Copy Elision and Side Effects

Including side effects in copy constructors or destructors can lead to unexpected behavior when copy elision or Return Value Optimization (RVO) is applied by the compiler.

Consider the following example:

1#include <iostream>
2
3class MyClass {
4public:
5  MyClass() = default;
6  MyClass(const MyClass& other) {
7    std::cout << "Copying object";
8    // Modify some global state or perform I/O
9  }
10};
11
12MyClass createObject() { return MyClass{}; }
13
14int main() {
15  MyClass obj = createObject();
16  // ...
17}

In this code, the copy constructor has a side effect of printing a message and potentially modifying global state or performing I/O operations.

However, when the compiler applies copy elision or RVO, the copy constructor may not be called at all. In the main() function, the object obj is likely to be directly constructed in place, bypassing the copy constructor entirely.

As a result, the expected side effects (printing the message and modifying global state) will not occur, leading to inconsistent behavior.

To avoid such issues, it's important to follow these guidelines:

1. Keep copy constructors and destructors focused on their primary purpose of copying/destroying objects. Avoid including side effects that modify global state or perform I/O operations.
2. If you need to perform additional actions when objects are copied or destroyed, consider using separate member functions or free functions that explicitly convey the intent.
3. Be aware that the compiler has the freedom to optimize away copies, so relying on side effects in copy constructors or destructors can lead to fragile and unpredictable code.

By keeping copy constructors and destructors free of side effects, you can write more robust and maintainable code that behaves consistently, regardless of compiler optimizations.