Best place to initialize default values in a pimpl class?

Question

I make pretty extensive use of PImpl and something that I've found myself waffling on is where exactly to to initialize members of the Pimpl struct. Options are to create a constructor for the Private struct and initialize them there, or to initialize them in the main class's constructor.

myclass.hpp:

class MyClass {
public:
    MyClass();
    ~MyClass();
private:
    struct Private; unique_ptr<Private> p;
};

myclass.cpp:

#include "myclass.hpp"
#include <string>

struct MyClass::Private {
    int some_var;
    std::string a_string;

    // Option A
    Private() :
        some_var {42},
        a_string {"foo"}
    {}
};

MyClass::MyClass() : p(new MyClass::Private) {
    // Option B
    p->some_var = 42;
    p->a_string = "foo";
}

At present I don't really see a difference between the two other than if I were, for some reason, to want to create new Private objects or copy them around or something, then Option A might be preferable. It also is able to initialize the variables in an initialization list, for what that's worth. But, I find that Option B tends to be more readable and perhaps more maintainable as well. Is there something here I'm not seeing which might tilt the scales one way or the other?

Well, using the initializer list is slightly more efficient, but it won't make much difference. Just do whatever you find more readable. — Charles Salvia, Apr 02 '12 at 20:00
If the implementation class is an aggregate, you can say something like `p(new MyClass::Impl{'a', 1, true, { 1,2,3} })`. — Kerrek SB, Apr 02 '12 at 20:03

bitmask · Accepted Answer · 2012-04-02T20:19:19.600

By all means, follow the RAII approach and initialise members in your Private type. If you keep stuff local (and more importantly, at logical places), maintenance will thank you. More importantly, you will be able to have const members, if you use Option A.

If you have to pass values from your MyClass ctor, then do create a proper constructor for Private:

struct MyClass::Private {
    int const some_var; // const members work now
    std::string a_string;

    // Option C
    Private(int const some_var, std::string const& a_string) :
        some_var {some_var},
        a_string {a_string}
    {}
};

MyClass::MyClass() : p(new MyClass::Private(42,"foo")) {
}

Otherwise your Private members will be default constructed, only to be overwritten later (which is irrelevant for ints, but what about more complicated types?).

Accepted this answer because of the points about following RAII approach and having const members are exactly the kinds of things that I was looking for when I asked if there was something I wasn't seeing. Cheers. — Mike E, Apr 03 '12 at 15:40

score 1 · Answer 2 · answered Apr 02 '12 at 20:22

As already noted by @Charles Salvia above, assignment in any of the two constructors incurs some overhead, as the variables are default-constructed before the value is assigned. The amount of this overhead of course strongly depends on the type of your variables.

If you can accept this, I think going for the most readable version is the best. So, if you find assignment in MyClass's constructor the most readable, go for it.

However, take into account that there is no way around an initializer list (for the Private c'tor), namely when your member variables do not have a default constructor or when you use references or constants.

You might want to decide this from case to case, but "always" using initializer lists will keep things consistent and future-proof for newly added data members.

Best place to initialize default values in a pimpl class?

2 Answers2