I would like to split a class implementation into three parts, to avoid that users need to deal with the implementation details, e.g., the libaries that I use to implement the functionality:
impl.cpp
#include <api.h>
#include <impl.h>
Class::Class() {
init();
}
Class::init() {
myData = SomeLibrary::Type(42);
}
Class::doSomething() {
myData.doSomething();
}
impl.h
#include <somelibrary.h>
class Class {
public:
Class();
init();
doSomething();
private:
SomeLibary::Type myData;
}
api.h
class Class {
Class();
doSomething();
}
The problem is, that I am not allowed to redefine headers for the class definition. This does not work when I define Class() and doSomething() only in api.h, either.
A possible option is to define api.h and do not use it in the project at all, but install it (and do not install impl.h).
The obvious drawback is, that I need to make sure, that the common methods in api.h and impl.h always have the same signature, otherwise programs using the library will get linker errors, that I cannot predict when compiling the library.
But would this approach work at all, or will I get other problems (e.g. wrong pointers to class members or similar issues), because the obj file does not match the header?
The short answer is "No!"
The reason: any/all 'client' projects that need to use your Class class have to have the full declaration of that class, in order that the compiler can properly determine such things as offsets for member variables.
The use of private members is fine - client programs won't be able to change them - as is your current implementation, where only the briefest outlines of member functions are provided in the header, with all actual definitions in your (private) source file.
A possible way around this is to declare a pointer to a nested class in Class, where this nested class is simply declared in the shared header: class NestedClass and then you can do what you like with that nested class pointer in your implementation. You would generally make the nested class pointer a private member; also, as its definition is not given in the shared header, any attempt by a 'client' project to access that class (other than as a pointer) will be a compiler error.
Here's a possible code breakdown (maybe not error-free, yet, as it's a quick type-up):
// impl.h
struct MyInternal; // An 'opaque' structure - the definition is For Your Eyes Only
class Class {
public:
Class();
init();
doSomething();
private:
MyInternal* hidden; // CLient never needs to access this! Compiler error if attempted.
}
// impl.cpp
#include <api.h>
#include <impl.h>
struct MyInternal {
SomeLibrary::Type myData;
};
Class::Class() {
init();
}
Class::init() {
hidden = new MyInternal; // MUCH BETTER TO USE unique_ptr, or some other STL.
hidden->myData = SomeLibrary::Type(42);
}
Class::doSomething() {
hidden->myData.doSomething();
}
NOTE: As I hinted in a code comment, it would be better code to use std::unique_ptr<MyInternal> hidden. However, this would require you to give explicit definitions in your Class for the destructor, assignment operator and others (move operator? copy constructor?), as these will need access to the full definition of the MyInternal struct.
The private implementation (PIMPL) idiom can help you out here. It will probably result in 2 header and 2 source files instead of 2 and 1. Have a silly example I haven't actually tried to compile:
api.h
#pragma once
#include <memory>
struct foo_impl;
struct foo {
int do_something(int argument);
private:
std::unique_ptr<foo_impl> impl;
}
api.c
#include "api.h"
#include "impl.h"
int foo::do_something(int a) { return impl->do_something(); }
impl.h
#pragma once
#include <iostream>
struct foo_impl {
foo_impl();
~foo_impl();
int do_something(int);
int initialize_b();
private:
int b;
};
impl.c
#include <iostream>
foo_impl::foo_impl() : b(initialize_b()} { }
foo_impl::~foo_impl() = default;
int foo_impl::do_something(int a) { return a+b++; }
int foo_impl::initialize_b() { ... }
foo_impl can have whatever methods it needs, as foo's header (the API) is all the user will see. All the compiler needs to compile foo is the knowledge that there is a pointer as a data member so it can size foo correctly.
I recently got this idea to separate different platform specific implementations (could be Win32/X, opengl/dx/vulkan, etc...) using CRTP (curiously recurring template pattern): I thought of something like this:
IDisplayDevice.h
#pragma once
#include "OSConfig.h"
namespace cbn
{
template <class TDerived> // Win32 type here
struct IDisplayDevice
{
bool run_frame(void) {
return
static_cast<const TDerived*>(this)->run_frame();
}
// a lot of other methods ...
