C++ Constructors

In the previous blog post, I've talked about the L-Values & R-Values of C++. Now, let's explore the more advanced concepts like L-Value references, R-Value references and move constructor.

L-Value Reference

A L-value reference, denoted by &, is a reference that can bind to existing objects, typically L-values.

• L-Value references can be used to alias an existing object.
• They can also be used to implement pass-by-reference semantics.

For example:

void swap(int& x, int& y) {
    int tmp = x;
    x = y;
    y = tmp;
}

R-Value Reference

In contrast, a R-Value reference, denoted by &&, is a reference that can bind to temporary objects, typically R-values. R-value references play a crucial role in enabling move semantics, allowing for the efficient transfer of resources from temporary objects to newly constructed ones.

Temporary objects: An unnamed object created by the compiler to store a temporary value.

For example:

void printRefValue(int&& x) {
    cout << x << endl;
}

int main(void) {
    printRefValue(100);
    return 0;
}

Move Constructor

Move constructor are special member functions that facilitate the efficient transfer of resources from temporary objects (typically R-values) to newly constructed objects.

Syntax

class MyClass {
  public:
    MyClass(MyClass&& m) noexcept { /* Transfer resources from m to this */ }
}

Characteristics

• Efficient Resource Transfer: The primary purpose of a move constructor is to transfer resources (such as dynamically allocated memory, file handles, or other costly-to-copy resources) from the source object to the newly constructed object. This transfer typically involves pointer swaps or other lightweight operations, avoiding expensive deep copying.
• Declared as noexcept: Move constructors are often declared as noexcept, indicating that they do not throw exceptions. This is important for certain optimizations and guarantees provided by the C++ language and standard library.
• Automatically Generated: If a class does not explicitly define a move constructor, the compiler will generate one automatically under certain conditions.

Use Cases

First of all, it optimizes the performance, especially when dealing with large objects or resource-heavy operations. By efficiently transferring resources instead of copying them, move constructors can significantly reduce memory usage and improve execution speed.

Secondly, Many standard library containers and algorithms in C++ take advantage of move semantics to optimize memory management and performance. Using move constructors allows you to take advantage of these optimizations when working with containers like std::vector, std::string or std::unique_ptr.

For example:

class MyClass {
    int *data;          // Pointer to int
    MyClass(int v) {    // Constructor
        data = new int;
        *data = v;
    }
    MyClass(MyClass&& other) noexcept { // Move Constructor
        data = other.data;
        other.data = nullptr;
    }
    ~MyClass() { delete data; }     // Destructor
};

int main(void) {
    MyClass myclass(5);
    MyClass newclass(std::move(myclass));

    std::cout << "newclass.data = " << *newclass.data << std::endl;
    return 0;
}

Advanced Usage

Consider the following example:

class Obj {
  public:
    Obj() {
        // Constructor logic (e.g. allocate resources)
        std::cout << "Constructing Obj" << std::endl;
    }
    Obj(Obj&& other) noexcept {
        // Move constructor logic to transfer resources efficiently
        std::cout << "Move constructor invoked" << std::endl;
        // Transfer resources from 'other' to 'this' (e.g., pointer swaps)
    }
    // Other member functions ...
};

int main () {
    std::vector<Obj> vec;

    // Inserting Obj into the vector may invoke move constructor
    vec.push_back(Obj()); // Move constructor is potentially invoked
    return 0;
}

Expected output:

Constructing Obj
Move constructor invoked

Explanation: The default constructor is called first when the temporary object is created. Then, the move constructor is invoked when the temporary object is moved into the vector.

Other Constructor in C++

1. Default constructor

   class ClassName {
     public:
       ClassName() { /* Body of constructor */ }
   }
   

   A constructor that doesn’t take any argument.

2. Parameterized Constructor in C++

   class ClassName {
     public:
       ClassName(params...) { /* Body logic */ }
   }
   

   A constructor that accepts parameters.

3. Copy constructor

   class ClassName {
     public:
       ClassName(ClassName& className) { /* Body logic */ }
   }
   

   A constructor that initializes an object using another object of the same class.

Conclusion

Understanding the intricacies of move constructors in C++ is essential for optimizing performance, managing resources efficiently, and writing expressive and robust code. So, try move constructor in your next C++ project and propel your code towards new heights of speed, elegance, and effectiveness.

Happy coding !!!