Howto: c++ Function Pointer with default values

Question

I have:

typedef void (*RespExtractor) (const cv::Mat & image, cv::Mat & resp);

virtual void predict_image(const cv::Mat & src,
            cv::Mat & img_detect,cv::Size patch_size,
            RespExtractor );

void create_hough_features(const cv::Mat & image, cv::Mat & resp, FeatureParams & params =   FeatureParams() );

How would i define the RespExtractor to accept a function with default parameters, such i can call:

predict_image(im_in,im_out,create_hough_features);

I tried following, with no succes:

typedef void (*RespExtractor) (const cv::Mat & image, cv::Mat & resp,FeatureParams params, FeatureParams()); 

Solution 1

Function pointers themselves can't have default values. You'll either have to wrap the call via the function pointer in a function that does have default parameters (this could even be a small class that wraps the function pointer and has an operator() with default paremeters), or have different function pointers for the different overloads of your functions.

Solution 2

Default parameters aren't part of the function signature, so you can't do this directly.

However, you could define a wrapper function for create_hough_features, or just a second overload that only takes two arguments:

void create_hough_features(const cv::Mat & image, cv::Mat & resp, FeatureParams & params) {
    // blah
}

void create_hough_features(const cv::Mat & image, cv::Mat & resp) {
    create_hough_features(image, resp, DefaultParams());
}

Comments

[Oliver Charlesworth]

Default parameters aren't part of the function signature...

[Poul K. Sørensen]

Thanks. I did typedef void (*RespExtractor) (const cv::Mat & image, cv::Mat & resp,FeatureParams &); and predict_image(im_in,im_out,create_hough_features,FeaturePara‌​ms & par=DefaultParams() );

[Poul K. Sørensen]

Vent with the suggestion of @pmjordan

[Matthieu M.]

@s093294: it is, but I would advise against it. I would advise using a regular function if it needs be virtual... the question of course would be, why should it be virtual, since it already dispatches to a function pointer ?