我什麼時候應該選擇 C 中的引用和指標？

When to Utilize References and Pointers in C++

Passing data by reference or pointer in C++ poses a common dilemma. To provide clarity, let's delve into each option:

References

• Advantages: References provide the ease of use of pass-by-value semantics while maintaining the efficiency of pass-by-reference. They cannot be reassigned, ensuring stability during function execution.
• Disadvantages: References cannot be set to null, making it inconvenient to represent absent values or pass function parameters by default.

Pointers

• Advantages: Pointers allow for pass-by-reference or pass-by-value depending on the specific need. They provide flexibility in dereferencing to access the underlying value.
• Disadvantages: Pointers can point to null, requiring careful handling and checks for validity. They also introduce complex syntax, especially when working with pointer arithmetic.

General Guidelines

As a general rule of thumb, "Use references when you can and pointers when you have to." Here's a breakdown of common scenarios:

• Pass by reference:

  • Passing built-in types (int, double, etc.)
  • Passing class objects (if copying is expensive)
  • Passing shared_ptr objects (to avoid unnecessary copies)

• Pass by pointer:

  • When you need to pass a null value
  • When you need to modify the object's address (e.g., for dynamic memory allocation)
  • When you need to pass a pointer to a member function (requires special syntax)

Specific Example

The provided code snippet demonstrates the use of references and pointers:

map<string, shared_ptr<vector<string>> > adjacencyMap;
vector<string>* myFriends = new vector<string>();
myFriends->push_back(string("a"));
myFriends->push_back(string("v"));
myFriends->push_back(string("g"));
adjacencyMap["s"] = shared_ptr<vector<string>>(myFriends);

In this case, using a reference (myFriends) for the vector allows for direct manipulation without the overhead of copying. However, since myFriends is dynamically allocated, it's accessed through a pointer, avoiding the dangling pointer issue.

Remember, the choice between references and pointers depends on the specific requirements of the situation. By understanding the advantages and disadvantages of each, you can make informed decisions that optimize code performance and clarity.

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