assigning a class variable outsite constructor body

Question

I am very new to C++, and am following along with this tutorial and have come across this class constructor:

class Player: public Entity
{
private:
    std::string m_Name;
public:
    Player(const std::string& name)
        : m_Name(name) {}

    std::string GetName() override
    {
        return m_Name;
    }
};

So, from what I understand, when we create an instance of the class, we pass a name into the constructor, and it sets m_Name = name, but why is it defined in a weird way and not inside the {} body of the constructor?

I have tried the alternative below, and it works all the same:

Player(const std::string& name)
{
    m_Name = name;
}

I am just wondering what is going on in the first example.

Try `const std::string m_Name;` and you get why that is the only available option. — 273K, Mar 08 '23 at 22:19

Vlad from Moscow · Accepted Answer · 2023-03-08T22:33:48.067

In this constructor

Player(const std::string& name)
    : m_Name(name) {}

the object m_Name is created using the copy constructor of the class std::string.

In this constructor

Player(const std::string& name)
{
    m_Name = name;
}

At first the object m_Name is created using the default constructor of the class std::string and then within the body of the constructor of the class Player there is used the copy assignment operator of the class std::string to assign the value of name to the object m_Name.

That is within the body of the constructor of the class Player the data member m_Name shall be already created.

So in general it is more efficient to create an object using only the copy constructor instead of calling the default constructor and the copy assignment operator..

Another problem with using the second implementation of the constructor is that you can not such a way to create a constant data member of the class or you also can not to initialize a data member that has a referenced type.

Constant objects and references shall be initialized when they are created.

For example if you will declare the following class

class A
{
private:
    std::string &m_Name;

public:
    A( std::string &name )
    {
        m_Name = name;
    }
};

then the compiler will issue an error saying that references must be initialized.

But if you will change the constructor like

    A( std::string &name ) : m_Name( name )
    {
    }

then the code will be compiled successfully.

assigning a class variable outsite constructor body

1 Answers1