The copy constructor is a special constructor in C used to create copy of an object. It takes a reference of the class type as a parameter and will be automatically called in scenarios such as initializing a new object, passing a parameter to a value, and returning a value. The default version performs shallow copy, and if the class contains pointers or resource management, deep copy needs to be manually implemented. Note when writing: 1. The parameters must be referenced to avoid recursion; 2. If the class contains dynamic resources, it should be implemented in a custom way; 3. The destructor and assignment operators need to be synchronized to follow the "three rules"; 4. Copying can be disabled through private declarations or =delete.
The copy constructor in C is part of the class and is used to create a new object as a copy of the existing object. Many people think that this thing is not used much when they first learn it, but in fact it is not the case - for example, when passing parameters, returning values, and initializing a new object, it may be automatically called. If you cannot write a copy constructor well, the program will cause errors at the least, and the resource will be leaked at the worst.
What is a copy constructor?
Simply put, a copy constructor is a special constructor whose parameters are references to the class type and are used to initialize a new object. If you don't write it yourself, the compiler will help you generate a default version.
For example:
class MyClass {
public:
MyClass(); // Normal constructor MyClass(const MyClass&); // Copy constructor};This function will be called in the following common scenarios:
- Initialize a new object with an existing object:
MyClass obj2 = obj1; - Pass the object into the function as a value:
func(obj); - The function returns the object as a value:
return obj;
Note that the default copy constructor makes "shallow copy", which means that if there are pointers or dynamically allocated resources in the class, there will be problems.
When do you need to write it yourself?
In most cases, when there is no complex data structure or resource management in your class, just use the default one. But if your class contains:
- Original pointer (such as
int*) - Open file handle
- External resources such as network connections
Then you must implement the copy constructor yourself, otherwise it is easy to cause multiple objects to share the same piece of memory, which will eventually lead to repeated releases, crashes and other errors.
For example:
class MyString {
char* data;
public:
MyString(const char* str) {
data = new char[strlen(str) 1];
strcpy(data, str);
}
MyString(const MyString& other) {
data = new char[strlen(other.data) 1];
strcpy(data, other.data);
}
};Here we manually copied the string content instead of directly letting the two objects point to the same address. This is what is called a "deep copy".
Several precautions for writing copy constructors
You must use reference to pass parameters , otherwise the copy constructor will be called when passed in, causing infinite recursion.
If you don't want others to copy your class object, you can declare the copy constructor as
privateor use C 11's= delete:MyClass(const MyClass&) = delete;
Don't forget to synchronize other resource management logics, such as destructors and assignment operators (operator=), which usually need to be processed together to avoid resource leakage. This is the so-called "Rule of Three".
Let's summarize
Although the copy constructor looks inconspicuous, it is called implicitly in many places. Writing well can make your class safer and more robust; if you don’t write well, just wait for debugging.
Basically all that is it. Remember to consider whether you really need a deep copy when writing it, and don’t miss the relevant assignment operators and destructors.
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