How to write an inheritable template class with operators

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I am hoping that it is possible to write a template class that will be inherited for several type-specific sub-classes. I want the inherited methods and operators to return the type of the sub-class rather than the parent template type. This is in hopes of saving lots of development and maintenance effort if I only have to modify one base class.

Here is an example of what I have already:

template<typename T> struct TMonoPixel
{
    T value;

    TMonoPixel(T v) { value = v; }

    // the template has some pure virtual functions here...

    TMonoPixel operator+ (const TMonoPixel& other)
    { return TMonoPixel(value + other.value); }
}

struct Mono8Pixel : TMonoPixel<uint8_t>
{
    using TMonoPixel::TMonoPixel;    // I want to inherit the constructor
    // each pixel type implements the virtual functions in the template
}

As you can see the Mono8Pixel struct inherits the + operator which accepts TMonoPixel, but using this operator returns TMonoPixel<uint8_t> rather than Mono8Pixel because it is defined in the base class.

I am planning to use these structs for iterating over pixels in an image:

Image* img; // img has an unsigned char* pointer to its pixel data
for (int row=0; row<img->height; row++) {
    for (int col=0; col<img->width; col++) {
        int i = (row*img->width + col);
        Mono8Pixel* pixel = reinterpret_cast<Mono8Pixel*>(img->dataPtr + sizeof(unsigned char)*i);
        // modify the pixel ...
    }
}

Is there any way to change just the template class to ensure that Mono8Pixel(2) + Mono8Pixel(2) is returning a Mono8Pixel?

Note that whatever the solution is, these structs must maintain standard layout because of how I wish to use them.

templates operator-overloading inheritance c++
1 Answers

What you want can be done using the curiously recurring template pattern (CRTP). The basic idea is this:

template<class Pixel> struct TMonoPixel {
    ...

    // not virtual
    std::string GetSomeProperty() const {
        return static_cast<const Pixel&>(*this).GetSomeProperty();
    }

    Pixel operator+(const TMonoPixel& other) const {
        return Pixel(value + other.value);
    }
};

struct Mono8Pixel : TMonoPixel<Mono8Pixel> {
    using TMonoPixel::TMonoPixel;

    std::string GetSomeProperty() const {
        return "My name is Mono8Pixel";
    }
};

Thanks to implicit derived-to-base conversion now you can use it like this:

template<class T>
void foo(const TMonoPixel<T>& number) {
    std::cout << number.GetSomeProperty();    
}

Mono8Pixel i;
foo(i);

Note that inside TMonoPixel, Pixel is an incomplete type, so you have some limitations on how it can be used. For example, you can't do this:

template<class Pixel> struct TMonoPixel {
    Pixel::Type operator+(const TMonoPixel& other);
};

struct Mono8Pixel : TMonoPixel<Mono8Pixel> {
    using Type = std::uint8_t;
};

Type traits is a useful technique to overcome such limitations:

struct Mono8Pixel;

template<class Pixel> struct ValueType;

template<> struct ValueType<Mono8Pixel> {
    using Type = std::uint8_t;
};

template<class Pixel> struct TMonoPixel {
    using Type = typename ValueType<Pixel>::Type;
    Type value;

    TMonoPixel(Type value) : value(value)
    {}

    Pixel operator+(const TMonoPixel& other) const {
        return Pixel(value + other.value);
    }
};

struct Mono8Pixel : TMonoPixel<Mono8Pixel> {
    using TMonoPixel::TMonoPixel;
};

The type of Mono8Pixel(2) + Mono8Pixel(2) is Mono8Pixel.

So I guess I'm asking whether these CRTP-based structs have standard layout after all of these changes to the type of value.

They do:

static_assert(std::is_standard_layout_v<Mono8Pixel>);

Complete example: https://godbolt.org/z/8z0CKX

user_16251870
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