};
}
Win32DisplayDevice.h:
#pragma once
#include "OSConfig.h"
// make sure it only gets compiled on win32/64
#if defined(CBN_OS_WINDOWS)
namespace cbn
{
class CWin32DisplayDevice
: public IDisplayDevice<CWin32DisplayDevice> {
public:
bool run_frame(void) {
call_hInstance();
call_hWnd();
#ifdef CBN_RENDERAPI_DX11
call_dx11_bufferswap();
#endif
return some_state;
}
private:
};
}
#endif
I would then provide an other implementation the same way in XDisplayDevice.h.
Finally, I would make a common interface in DisplayDevice.h:
#include "Win32DisplayDevice.h"
#include "XDisplayDevice.h"
namespace cbn
{
class CDisplayDevice
{
public:
CBN_INLINE
bool run_frame(void) { return device_->run_frame(); }
private:
#if defined(CBN_OS_WINDOWS)
CWin32DisplayDevice device_;
#elif defined(CBN_OS_LINUX)
CXDisplayDevice device_;
#elif // and so on
#else
// does nothing ...
CNillDisplayDevice device_;
#endif
}
}
So I could call it in main.cpp like:
int main()
{
CDisplayDevice my_device;
while(my_device->run_frame())
{
do_some_magic();
}
}
Do you think this would be a good way to deal with platform specific code ?
PS: I avoid victuals and polymorphism because of platform restraints (android, ps4, etc...) where pointer calls matter.
Consider this code:
struct OpenGLTraits // keep this in it's own files (.h and .cpp)
{
bool run_frame() { /* open gl specific stuff here */ }
};
struct VulkanTraits // keep this in it's own files (.h and .cpp)
{
bool run_frame() { /* vulkan specific stuff here */ }
};
template<typename T>
class DisplayDevice
{
using graphic_traits = T;
graphic_traits graphics; // maybe inject this in constructor?
void do_your_operation()
{
if(!graphics.run_frame()) // subsystem-specific call
{ ... }
}
};
This will use subsystem-specific calls, and abstract them away between a common API, without a virtual call involved. You can even inline the run_frame() implementations.
Edit (address comment question):
consider this:
#ifdef FLAG_SPECIFYING_OPEN_GL
using Device = DisplayDevice<OpenGLTraits>;
#elif FLAG_SPECIFYING_VULKAN
using Device = DisplayDevice<VulkanTraits>;
...
#endif
client code:
Device device;
device.do_your_operation();
I don't really see the benefit of CRTP here, you still have platform specific (as opposed to feature specific) ifdefs within the code, and this tends to make things harder to read and maintain. I usually prefer having different implementations in different source files - and in fact, generally having seperate directories for each platform.
such as:
platform/win64
platform/win32
platform/gnu-linux
platform/freebsd
In this way you can largely avoid the ifdef clutter, and you generally know where to find the platform specific things. You also know what you need to write in order to port things to another platform. The build system can then be made to select the correct source rather than the preprocessor.
Given two IDL definitions: (I'm only implementing a client, the server side is fixed.)
// Version 1.2
module Server {
interface IObject {
void Foo1();
void Foo2() raises(EFail);
string Foo3();
// ...
}
};
// Version 2.3
module Server {
interface IObject {
// no longer available: void Foo1();
void Foo2(string x) raises(ENotFound, EFail); // incompatible change
wstring Foo3();
// ...
}
};
(Edit Note: added Foo3 method that cannot be overloaded because the return type changed.)
Is it somehow possible to compile both stub code files in the same C++ CORBA Client App?
Using the defaults of an IDL compiler, the above two IDL definitions will result in stub code that cannot be compiled into the same C++ module, as you'd get multiple definition errors from the linker. The client however needs to be able to talk to both server versions.
What are possible solutions?
(Note: We're using omniORB)
(Adding answer from one Stefan Gustafsson, posted in comp.object.corba 2011-03-08)
If you look at it as a C++ problem instead of a CORBA problem, the
solution is C++ namespaces.
