Pimpl idiom without using dynamic memory allocation

Question

we want to use pimpl idiom for certain parts of our project. These parts of the project also happen to be parts where dynamic memory allocation is forbidden and this decisio

Answer 1

pimpl bases on pointers and you can set them to any place where your objects are allocated. This can also be a static table of objects declared in the cpp file. The main point of pimpl is to keep the interfaces stable and hide the implementation (and its used types).

Answer 2

The point of using pimpl is to hide the implementation of your object. This includes the size of the true implementation object. However this also makes it awkward to avoid dynamic allocation - in order to reserve sufficient stack space for the object, you need to know how big the object is.

The typical solution is indeed to use dynamic allocation, and pass the responsibility for allocating sufficient space to the (hidden) implementation. However, this isn't possible in your case, so we'll need another option.

One such option is using alloca(). This little-known function allocates memory on the stack; the memory will be automatically freed when the function exits its scope. This is not portable C++, however many C++ implementations support it (or a variation on this idea).

Note that you must allocate your pimpl'd objects using a macro; alloca() must be invoked to obtain the necessary memory directly from the owning function. Example:

// Foo.h
class Foo {
    void *pImpl;
public:
    void bar();
    static const size_t implsz_;
    Foo(void *);
    ~Foo();
};

#define DECLARE_FOO(name) \
    Foo name(alloca(Foo::implsz_));

// Foo.cpp
class FooImpl {
    void bar() {
        std::cout << "Bar!\n";
    }
};

Foo::Foo(void *pImpl) {
    this->pImpl = pImpl;
    new(this->pImpl) FooImpl;
}

Foo::~Foo() {
    ((FooImpl*)pImpl)->~FooImpl();
}

void Foo::Bar() {
    ((FooImpl*)pImpl)->Bar();
}

// Baz.cpp
void callFoo() {
    DECLARE_FOO(x);
    x.bar();
}

This, as you can see, makes the syntax rather awkward, but it does accomplish a pimpl analogue.

If you can hardcode the size of the object in the header, there's also the option of using a char array:

class Foo {
private:
    enum { IMPL_SIZE = 123; };
    union {
        char implbuf[IMPL_SIZE];
        double aligndummy; // make this the type with strictest alignment on your platform
    } impl;
// ...
}

This is less pure than the above approach, as you must change the headers whenever the implementation size changes. However, it allows you to use normal syntax for initialization.

You could also implement a shadow stack - that is, a secondary stack separate from the normal C++ stack, specifically to hold pImpl'd objects. This requires very careful management, but, properly wrapped, it should work. This sort of is in the grey zone between dynamic and static allocation.

// One instance per thread; TLS is left as an exercise for the reader
class ShadowStack {
    char stack[4096];
    ssize_t ptr;
public:
    ShadowStack() {
        ptr = sizeof(stack);
    }

    ~ShadowStack() {
        assert(ptr == sizeof(stack));
    }

    void *alloc(size_t sz) {
        if (sz % 8) // replace 8 with max alignment for your platform
            sz += 8 - (sz % 8);
        if (ptr < sz) return NULL;
        ptr -= sz;
        return &stack[ptr];
    }

    void free(void *p, size_t sz) {
        assert(p == stack[ptr]);
        ptr += sz;
        assert(ptr < sizeof(stack));
    }
};
ShadowStack theStack;

Foo::Foo(ShadowStack *ss = NULL) {
    this->ss = ss;
    if (ss)
        pImpl = ss->alloc(sizeof(FooImpl));
    else
        pImpl = new FooImpl();
}

Foo::~Foo() {
    if (ss)
        ss->free(pImpl, sizeof(FooImpl));
    else
        delete ss;
}

void callFoo() {
    Foo x(&theStack);
    x.Foo();
}

With this approach it is critical to ensure that you do NOT use the shadow stack for objects where the wrapper object is on the heap; this would violate the assumption that objects are always destroyed in reverse order of creation.

Answer 3

If you can use boost, consider boost::optional<>. This avoids the cost of dynamic allocation, but at the same time, your object will not be constructed until you deem necessary.

Answer 4

One technique I've used is a non-owning pImpl wrapper. This is a very niche option and isn't as safe as traditional pimpl, but it can help if performance is a concern. It may require some re-architecture to more functional like apis.

You can create a non-owning pimpl class, as long as you can (somewhat) guarantee the stack pimpl object will outlive the wrapper.

For ex.

/* header */
struct MyClassPimpl;
struct MyClass {
    MyClass(MyClassPimpl& stack_object); // Initialize wrapper with stack object.

private:
    MyClassPimpl* mImpl; // You could use a ref too.
};


/* in your implementation code somewhere */

void func(const std::function<void()>& callback) {
    MyClassPimpl p; // Initialize pimpl on stack.

    MyClass obj(p); // Create wrapper.

    callback(obj); // Call user code with MyClass obj.
}

The danger here, like most wrappers, is the user stores the wrapper in a scope that will outlive the stack allocation. Use at your own risk.

Answer 5

One way would be to have a char[] array in your class. Make it large enough for your Impl to fit, and in your constructor, instantiate your Impl in place in your array, with a placement new: new (&array[0]) Impl(...).

You should also ensure you don't have any alignment problems, probably by having your char[] array a member of an union. This:

union { char array[xxx]; int i; double d; char *p; };

for instance, will make sure the alignment of array[0] will be suitable for an int, double or a pointer.

Answer 6

See The Fast Pimpl Idiom and The Joy of Pimpls about using a fixed allocator along with the pimpl idiom.