Object Slicing in C
Object slice refers to the phenomenon that only part of the base class data is copied when assigning or passing a derived class object to a base class object, resulting in the loss of new members of the derived class. 1. Object slices occur in containers that directly assign values, pass parameters by value, or store polymorphic objects in storage base classes; 2. The consequences include data loss, abnormal behavior and difficult to debug; 3. Avoiding methods include passing polymorphic objects using pointers or references, or using smart pointers to manage the object life cycle.
In C, Object Slicing is a language feature that is easily overlooked but can cause serious problems. It usually occurs when receiving derived class objects with base class objects, causing the newly added member variables in the derived class to be "cut off", retaining only the part of the base class. This can cause data loss and abnormal behavior problems.
What is object slicing?
Object slices refer to when you assign or pass a derived class object to an object of the base class type, the compiler will only copy the data of the base class part, and the extended members in the derived class will be "cut off".
For example:
struct Base {
int a;
};
struct Derived : Base {
int b;
};
Derived d;
Base b = d; // Object slice happens here, there is no db in b At this time, b only contains a , and the information of db is lost.
What situations are prone to trigger object slicing?
Direct assignment or initialization
Base base = derived; // Slicing happens here
Pass parameters by value
void foo(Base b); Derived d; foo(d); // The parameters are passed by value, and slices will occur
Container storage base class object
std::vector<Base> vec; Derived d; vec.push_back(d); // The stored type is Base, not Derived
In these scenarios, you may mistakenly think that you are operating with complete derived class objects, but in fact only the base class part is preserved.
How to avoid object slicing?
To avoid this problem, the key is not to deal with polymorphic objects in a "value" way, but use pointers or references:
✅ Use pointers (naked pointers or smart pointers)
std::unique_ptr<Base> ptr = std::make_unique<Derived>();
✅ Pass parameters using references
void foo(const Base& b); // Can't slice
✅ If multiple types of objects must be stored in the container, you can:
std::vector<std::unique_ptr<Base>> objects; objects.push_back(std::make_unique<Derived>());
Note: If you just want to access virtual functions, both references and pointers can be dynamically bound correctly; but if member variables are involved, there will be problems with the copy of the value.
What are the consequences of object slicing?
- Data Loss: Additional members in the derived class will not be copied.
- Behavior error: If the derived class overrides the virtual function but is sliced during the value transfer process, although the function can be called normally, it operates in the wrong object state.
- Difficult to debug: the slice will not report an error, the running result may seem "reasonable", but it is logically wrong.
for example:
struct Animal {
virtual void speak() { cout << "..." << endl; }
};
struct Dog : Animal {
void speak() override { cout << "Woof!" << endl; }
};
void makeSpeak(Animal a) {
a.speak();
}
Dog dog;
makeSpeak(dog); // The output may be "...", not "Woof!" Here, even if speak() is a virtual function, it may not be able to show the expected behavior because it becomes an Animal type object after slice.
Basically that's it. Object slicing is not a syntax error, but can cause program behavior abnormalities, especially in polymorphic programming. Just remember: use pointers or references to process polymorphic objects, and do not use values to copy them , to effectively avoid this pit.
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