C ++类成员初始化两次

Question

Are there two MyVec 's being created? Does a temporarty, default constructed, MyVec exist before the assinment in Foo 's constructor?

struct Foo {

  typedef std::vector<int> MyVec;

  Foo () {

    // determine how big the vector needs to be.
    // .....

    m_vec = MyVec(12);
  }

  MyVec m_vec; 
};

I know I can do it using pointers.

struct Foo {

  typedef std::vector<int> MyVec;

  Foo () {
    m_vec = new MyVec();
  }

  ~Foo () {
    delete m_vec;
  }

  MyVec * m_vec;
};

But I'd like to avoid that if possible.

Edit:

I forgot to mention. I can't use initializer lists because I need to do stuff in the constructor before the assignment.

Answer 1

Try this syntax instead:

struct Foo {

  typedef std::vector<int> MyVec;

  Foo ()
  : m_vec(12)
  {
  }

  MyVec m_vec; 
};

It's called a c++ member initialization list .

---

I can't use initializer lists because I need to do stuff in the constructor before the assignment.

If you need to calculate how big the vector is, perhaps you can do that either before you call the constructor, or in a static method which you call from the constructor:

struct Foo {

  typedef std::vector<int> MyVec;

  Foo ()
  : m_vec(calculateVectorSize())
  {
  }

  static int calculateVectorSize()
  {
      // determine how big the vector needs to be.
      // .....
  }

  MyVec m_vec; 
};

If those are impossible too, then a cheaper solution than your original post is:

struct Foo {

  typedef std::vector<int> MyVec;

  Foo () {

    // determine how big the vector needs to be.
    // .....

    m_vec.resize(12);
  }

  MyVec m_vec; 
};

Answer 2

Yes.

And yes you can avoid it:

Foo():
  m_vec(/*args_to_construct_m_vec*/)
{
  // body goes here
}

via the above syntax.

If you cannot use initializer list because you want to do work what to put into m_vec , another approach is to split your class.

The part of the class that needs to be initialized before m_vec 's size is calculated gets stuck into a parent class. You construct it, then construct m_vec in the initializer list.

Note that std::vector default initialization is pretty damn cheap (usually " memset " sizeof ( 3 pointers ) to 0 ), so your concern is almost certainly misplaced.

Answer 3

I believe the typedef is just telling the compiler that MyVec can be seen as vector<int> gvien that the vector library is included.

However, no actual MyVec is constructed yet.

Also you should try the alternative and more preferred syntax for initializing member fields.