C++ noexcept Specifier: Performance Impact

#Why should we use noexcept?

The noexcept specifier – when present – indicates that a function cannot throw exceptions.

#What is a move constructor?

A move constructor creates a new object by transferring resources from one object to another without making a copy. It’s particularly useful when dealing with large objects or resource-intensive operations.

class MyClass {
public:
    // Possible implementation of MyClass move constructor
    MyClass(MyClass&& other) noexcept {
        // Transfer resources from 'other' to 'this'
        this->data = other.data;
        other.data = nullptr; // Leave 'other' in a valid but unspecified state
    }
};

The MyClass&& other is the syntax for an rvalue reference in C++.

Rvalue references allow you to distinguish between temporary objects (rvalues) and objects that persist (lvalues, i.e. variables). This is crucial for move semantics, as it enables the efficient transfer of resources from temporary objects.

When you write MyClass&& other, you’re declaring that other is a rvalue reference to a MyClass object. This implies other can bind to a temporary MyClass object, enabling you to “steal” its resources.

#What is a move assignment operator?

The move assignment operator moves resources from an existing object to another, releasing any resources held by the destination.

class MyClass {
public:
    // Possible implementation of a move assignment operator
    MyClass& operator=(MyClass&& other) noexcept {
        if (this != &other) {
            // Free existing resources
            delete[] this->data;

            // Transfer resources from 'other' to 'this'
            this->data = other.data;
            other.data = nullptr; // Leave 'other' in a valid but unspecified state
        }
        return *this;
    }
};

Because the move assignment operator releases all of the resources that the destination object may be holding, it must also ensure that the target and source are separate objects! Without the check, the method would destroy the target’s contents and leave the object in an undetermined state, potentially triggering memory crashes or leaks.

#How does std::move() really do?

Move constructors and move assignment operators are automatically called when you use std::move:

MyClass a;
MyClass b(std::move(a)); // Calls move constructor
MyClass c;
c = std::move(b); // Calls move assignment operator

std::move is essentially a cast that converts an object into an rvalue reference, enabling the move semantics. It doesn’t actually move anything by itself – it simply marks an object as ready to be moved.

#Why using noexcept with the move constructors and move assignment operators?

Marking move constructors and move assignment operators as noexcept is a smart choice for several reasons:

• Efficiency: When a move operation is marked noexcept, it signals to the compiler that the operation won’t throw exceptions. This allows certain optimizations, like using move operations in STL containers, improving performance by avoiding unnecessary copies. Standard Template Library (STL) containers like std::vector will prefer move operations over copy operations if they are noexcept. If a move operation can throw, the container might fall back to the slower copy operation to ensure strong exception safety guarantees (please, check the documentation about std::move_if_noexcept ).
• Exception Safety: Declaring move operations as noexcept helps maintaining exception safety in your programs, reducing the complexity of handling exceptions and avoiding unexpected behaviors.
• Code Clarity: noexcept makes it clear to anyone reading the code that the function is guaranteed not to throw exceptions, making your intentions explicit and the code easier to understand.

#Conclusions

Understanding move semantics is a game changer for C++ developers. Use move constructors and move assignment operators, as well as features like std::move and noexcept, to make your code more efficient and responsive. Rvalue references allow you to smoothly move resources, lowering overhead and increasing efficiency.