copy_elision.md

Copy elision

Copy elision in C++ is a compiler optimization technique that reduces the overhead of copying and moving objects. It is especially important in C++ because it can significantly improve performance by eliminating unnecessary copy and move operations. Here's a deeper dive into the concept, along with various examples:

1. Return Value Optimization (RVO)

RVO is a form of copy elision that occurs when a function returns a local object. The compiler can construct the return value directly in the memory space allocated for it in the caller's context, avoiding the need to copy or move the object.

Example:

class MyClass {
public:
    MyClass() {}
    MyClass(const MyClass&) {
        std::cout << "Copy constructor called" << std::endl;
    }
};

MyClass createObject() {
    MyClass obj;
    return obj; // RVO applies here
}

int main() {
    MyClass myObject = createObject(); // No copy is made due to RVO
}

In this example, without RVO, you would expect the copy constructor to be called when returning obj from createObject(). However, with RVO, the compiler can optimize away this copy.

2. Named Return Value Optimization (NRVO)

NRVO is similar to RVO but applies when the returned object has a name.

Example:

MyClass createObject(bool condition) {
    MyClass obj1, obj2;
    if (condition) {
        return obj1; // NRVO can apply here
    } else {
        return obj2; // NRVO can also apply here
    }
}

In this case, the compiler may directly construct obj1 or obj2 in the return value's space, depending on the condition.

3. Elision of Copy/Move Constructors in Initialization

Copy elision can also occur when an object is initialized directly from a temporary object.

Example:

MyClass obj = MyClass(); // Copy/move constructor may be elided

Here, the compiler can construct the temporary MyClass() object directly in the space allocated for obj, eliminating the need for a copy or move.

4. Elision in Throwing and Catching Exceptions

When an exception is thrown and caught, the compiler may elide the copy construction of the exception object.

Example:

try {
    throw MyClass(); // Copy may be elided
} catch (MyClass obj) {
    // ...
}

In the above scenario, the compiler can optimize by constructing the MyClass exception object directly in the exception handling area.

Important Notes:

• Copy elision is permitted by the C++ standard, but not guaranteed. It depends on the compiler and its optimization settings.
• Since C++17, the standard mandates certain cases of copy elision, particularly in returning and throwing situations, making it more predictable.
• It's important to write code that doesn't rely on copy elision for correctness, even though it can be used for performance optimization.

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