Stdlaunder In C++

In this article, we will discuss the std::launder method in C++ with its syntax and examples.

What is the std::launder function in C++?

The launder function was introduced in C++17. It is a utility function related to pointer provenance and type-based aliasing optimizations.

When there is a pointer named "pointer1" which is of type "type1" and the user wants to reinterpret thacpp tutorialer to another type, "type2", the compiler assumes that the memory location pointed to by "pointer1" is still of type "type1". This assumption can lead to undefined behavior when accessing the memory location through the reinterpreted pointer.

This launder function is used to inform the compiler that it should not rely on any previous pointer information of "pointer1". It treats the "pointer 1" as a freshly allocated pointer and has the type "type2".

For example, there is a class named "Shape" and another class named "Circle". The class "Circle" is derived from the class "Shape". The user creates an object of type "Circle" and stores a pointer to it in a variable of type "Shape". If that variable is reinterpreted into "Circle", without any improper behavior of that variable, we use the launder function so that it treats the pointer as newly created and allocated.

Syntax:

It has the following syntax:

#include <new>
T* std::launder(T* ptr);

Parameters:

This function will take a pointer as parameter and returns a pointer. Both the input and return pointers are of same type.

Example 1:

Let us take a C++ program to illustrate the launder function.

#include <iostream>
#include <new>
struct A {
    int x;
};
int main() {
    alignas(A) char buffer[sizeof(A)]; 
    A* objectA = new (buffer) A(); 
    int* incpptutorialer = reinterpret_cast<int*>(objectA);
    int value = *std::launder(incpptutorialer);
    std::cout << "Value: " << value << std::endl;
    return 0;
}

Output:

Explanation:

• ✅ This program illustrates the launder function. The variables used in the program are "buffer", which represents the memory buffer allocated to store objects of type 'A'.
• ✅ The "objectA" is a pointer to the object created, which is of type 'A'.
• ✅ The "incpptutorialer" represents a pointer to an "int".
• ✅ The "value" is a variable to store the value after dereferencing the "incpptutorialer".
• ✅ A structure is created with an integer member named "x". The main function creates an object of type A. After that, it is reinterpreted to an objectA pointer, which is an incpp tutorialer. There may be a chance of undefined behavior as the user dereferences the incpp tutorialer directly. The launder function makes the pointer treated as valid.

Applications of Launder function:

There are several applications where we use the launder function in C++. Some of them are as follows:

• ✅ Custom memory management
• ✅ Low level data manipulation
• ✅ Serialization and Deserialization
• ✅ Dynamic polymorphism
• ✅ Pointer wrappers and proxies
• ✅ Cross-Language interoperability

Example 2:

Let us take another C++ program for custom memory allocator.

#include <iostream>
#include <memory>
using namespace std;
// Custom memory allocator
class MyAllocator {
public:
    static void* allocate(std::size_t size) {
        // Allocate memory using operator new
        return ::operator new(size);
    }
    static void deallocate(void* ptr, std::size_t /*size*/) {
        // Deallocate memory using operator delete
        ::operator delete(ptr);
    }
};
// Custom container using custom allocator
template<typename T>
class MyContainer {
private:
    T* data_;
public:
    MyContainer() : data_(nullptr) {}
    void allocate() {
        data_ = reinterpret_cast<T*>(MyAllocator::allocate(sizeof(T)));
        new (data_) T(); // Placement new to construct object in allocated memory
    }
    T* getData() { return std::launder(data_); }
};
int main() {
    MyContainer<int> container;
    container.allocate();
    int* ptr = container.getData();
    *ptr = 100;
    std::cout << "Value: " << *ptr << std::endl;
    return 0;
}

Output:

Explanation:

• ✅ This program demonstrates a custom memory allocator and a custom container.
• ✅ The "MyAllocator" is a memory allocator class that allocates and deallocates functions.
• ✅ The "MyContainer" is a custom container class with two functions: allocated and getData. The getData function returns a pointer to a stored object, ensuring its validity using the launder function.
• ✅ In the main function, an object for MyContainer is created and named "container". The memory for the above object is allocated and initialized using the allocate function.
• ✅ The pointer "ptr" is obtained by calling getData on the container. After that, a value of 100 is assigned to the int objeccpp tutorialer to "ptr".
• ✅ Finally, the value pointed to "ptr" is printed.

Conclusion:

In conclusion, this article demonstrates the functionality and importance of the launder function in C++. This function informs the compiler to remove or disregard assumptions about a pointer's previous type and treat it as freshly created. This function helps developers handle the pointer and maintain type safety.