Returning inline defined structure with uninitialised member. C++

Question

This questions received too little attention in forums.

It has been asked before but no one captured this little detail. As a result I am not sure if I am doing the right thing:

Simple example:

struct TEST {
    bool a;
    int b;
};

TEST func() {
    return { false };
}

Is this correct? Note that int value is not initialized. While it would not be a problem to just set it to 0, more problems arise:

struct _FILE_MUTEX {
    bool locked;
    HANDLE handle;
};

And the last member of the structure could become more and more complex from vector,arrays to function pointers.

Can I leave some of the members empty when returning an inline-initialised structure?

Answer 1

This is called aggregate initialization.

https://en.cppreference.com/w/cpp/language/aggregate_initialization

If the number of initializer clauses is less than the number of members and bases (since C++17) or initializer list is completely empty, the remaining members and bases (since C++17) are initialized by their default member initializers, if provided in the class definition, and otherwise (since C++14) by empty lists, in accordance with the usual list-initialization rules (which performs value-initialization for non-class types and non-aggregate classes with default constructors, and aggregate initialization for aggregates). If a member of a reference type is one of these remaining members, the program is ill-formed.

Thus, in aggregate initialization you allowed to provide less clauses than members in structure. Every remaining member (e.g. X), that doesn't have default initializer, would be initialized as X{}.