You could try to wrap the different implementations in different C++
namespaces.
Like:
namespace v1 {
#include "v1/foo.h" // From foo.idl version 1
}
namespace v2 {
#include "v2/foo.h" // from foo.idl version 2
}
And to be able to compile the C++ proxy/stub code you need to create C++
main files like:
// foo.cpp
namespace v1 {
#include "v1/foo_proxy.cpp" // filename depend on IDL compiler
}
namespace v2 {
#include "v2/foo_proxy.cpp"
}
This will prevent the C++ linker complaining since the names will be
different. Of course you
could run into problems with C++ compilers not supporting nested
namespaces..
A second solution is to implement the invocation using DII, you could
write a C++ class
class ServerCall {
void foo2_v1() {
// create request
// invoke
}
void foo2_v2(String arg) {
// create_list
// add_value("x",value,ARG_IN)
// create_request
// invoke
}
}
By using DII you can create any invocation you like, and can keep full
control of your client code.
I think this is a good idea, but I haven't been able to try it out yet, so there may lurk some unexpected surprises wrt to things no longer being in the global namespace.
What comes to my mind would be splitting the client code into separate libraries for each version.
Then you can select the correct client depending on the version to be used.
In a recent project we handled this by introducing a service layer with no dependency to the CORBA IDL.
For example:
class ObjectService
{
public:
virtual void Foo1() = 0;
virtual void Foo2() = 0;
virtual void Foo2(const std::string &x) = 0;
};
For each version, create a class derived from ObjectService and implement the operations by
calling the CORBA::Object. Each derived class must be in separate library.
In the client implementation, you only operate on instances of ObjectService.
CORBA::Object_var remoteObject=... // How to get the remote object depends on your project
ObjectService *serviceObject=0;
// create a service object matching the remote object version
// Again, this is project specific
switch (getRemoteObjectVersion(remoteObject))
{
case VERSION_1_2:
serviceObject=new ServiceObjectImpl12(remoteObject);
break;
case VERSION_2_3:
serviceObject=new ServiceObjectImpl23(remoteObject);
break;
default:
// No matching version found, throw exception?
break;
}
// Access remote object through service object
serviceObject->Foo2("42");
Anyone knows if is possible to have partial class definition on C++ ?
Something like:
file1.h:
class Test {
public:
int test1();
};
file2.h:
class Test {
public:
int test2();
};
For me it seems quite useful for definining multi-platform classes that have common functions between them that are platform-independent because inheritance is a cost to pay that is non-useful for multi-platform classes.
I mean you will never have two multi-platform specialization instances at runtime, only at compile time. Inheritance could be useful to fulfill your public interface needs but after that it won't add anything useful at runtime, just costs.
Also you will have to use an ugly #ifdef to use the class because you can't make an instance from an abstract class:
class genericTest {
public:
int genericMethod();
};
Then let's say for win32:
class win32Test: public genericTest {
public:
int win32Method();
};
And maybe:
class macTest: public genericTest {
public:
int macMethod();
};
Let's think that both win32Method() and macMethod() calls genericMethod(), and you will have to use the class like this:
#ifdef _WIN32
genericTest *test = new win32Test();
#elif MAC
genericTest *test = new macTest();
#endif
test->genericMethod();
Now thinking a while the inheritance was only useful for giving them both a genericMethod() that is dependent on the platform-specific one, but you have the cost of calling two constructors because of that. Also you have ugly #ifdef scattered around the code.
That's why I was looking for partial classes. I could at compile-time define the specific platform dependent partial end, of course that on this silly example I still need an ugly #ifdef inside genericMethod() but there is another ways to avoid that.
This is not possible in C++, it will give you an error about redefining already-defined classes. If you'd like to share behavior, consider inheritance.
Try inheritance
Specifically
class AllPlatforms {
public:
int common();
};
and then
class PlatformA : public AllPlatforms {
public:
int specific();
};
You can't partially define classes in C++.
