where the types are known at compile time.

Comparison Summary:

Feature std::any Templates
Type Handling Runtime type erasure Compile-time type resolution
Type Safety Type safety only at runtime Type safety at compile-time
Performance Slower, with potential overhead Generally faster, optimized code
Flexibility Can hold any type, very flexible Generic, but types must be known at compile time
Usage Scenario Heterogeneous type containers, APIs Generic programming, containers, algorithms

When to Use Which:

  • Use std::any when you need to store or pass around objects of unknown types at runtime and are willing to accept the associated performance costs.

  • Use templates when you need to write type-safe, efficient, and generic code where types are known at compile time.

Choosing between std::any and templates depends on whether you need runtime flexibility (std::any) or compile-time efficiency and type safety (templates).