How To Avoid Object Slicing In C++

How To Avoid Object Slicing In C++

Object slicing in C++ happens when you assign or copy a derived class object to a base class object by value, causing the derived part of the object to be “sliced off.” This means only the base class portion is copied, and any extra data or overridden methods in the derived class are lost.

What is Object Slicing?

cppCopyEditclass Base {
public:
    int base_data;
};

class Derived : public Base {
public:
    int derived_data;
};

Derived d;
d.base_data = 1;
d.derived_data = 2;

Base b = d;  // Object slicing happens here!

In the last line, only base_data is copied to b. The derived_data is lost because b is a Base object, not a Derived.

How To Avoid Object Slicing

1. Use pointers or references to base class instead of objects by value:

cppCopyEditvoid foo(Base& b) { /* ... */ }   // Pass by reference
void bar(Base* b) { /* ... */ }   // Pass by pointer

Derived d;
foo(d);  // No slicing, polymorphism preserved
bar(&d); // Also safe

2. Use smart pointers like std::unique_ptr<Base> or std::shared_ptr<Base>:

cppCopyEditstd::unique_ptr<Base> p = std::make_unique<Derived>();

3. Use virtual functions for polymorphism to ensure derived behavior is accessible even when accessed through base pointers or references.
4. Avoid object copying by value when polymorphism is involved.
5. If copying objects is necessary, consider implementing a virtual clone() method:

cppCopyEditclass Base {
public:
    virtual ~Base() = default;
    virtual Base* clone() const = 0;
};

class Derived : public Base {
public:
    Derived* clone() const override { return new Derived(*this); }
};

Now you can copy polymorphically:

cppCopyEditBase* b = d.clone();

Summary

• Never assign or pass derived objects to base objects by value if you want to preserve the full object.
• Use references, pointers, or smart pointers.
• Use virtual functions and clone idioms for safe copying.