Using C++ class in Objective-C - Why use PImpl idiom?

Question

Suppose I have a simple C++ class:

// MyCPPClass.hpp
class MyCPPClass {
    std::uint8_t m_num1 = 0;
    std::uint8_t m_num2 = 0;

    std::string GetText() const;
}

and I want to use this in an Objective-C class:

// MyObjCClass.h
@interface MyObjCClass {


    @public
        MyCPPClass _cppObj;
}

What do I really gain from using PImpl idiom over just embedding it directly into the Obj-C class (as above), aside from having to write more code for the PImpl part ?

newacct · Accepted Answer · 2017-02-08T02:56:40.710

The downside is this C++ type MyCPPClass is now in the header file MyObjCClass.h, which other Objective-C classes that use MyObjCClass will have to import. Importing a header is like putting it into the code, so these other Objective-C classes now have C++ in their code, and so will now have to be changed to Objective-C++ files or they won't compile. This is often not desirable, as these other files shouldn't need C++. Although this problem can be solved by declaring your instance variable not in the main class declaration in the header, but in the "extension" (declared like a category but with no name in the parentheses) or in the @implementation, which are in the implementation file, so other classes don't have to import it.

Another problem with C++ classes as instance variables of Objective-C classes is that your C++ object can only be constructed using the default constructor (i.e. no constructor arguments), as there is no way to provide an initializer for an instance variable of an Objective-C class. So if you want to construct it with constructor arguments, you must use pimpl.

Thanks for detailed answer. Would you recommend always using PImpl, even when you only need default constructor (so PImpl becomes second nature), or would you use the category/extension solution to avoid the extra PImpl code in this case ? — DarkMatter, Feb 08 '17 at 07:48

Using C++ class in Objective-C - Why use PImpl idiom?

1 Answers1