Constructor member initialization lists explained

Question

The question is in the topic - I don't think I understand how member initialization list is different from direct initialization in the constructor body.

Suppose the following code:

template<typename T>
class MyClass
{
public:
    explicit MyClass(const T& val) : val_(val) {}
private:
    T val_;
    MyClass(const MyClass &other) = delete;
    MyClass(const MyClass &&other) = delete;
    MyClass& operator=(const MyClass &other) = delete;
    MyClass& operator=(const MyClass &&other) = delete;
};

template<typename R>
class MyOtherClass
{
public:
    explicit MyOtherClass(const R& val)
    : stuff_(val)
    {
        //stuff_(val); <-- does not work
    }   
private:
    MyClass<R> stuff_;
};

int main()
{
    MyOtherClass<int> my_value(34);
}

So why does : stuff_(val) work while ...{ stuff_(val); } gives following error:

error: no matching function for call to ‘MyClass<int>::MyClass()’
  {
  ^
test.cpp:5:11: note: candidate: MyClass<T>::MyClass(const T&) [with T = int]
  explicit MyClass(const T& val) : val_(val) {}
           ^
test.cpp:5:11: note:   candidate expects 1 argument, 0 provided

[P.S.] The code is with templates because from the start I wanted to ask a different question, but was not able to reproduce it on this code snippet. In my production code I have something similar to this:

template<typename R>
class MyOtherClass
{
public:
    explicit MyOtherClass(const R& val)
    {
        stuff_(val); 
        another_stuff_(56);
    }   
private:
    MyClass<R> stuff_;
    MyClass<int> another_stuff_;
};

And it gives me error only on another_stuff_(56). Trying to figure out what is the difference for now.

Answer 1

stuff_(val); doesn't match any initialization syntax. It's trying call stuff_ as a functor and pass val as argument.

Furthermore, when goes into the body of the contructor, the initialization of stuff_ has been finished. In other words you cannot perform initialization in the body of the constructor (you might perform assignment or something else). If you specify the member initializer list, the appropriate constructor is called to initialize it before entering constructor. If not, it'll be default initialized (In your example it fails because MyClass doesn't have default constructor, as the error message said).