Default initialization confusion

Question

In my C++ Primer, 5th Edition, they say on page 43 about default initialization (emphasis mine):

The value of an object of built-in type that is not explicitly initialized depends on where it is defined. Variables defined outside any function body are initialized to zero.

Later, on page 73, they define a new class like this:

struct Sales_data {
  std::string bookNo;
  unsigned units_sold = 0;
  double revenue = 0.0;
};

This is supposed to provide an example of the new standard's in-class initializers. In this case, units_sold and revenue will be initialized to zero. If they are not, they say, the variables will be default-initialized.

But this is my question: What is the point of this in-class initialization? What's wrong with letting them just default-initialize? The reason I ask is that they already mentioned that variables defined outside any function body are initialized to zero, and in this case, these variables are not inside any function - they are in a class definition. So the default-initialization should initialize these variables to zero, shouldn't it?

Answer 1

The quote seems to be in wrong context. A variable can have at least 3 states:

1. Default initialized: Variable is initialized inside the constructor based on argument or no argument. Value initialized is a special case of this type
2. In-class initialized: The C++11 feature which you have presented in your code
3. Uninitialized: Variable's initialization is not addressed anywhere and it can contain any garbage value. Some compilers may automatically make it 0 or give a warning

Answer 2

The first statement about "Variables defined outside any function body" refers to objects with static linkage, i.e., variables declared in namespaces: These are zero initialized. The members in the struct get initialized wherever this struct lives. If it lives on the stack or is allocated on the heap, the built-in variable won't get initialized without the assignments, e.g., when used like this:

void f() {
    Sales_data data;
}

Even without the initialization in the declaration, they would get zero-initialized if the struct is used like this, though:

Sales_data global; // <--- the "outside any function body" case
void f() {
    Sales_data data0 = {};
    Sales_data data1 = Sales_data();
    Sales_data data2{};
}

However, these all require cooperation by the user of the struct and initializing them explicitly makes sure the values are set.