Here's a way to get the "polymorphism, where there's only one subclass" effect you're after without overhead and with a bare minimum of #define or code duplication. It's called simulated dynamic binding:
template <typename T>
class genericTest {
public:
void genericMethod() {
// do some generic things
std::cout << "Could be any platform, I don't know" << std::endl;
// base class can call a method in the child with static_cast
(static_cast<T*>(this))->doClassDependentThing();
}
};
#ifdef _WIN32
typedef Win32Test Test;
#elif MAC
typedef MacTest Test;
#endif
Then off in some other headers you'll have:
class Win32Test : public genericTest<Win32Test> {
public:
void win32Method() {
// windows-specific stuff:
std::cout << "I'm in windows" << std::endl;
// we can call a method in the base class
genericMethod();
// more windows-specific stuff...
}
void doClassDependentThing() {
std::cout << "Yep, definitely in windows" << std::endl;
}
};
and
class MacTest : public genericTest<MacTest> {
public:
void macMethod() {
// mac-specific stuff:
std::cout << "I'm in MacOS" << std::endl;
// we can call a method in the base class
genericMethod();
// more mac-specific stuff...
}
void doClassDependentThing() {
std::cout << "Yep, definitely in MacOS" << std::endl;
}
};
This gives you proper polymorphism at compile time. genericTest can non-virtually call doClassDependentThing in a way that gives it the platform version, (almost like a virtual method), and when win32Method calls genericMethod it of course gets the base class version.
This creates no overhead associated with virtual calls - you get the same performance as if you'd typed out two big classes with no shared code. It may create a non-virtual call overhead at con(de)struction, but if the con(de)structor for genericTest is inlined you should be fine, and that overhead is in any case no worse than having a genericInit method that's called by both platforms.
Client code just creates instances of Test, and can call methods on them which are either in genericTest or in the correct version for the platform. To help with type safety in code which doesn't care about the platform and doesn't want to accidentally make use of platform-specific calls, you could additionally do:
#ifdef _WIN32
typedef genericTest<Win32Test> BaseTest;
#elif MAC
typedef genericTest<MacTest> BaseTest;
#endif
You have to be a bit careful using BaseTest, but not much more so than is always the case with base classes in C++. For instance, don't slice it with an ill-judged pass-by-value. And don't instantiate it directly, because if you do and call a method that ends up attempting a "fake virtual" call, you're in trouble. The latter can be enforced by ensuring that all of genericTest's constructors are protected.
or you could try PIMPL
common header file:
class Test
{
public:
...
void common();
...
private:
class TestImpl;
TestImpl* m_customImpl;
};
Then create the cpp files doing the custom implementations that are platform specific.
#include will work as that is preprocessor stuff.
class Foo
{
#include "FooFile_Private.h"
}
////////
FooFile_Private.h:
private:
void DoSg();
How about this:
class WindowsFuncs { public: int f(); int winf(); };
class MacFuncs { public: int f(); int macf(); }
class Funcs
#ifdef Windows
: public WindowsFuncs
#else
: public MacFuncs
#endif
{
public:
Funcs();
int g();
};
Now Funcs is a class known at compile-time, so no overheads are caused by abstract base classes or whatever.
As written, it is not possible, and in some cases it is actually annoying.
There was an official proposal to the ISO, with in mind embedded software, in particular to avoid the RAM ovehead given by both inheritance and pimpl pattern (both approaches require an additional pointer for each object):
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0309r0.pdf
Unfortunately the proposal was rejected.
As written, it is not possible.
You may want to look into namespaces. You can add a function to a namespace in another file. The problem with a class is that each .cpp needs to see the full layout of the class.
Nope.
But, you may want to look up a technique called "Policy Classes". Basically, you make micro-classes (that aren't useful on their own) then glue them together at some later point.
Either use inheritance, as Jamie said, or #ifdef to make different parts compile on different platforms.
For me it seems quite useful for definining multi-platform classes that have common functions between them that are platform-independent.
Except developers have been doing this for decades without this 'feature'.
I believe partial was created because Microsoft has had, for decades also, a bad habit of generating code and handing it off to developers to develop and maintain.
Generated code is often a maintenance nightmare. What habits to that entire MFC generated framework when you need to bump your MFC version? Or how do you port all that code in *.designer.cs files when you upgrade Visual Studio?
