既然可以在C ++中以两种方式声明复制构造函数，那是正确的方法吗？

Question

Say you have the following class:

class Person{
    string name;
    int age;
    int niNo;

public:
    Person(const string & _name, const int & _age, const int & ni) : name(_name), age(_age), niNo(ni) {}

    string getName() const{
        return name;
    }

    int getAge() const{
        return age;
    }

    int getNi() const{
        return niNo;
    }

    bool operator==(const Person &p){
        return name == p.getName() && age == p.getAge() && niNo == p.getNi();
    }

And then you can define the copy constructor as follows:

    Person (const Person &other){
        name = other.name;
        age = other.age;
        niNo = other.age;
    }

or like this

    Person (const Person &other) : name(other.getName()), age(other.getAge()), niNo(other.getNi()) {}

Which is the best way to do it? Or does it not matter?

Answer 1

As your class is trivially copy constructable, the best way is in C++11 :

Person (const Person &other) = default;

Otherwise, the second one is he better, because there is no default init and affection as in the first one.

Answer 2

The second is more efficient. In the first, the member name is first default constructed and then assigned.

Answer 3

In general (not just for copy constructors) it's better to initialise members directly in the initialiser list, as your second example does. The first example default-initialises them, then reassigns them. This is potentially less efficient, and won't work for all types: it requires that they be default-initialisable and copy-assignable, while direct initialisation doesn't.

In this case, where you just want a simple member-wise copy, don't declare a copy constructor at all. The implicitly generated one will do the right thing, and will be less error-prone since it doesn't need updating if you add new members.

Answer 4

One important aspect to add. If one of your private data members is const let's say name

class Person{
   const string name;
    int age;
    int niNo;
...

You have no option and you must initialize name in the constructor initializer list!

If it is constant it has been default constructed before entering the body of the constructor and you can't redefine it as it constant...so there you must use the initializer list

Answer 5

Of the two ways you described, I would strongly recommend using the initializer list.

When control enters the body of your constructor, standards dictate that the object is fully initialized at that time, therefore all its members must be initialized as well. What your first example does is that it initializes all the members in some default way and then assigns to them later.

This is potentially ineffective if one of the class members is a huge object, and impossible to do if a member cannot be initialized without parameters, ie the member is a const , a reference, or is a class with no default constructor.

In that case, you must use the initializer list to customize the initialization process. Initializer list syntax can also be used to delegate construction (C++11) and initialize base classes.