Pointers vs References for Component Communication in C++: Safety and Use Cases

Neither raw pointers nor references are inherently "safer" than the other in C++. Both can lead to undefined behavior if misused, specifically through accessing invalid memory (dangling pointers/references or null pointer dereferences). Safety comes from careful design, particularly regarding object lifetimes and ownership.

Let's compare them in the context of parent-child communication, like our Button needing to know about its UI manager.

As used in the lesson after the modification:

1// Button.h
2class UI;
3class Button : public Rectangle {
4  // ...
5  Button(UI& UIManager, const SDL_Rect& Rect);
6private:
7  UI& UIManager; // Reference member
8};
9
10// Button.cpp
11Button::Button(UI& UIManager, const SDL_Rect& Rect) :
12  UIManager{UIManager}, // Must be initialized
13  Rectangle{Rect}
14{}
15
16void Button::SomeMethod() {
17  // Can directly use UIManager, assume it's valid
18  UIManager.SetRectangleColors(...); // Use '.'
19}
20
21// UI.h
22class Button;
23class UI {
24  // ...
25private:
26  // Button member initialized with *this
27  Button C{*this, {250, 50, 50, 50}};
28};

Pros:

• Guaranteed Initialization: References must be initialized when the object (Button) is created. This guarantees that the Button starts its life referring to a specific UI object.
• Implicit Non-Null: References cannot be null. This eliminates the need for null checks before using UIManager, making the code slightly cleaner.
• Clear Intent: Often signals a mandatory, non-changing relationship (a Button must belong to a UI).

Cons:

• Dangling Risk: The biggest danger. If the UI object is destroyed while the Button still exists, the UIManager reference becomes dangling, and using it leads to undefined behavior (as discussed in another FAQ). Lifetime management is critical.
• No Reassignment: Cannot be reseated to refer to a different UI object later (usually not a requirement for a parent link).

Alternatively, the Button could store a pointer:

1// Button.h
2class UI;
3class Button : public Rectangle {
4  // ...
5  Button(UI* pUIManager, const SDL_Rect& Rect);
6private:
7  UI* pUIManager; // Pointer member
8};
9
10// Button.cpp
11Button::Button(UI* pUIManager, const SDL_Rect& Rect) :
12  pUIManager{pUIManager}, // Initialize pointer
13  Rectangle{Rect}
14{}
15
16void Button::SomeMethod() {
17  // MUST check for null before use!
18  if (pUIManager) { 
19    pUIManager->SetRectangleColors(...); // Use '->' 
20  } else {
21    // Handle case where manager is null (optional?)
22  }
23}
24
25// UI.h
26class Button;
27class UI {
28  // ...
29private:
30  // Button member initialized with 'this' (address-of)
31  Button C{this, {250, 50, 50, 50}};
32};

Pros:

• Optional Relationship: Can be set to nullptr, clearly indicating that the relationship might be optional or not yet established.
• Reassignable: Can be changed to point to a different UI object or nullptr later (less common for parent links).
• Explicit Address: Makes it slightly more explicit that you're dealing with memory addresses and indirection.

Cons:

• Null Pointer Dereference Risk: You must check if the pointer is nullptr before dereferencing it (pUIManager->...). Forgetting this check leads to undefined behavior (often a crash).
• Dangling Risk: Pointers can also dangle. If the UI object is destroyed, pUIManager still holds the old memory address (now invalid). Accessing it is undefined behavior, just like with a dangling reference.
• Slightly Clumsier Syntax: Requires the > operator instead of ., and explicit null checks.

Safety: Relies entirely on how you manage object lifetimes and whether you perform necessary checks (null checks for pointers). A dangling reference is just as bad as a dangling pointer or a null pointer dereference.

Use Case:

• Use references when the relationship is mandatory (the Button must have a UI manager) and established at construction, and the lifetime of the referenced object (UI) is guaranteed to exceed or equal the lifetime of the referencing object (Button). The non-null guarantee simplifies usage code.
• Use pointers when the relationship is optional (a Button might have a UI manager, or it might be set later), or if you need the ability to reassign the relationship. Remember to perform null checks diligently.

For the lesson's scenario where the Button is a member of UI, the UI's lifetime naturally includes the Button's lifetime. Using a reference (UI&) passed via *this during construction is a very common and reasonable pattern, clearly signaling the mandatory, non-null relationship. The primary responsibility remains ensuring the UI object itself isn't destroyed prematurely.