Most other platforms rely more heavily on generating configuration files instead that the user/developer can modify. Those, having a more limited vocabulary and not prone to be mixed with unrelated code. The configuration files can even be inserted in the binary as a resource file if deemed necessary.
I have never seen 'partial' used in a place where inheritance or a configuration resource file wouldn't have done a better job.
Since headers are just textually inserted, one of them could omit the "class Test {" and "}" and be #included in the middle of the other.
I've actually seen this in production code, albeit Delphi not C++. It particularly annoyed me because it broke the IDE's code navigation features.
Dirty but practical way is using #include preprocessor:
Test.h:
#ifndef TEST_H
#define TEST_H
class Test
{
public:
Test(void);
virtual ~Test(void);
#include "Test_Partial_Win32.h"
#include "Test_Partial_OSX.h"
};
#endif // !TEST_H
Test_Partial_OSX.h:
// This file should be included in Test.h only.
#ifdef MAC
public:
int macMethod();
#endif // MAC
Test_Partial_WIN32.h:
// This file should be included in Test.h only.
#ifdef _WIN32
public:
int win32Method();
#endif // _WIN32
Test.cpp:
// Implement common member function of class Test in this file.
#include "stdafx.h"
#include "Test.h"
Test::Test(void)
{
}
Test::~Test(void)
{
}
Test_Partial_OSX.cpp:
// Implement OSX platform specific function of class Test in this file.
#include "stdafx.h"
#include "Test.h"
#ifdef MAC
int Test::macMethod()
{
return 0;
}
#endif // MAC
Test_Partial_WIN32.cpp:
// Implement WIN32 platform specific function of class Test in this file.
#include "stdafx.h"
#include "Test.h"
#ifdef _WIN32
int Test::win32Method()
{
return 0;
}
#endif // _WIN32
Suppose that I have:
MyClass_Part1.hpp, MyClass_Part2.hpp and MyClass_Part3.hpp
Theoretically someone can develop a GUI tool that reads all these hpp files above and creates the following hpp file:
MyClass.hpp
class MyClass
{
#include <MyClass_Part1.hpp>
#include <MyClass_Part2.hpp>
#include <MyClass_Part3.hpp>
};
The user can theoretically tell the GUI tool where is each input hpp file and where to create the output hpp file.
Of course that the developer can theoretically program the GUI tool to work with any varying number of hpp files (not necessarily 3 only) whose prefix can be any arbitrary string (not necessarily "MyClass" only).
Just don't forget to #include <MyClass.hpp> to use the class "MyClass" in your projects.
Declaring a class body twice will likely generate a type redefinition error. If you're looking for a work around. I'd suggest #ifdef'ing, or using an Abstract Base Class to hide platform specific details.
You can get something like partial classes using template specialization and partial specialization. Before you invest too much time, check your compiler's support for these. Older compilers like MSC++ 6.0 didn't support partial specialization.
This is not possible in C++, it will give you an error about redefining already-defined
classes. If you'd like to share behavior, consider inheritance.
I do agree on this. Partial classes is strange construct that makes it very difficult to maintain afterwards. It is difficult to locate on which partial class each member is declared and redefinition or even reimplementation of features are hard to avoid.
Do you want to extend the std::vector, you have to inherit from it. This is because of several reasons. First of all you change the responsibility of the class and (properly?) its class invariants. Secondly, from a security point of view this should be avoided.
Consider a class that handles user authentication...
partial class UserAuthentication {
private string user;
private string password;
public bool signon(string usr, string pwd);
}
partial class UserAuthentication {
private string getPassword() { return password; }
}
A lot of other reasons could be mentioned...
Let platform independent and platform dependent classes/functions be each-others friend classes/functions. :)
And their separate name identifiers permit finer control over instantiation, so coupling is looser. Partial breaks encapsulation foundation of OO far too absolutely, whereas the requisite friend declarations barely relax it just enough to facilitate multi-paradigm Separation of Concerns like Platform Specific aspects from Domain-Specific platform independent ones.
