How can the synthesized default constructor initialize already initialized class members?

Question

I am learning about classes, and chapter 7.1.4 of the book C++ Primer says the following:

The compiler-generated constructor is known as the synthesized default constructor. For most classes, this synthesized constructor initializes each data member of the class as follows:

If there is an in-class initializer (§ 2.6.1, p. 73), use it to initialize the member.

Otherwise, default-initialize (§ 2.2.1, p. 43) the member.

If the class members already have an in-class initializer, why would this synthesized default constructor initialize them again? Also, I thought that initialization is a thing that can only happen once. The above suggest that the members are initialized twice, once explicitly, and once implicitly by the synthesized default constructor.

It doesn't initialize them again. It uses the in-class initializer to initialize the class's member variable. — Eljay
The constructor initializes all member data. An in-class initializer describes how to initialize a member, but it doesn’t do anything on its own. The constructor uses it to initialize the corresponding member. — Pete Becker

NathanOliver NathanOliver · Accepted Answer · 2020-08-10T19:32:51

In class initializers don't actually initialize anything. All they are are syntactic sugar for use this initializer if none is provided. So, with

struct foo
{
    int a = 42;
    int b;
};

The compiler is going to generate a constructor like

foo() : a(42)/*, b()*/ {}

Since a has an "initializer" and b does not. note the b() is just exposition. since b is an int no initialization happens to it.

In a more complicated constructor like

struct foo
{
    int a = 42;
    int b;
    foo() : b(21) {}
};

Since the a member is missing from the class member initialization list, the compiler will add a(42) to the list because that is the default initializer to use.

If you had

struct foo
{
    int a = 42;
    int b;
    foo() : a(3), b(21) {}
};

Then the default initializer is ignored and a will have the value of 3 instead of 42.

How can the synthesized default constructor initialize already initialized class members?

2 Answers