类的初始化列表中的向量

Question

I have a learner list/vector defined in one .hpp file like this:

std::vector<DecisionTree> learners_; .

Now in the constructor, this is used like this:

Bagging::Bagging(const DataReader& dr, const int ensembleSize, uint seed) : 
  dr_(dr), 
  ensembleSize_(ensembleSize),
  learners_({}) {
  random_number_generator.seed(seed);
  buildBag();
}

What is happening in the constructor?

Does the program try to fill only two instances within that vector where the first one is a random number and second one is a call to a function? But how does it work because buildBag returns a decision tree but the second argument returns an integer and we declared it as a vector of decisionTrees?

Why are there curly brackets ({}) ?

As I have always programmed in java and python, this is hard to grasp and it does not come from any book but from the code example.

Answer 1

dr_(dr), ensembleSize_(ensembleSize), learners_({}) : These are used to initialize the object variables of Bagging Class to the specific values provided when the constructor is called.

({}) means that the std::vector<DecisionTree> learners_ is initialized to just an empty vector through an initializer list (it's a C++11 thing).

{
  random_number_generator.seed(seed);
  buildBag();
}

Finally, these statements are the body of the constructor of Bagging Class. Plain commands that are executed upon creation of an object.