C.48: Prefer default member initializers, also for non-constant initializers

The restriction "for constant initializers" on guideline C.48: Prefer default member initializers to member initializers in constructors for constant initializers appears unnecessary and undesirable.

For example, suppose a class has a creationTime member, that is initialized by now() in each of its constructors:

    class MyContainer {
        time_point<system_clock> creationTime;  // No default member initializer! Looks BAD to me.
        vector<int> data{};
    public:
        MyContainer() : creationTime{ system_clock::now() } {}
        explicit MyContainer(size_t n) : creationTime{ system_clock::now() }, data(n) {}
        MyContainer(initializer_list<int> values) : creationTime{ system_clock::now() }, data{ values } {}
    };

It appears preferable to use a default member initializer for MyContainer::creationTime, even though now() is not a constant. As follows:

    class MyContainer {
        time_point<system_clock> creationTime{ system_clock::now() };  // Preferable, right?
        vector<int> data{};
    public:
        MyContainer() = default;
        explicit MyContainer(size_t n) : data(n) {}
        MyContainer(initializer_list<int> values) : data{ values } {}
    };

In this example, using a default member initializer also avoids unnecessary code repetition, and it allows defaulting the default constructor.

In general, the guideline appears applicable to both constant and non-constant initializers.

• Proposed fix: pull request #2333

Start with guideline C.48 in the linked C++ Core Guidelines and review proposed fix pull request #2333. Done means the restriction to constant initializers is addressed so the guideline clearly covers the non-constant initializer example as well.