NOTE: class member initialization order in C++

There are some trap when using class in C++. One of them is the initialization order of members. This can be annoying, therefore, I want to record this: Declare order would affect initialization order for C++ class members. For example:

class Foo {
public:
    int i;
    int j = 1;
    Foo(): i{j} {}
};

can lead to an unexpected result, i would be 0 if you create a Foo instance. From reader view, j already initialized with value 1, however, j wasn't initialized yet! It is hard to find out, but we only need a small change to fix it:

class Foo {
public:
    int j = 1;
    int i;
    Foo(): i{j} {}
};

This is unfortunate but happened. I hope this can help to figure out what happened faster next time XD.

Reference to cppreference.com for more information:

The order of member initializers in the list is irrelevant: the actual order of initialization is as follows:

1. If the constructor is for the most-derived class, virtual base classes are initialized in the order in which they appear in depth-first left-to-right traversal of the base class declarations (left-to-right refers to the appearance in base-specifier lists)
2. Then, direct base classes are initialized in left-to-right order as they appear in this class's base-specifier list
3. Then, non-static data members are initialized in order of declaration in the class definition.
4. Finally, the body of the constructor is executed