What is an opaque pointer?
Opaque as the name suggests is something we can’t see through. e.g. wood is opaque. Opaque pointer is a pointer which points to a data structure whose contents are not exposed at the time of its definition.
Following pointer is opaque. One can’t know the data contained in STest structure by looking at the definition.
struct STest* pSTest;
It is safe to assign NULL to an opaque pointer.
pSTest = NULL;
Why Opaque pointer?
There are places where we just want to hint the compiler that “Hey! This is some data structure which will be used by our clients. Don’t worry, clients will provide its implementation while preparing compilation unit”. Such type of design is robust when we deal with shared code. Please see below example:
Let’s say we are working on an app to deal with images. Since we are living in a world where everything is moving to cloud and devices are very affordable to buy, we want to develop apps for windows, android and apple platforms. So, it would be nice to have a good design which is robust, scalable and flexible as per our requirements. We can have shared code which would be used by all platforms and then different end-point can have platform specific code.
To deal with images, we have a CImage class exposing APIs to deal with various image operations (scale, rotate, move, save etc).
Since all the platforms will be providing same operations, we would define this class in a header file. But the way an image is handled might differ across platforms. Like Apple can have different mechanism to access pixels of an image than Windows does. This means that APIs might demand different set of info to perform operations. So to work on shared code, this is what we would like to do:
Image.h : A header file to store class declaration.
Image.cpp : Code that will be shared across different end-points
Image_windows.cpp : Code specific to Windows will reside here
Image_apple.cpp : Code specific to Apple will reside here
As it can be seen from the above example, while defining blueprint of the CImage class we are only mentioning that there is a SImageInfo data structure.
The content of SImageInfo is unknown. Now it is the responsibility of clients(windows, apple, android) to define that data structure and use it as per their requirement. If in future we want to develop app for a new end-point ‘X’, the design is already there. We only need to define SImageInfo for end-point ‘X’ and use it accordingly.
Please note that the above explained example is one way of doing this. Design is all about discussion and requirement. A good design is decided taking many factors into account. We can also have platform specific classes like CImageWindows, CImageApple and put all platform specific code there.
Questions? Keep them coming. We would love to answer.