Explicit vs Implicit Copy Operations

In C++, the terms "explicit" and "implicit" in the context of copy operations refer to how these operations are defined and invoked. Understanding the difference is crucial for controlling object behavior and preventing unintended copies.

Implicit Copy Operations

Implicit copy operations are those that the compiler generates automatically if you don't define them yourself. These include:

1. The default copy constructor
2. The default copy assignment operator

Here's an example of implicit copy operations:

1#include <iostream>
2
3class Player {
4public:
5  int health{100};
6};
7
8int main() {
9  Player p1;
10  Player p2 = p1; // Implicit copy constructor
11  Player p3;
12  p3 = p1; // Implicit copy assignment
13
14  std::cout << "p2 health: " << p2.health <<
15    "\n";
16  std::cout << "p3 health: " << p3.health <<
17    "\n";
18}

1p2 health: 100
2p3 health: 100

In this case, the compiler generates a copy constructor and copy assignment operator that perform member-wise copying.

Explicit Copy Operations

Explicit copy operations are those that you define yourself. You might do this to implement deep copying, manage resources, or prevent copying altogether. Here's an example:

1#include <iostream>
2#include <memory>
3
4class Weapon {
5public:
6  virtual ~Weapon() = default;
7  virtual std::unique_ptr<Weapon> clone() const
8  = 0;
9};
10
11class Player {
12public:
13  Player() : weapon(nullptr) {}
14
15  // Explicit copy constructor
16  Player(const Player& other) : health(
17    other.health) {
18    if (other.weapon) {
19      weapon = other.weapon->clone();
20    }
21    std::cout <<
22      "Explicit copy constructor called\n";
23  }
24
25  // Explicit copy assignment operator
26  Player& operator=(const Player& other) {
27    if (this != &other) {
28      health = other.health;
29      if (other.weapon) {
30        weapon = other.weapon->clone();
31      } else { weapon = nullptr; }
32    }
33    std::cout <<
34      "Explicit copy assignment operator called\n";
35    return *this;
36  }
37
38  int health{100};
39  std::unique_ptr<Weapon> weapon;
40};
41
42int main() {
43  Player p1;
44  // Calls explicit copy constructor
45  Player p2 = p1;
46
47  Player p3;
48  // Calls explicit copy assignment operator
49  p3 = p1;
50}

1Explicit copy constructor called
2Explicit copy assignment operator called

Key Differences

1. Control: Explicit operations give you full control over the copying process.
2. Resource Management: Explicit operations allow proper handling of resources like dynamic memory.
3. Performance: Explicit operations can be optimized for your specific use case.
4. Preventing Copies: You can make copying explicit-only by declaring the copy operations as private or deleting them.

Understanding and properly implementing explicit copy operations when needed can lead to more robust and efficient code, especially when dealing with complex objects or resource management.