Initialize const member variables

Question

I have C++ code that boils down to something like the following:

class Foo{
    bool bar;
    bool baz;
    Foo(const void*);
};
Foo::Foo(const void* ptr){
          


        

Answer 1

One option is a C++11 delegating constructor, as discussed in other answers. The C++03-compatible method is to use a subobject:

class Foo{
    struct subobject {
        const bool bar;
        const bool baz;
        subobject(const struct my_struct* s)
            : bar(calculate_bar(s))
            , baz(calculate_baz(s))
        {}
    } subobject;
    Foo(const void*);
};
Foo::Foo(const void* ptr)
    : subobject(complex_method(ptr))
{}

You can make bar and baz const, or make the subobject const, or both.

If you make only subobject const, then you can calculate complex_method and assign to bar and baz within the constructor of subobject:

class Foo{
    const struct subobject {
        bool bar;
        bool baz;
        subobject(const void*);
    } subobject;
    Foo(const void*);
};
Foo::Foo(const void* ptr)
    : subobject(ptr)
{}
Foo::subobject::subobject(const void* ptr){
    const struct my_struct* s = complex_method(ptr);
    bar = calculate_bar(s);
    baz = calculate_baz(s);
}

---

The reason that you can't mutate const members within a constructor body is that a constructor body is treated just like any other member function body, for consistency. Note that you can move code from a constructor into a member function for refactoring, and the factored-out member function doesn't need any special treatment.