Correct way to initialize vector member variable

Question

// Method One
class ClassName
{
public:
    ClassName() : m_vecInts() {}

private:
    std::vector<int> m_vecInts;
}

// Method Two
class ClassName
{
public:
    ClassName() {} // do nothing

private:
    std::vector<int> m_vecInts;
}

Question> What is the correct way to initialize the vector member variable of the class? Do we have to initialize it at all?

Answer 1

See http://en.cppreference.com/w/cpp/language/default_initialization

Default initialization is performed in three situations:

1. when a variable with automatic storage duration is declared with no initializer
2. when an object with dynamic storage duration is created by a new-expression without an initializer
3. when a base class or a non-static data member is not mentioned in a constructor initializer list and that constructor is called.

The effects of default initialization are:

• If T is a class type, the default constructor is called to provide the initial value for the new object.
• If T is an array type, every element of the array is default-initialized.
• Otherwise, nothing is done.

Since std::vector is a class type its default constructor is called. So the manual initialization isn't needed.

Answer 2

It depends. If you want a size 0 vector, then you don't have to do anything. If you wanted, say, a size N vector fill of 42s then use the constructor initializer lists:

ClassName() : m_vecInts(N, 42) {}

Answer 3

Since C++11, you can also use list-initialization of a non-static member directly inside the class declaration:

class ClassName
{
public:
    ClassName() {}

private:
    std::vector<int> m_vecInts {1, 2, 3}; // or = {1, 2, 3}
}

Answer 4

You do not have to initialise it explcitly, it will be created when you create an instance of your class.