I've been doing something similar in my rendering engine. I have a templated IResource interface class from which a variety of resources inherit (stripped down for brevity):
template <typename TResource, typename TParams, typename TKey>
class IResource
{
public:
virtual TKey GetKey() const = 0;
protected:
static shared_ptr<TResource> Create(const TParams& params)
{
return ResourceManager::GetInstance().Load(params);
}
virtual Status Initialize(const TParams& params, const TKey key, shared_ptr<Viewer> pViewer) = 0;
};
The Create static function calls back to a templated ResourceManager class that is responsible for loading, unloading, and storing instances of the type of resource it manages with unique keys, ensuring duplicate calls are simply retrieved from the store, rather than reloaded as separate resources.
template <typename TResource, typename TParams, typename TKey>
class TResourceManager
{
sptr<TResource> Load(const TParams& params) { ... }
};
Concrete resource classes inherit from IResource utilizing the CRTP. ResourceManagers specialized to each resource type are declared as friends to those classes, so that the ResourceManager's Load function can call the concrete resource's Initialize function. One such resource is a texture class, which further uses a pImpl idiom to hide its privates:
class Texture2D : public IResource<Texture2D , Params::Texture2D , Key::Texture2D >
{
typedef TResourceManager<Texture2D , Params::Texture2D , Key::Texture2D > ResourceManager;
friend class ResourceManager;
public:
virtual Key::Texture2D GetKey() const override final;
void GetWidth() const;
private:
virtual Status Initialize(const Params::Texture2D & params, const Key::Texture2D key, shared_ptr<Texture2D > pTexture) override final;
struct Impl;
unique_ptr<Impl> m;
};
Much of the implementation of our texture class is platform-independent (such as the GetWidth function if it just returns an int stored in the Impl). However, depending on what graphics API we're targeting (e.g. Direct3D11 vs. OpenGL 4.3), some of the implementation details may differ. One solution could be to inherit from IResource an intermediary Texture2D class that defines the extended public interface for all textures, and then inherit a D3DTexture2D and OGLTexture2D class from that. The first problem with this solution is that it requires users of your API to be constantly mindful of which graphics API they're targeting (they could call Create on both child classes). This could be resolved by restricting the Create to the intermediary Texture2D class, which uses maybe a #ifdef switch to create either a D3D or an OGL child object. But then there is still the second problem with this solution, which is that the platform-independent code would be duplicated across both children, causing extra maintenance efforts. You could attempt to solve this problem by moving the platform-independent code into the intermediary class, but what happens if some of the member data is used by both platform-specific and platform-independent code? The D3D/OGL children won't be able to access those data members in the intermediary's Impl, so you'd have to move them out of the Impl and into the header, along with any dependencies they carry, exposing anyone who includes your header to all that crap they don't need to know about.
API's should be easy to use right and hard to use wrong. Part of being easy to use right is restricting the user's exposure to only the parts of the API they should be using. This solution opens it up to be easily used wrong and adds maintenance overhead. Users should only have to care about the graphics API they're targeting in one spot, not everywhere they use your API, and they shouldn't be exposed to your internal dependencies. This situation screams for partial classes, but they are not available in C++. So instead, you might simply define the Impl structure in separate header files, one for D3D, and one for OGL, and put an #ifdef switch at the top of the Texture2D.cpp file, and define the rest of the public interface universally. This way, the public interface has access to the private data it needs, the only duplicate code is data member declarations (construction can still be done in the Texture2D constructor that creates the Impl), your private dependencies stay private, and users don't have to care about anything except using the limited set of calls in the exposed API surface:
// D3DTexture2DImpl.h
#include "Texture2D.h"
struct Texture2D::Impl
{
/* insert D3D-specific stuff here */
};
// OGLTexture2DImpl.h
#include "Texture2D.h"
struct Texture2D::Impl
{
/* insert OGL-specific stuff here */
};
// Texture2D.cpp
#include "Texture2D.h"
#ifdef USING_D3D
#include "D3DTexture2DImpl.h"
#else
#include "OGLTexture2DImpl.h"
#endif
Key::Texture2D Texture2D::GetKey() const
{
return m->key;
}
// etc...