Pass generic function and its parameters to meta-function - c++

Question

First of all, I feel like this must have been asked already somewhere, but all my searches proved fruitless. If this is indeed a duplicate of something somewhere, I apologize in advance.
I am trying to mass-benchmark a bunch of functions from OpenCV and to do so I wanted to write a small meta-function that takes the function to run, its parameters (which vary according to the function passed) and essentially sets up the timing and runs the function in a loop.
Since I'm planning to pass lambdas to the meta-function as well later on (to benchmark composition of functions), I thought of using std::function.
This is the code I came up with after reading the parameter pack description:
template<typename ...Ts>
void run_test(std::string test_name, int num_repeats, std::function<void(Ts...)> f, Ts... fargs)
{
std::cout << std::endl << "Starting " << test_name << std::endl;
start_timing(test_name);
for(int i=0; i<num_repeats; i++)
{
f(fargs...);
}
end_timing(num_repeats);
}
As you see, functionality is down to a minimum. start_timing and end_timing are simple helper functions beyond the scope of this question.
In my main, I call:
// im_in defined and loaded elsewhere
cv::Mat out(im_in.size(), CV_8U);
run_test(
"erode 3x3",
100,
cv::erode,
im_in, out, cv::Mat::ones(3,3,CV_8U)
);
Now, if I try to compile this, I get:
error: no matching function for call to 'run_test(const char [10], const int&, void (&)(cv::InputArray, cv::OutputArray, cv::InputArray, cv::Point, int, int, const Scalar&), cv::Mat&, cv::Mat&, cv::MatExpr)'
);
^
note: candidate: template<class ... Ts> void run_test(std::__cxx11::string, int, std::function<void(Ts ...)>, Ts ...)
void run_test(std::string test_name, int num_repeats, std::function<void(Ts...)> f, Ts... fargs)
^~~~~~~~
note: template argument deduction/substitution failed:
note: mismatched types 'std::function<void(Ts ...)>' and 'void (*)(cv::InputArray, cv::OutputArray, cv::InputArray, cv::Point, int, int, const Scalar&) {aka void (*)(const cv::_InputArray&, const cv::_OutputArray&, const cv::_InputArray&, cv::Point_<int>, int, int, const cv::Scalar_<double>&)}'
);
So... what am I doing wrong? Why is it having a type mismatch instead of deducing the types in Ts... from the parameters list as I expected?
Update:
After writing the last question above, I realized it might have problems deducing Ts... for the std::function because the parameter comes before the actual parameter list to expand into Ts.... So, I changed my code as follows (essentialy, I moved the f parameter at the end):
void run_test(std::string test_name, int num_repeats, Ts... fargs, std::function<void(Ts...)> f)
{ ... }
Of course, I also adapted the function call accordingly:
cv::Mat out(im_in.size(), CV_8U);
run_test(
"erode 3x3",
NUM_REPEATS,
im_in, out, cv::Mat::ones(3,3,CV_8U),
cv::erode, // <<<<<<<<<<<<<<<<<<<<<<<
);
Now, if I compile, I get a different error:
error: expected primary-expression before ')' token );
The change in error makes me think the order of the parameters was indeed important. Is, however, this the correct one? If so, what am I doing wrong?
Update2:
Writing the first update, it struck me that probably my assumption that f can take a function and convert it to a std::function was probably wrong. After a quick research, it seems it is.
I tried replacing std::function<void(Ts...)> f with both auto f and auto& f (& compiling with C++14 enabled), but the expected primary-expression error is still there.
For all I could research, I couldn't find a way that allows me to just pass the function relying on the compiler to figure out the types.
I'm thinking of adapting C++17's std::apply function implementation shown here adding my timing loop around the invoke call, but I don't understand that code, so the chance of getting something wrong is high.

Answer 1

You might get rid of std::function (which add overhead BTW), and use generic for f too:
template<typename F, typename ...Ts>
void run_test(std::string test_name, int num_repeats, F f, const Ts&... fargs)
{
std::cout << std::endl << "Starting " << test_name << std::endl;
start_timing(test_name);
for(int i=0; i<num_repeats; i++)
{
f(fargs...);
}
end_timing(num_repeats);
}
I realized it might have problems deducing Ts... for the std::function
As cv::erode is not a std::function, Ts... cannot be deduced from it, but would be from extra parameters.
Your problem is that cv::erode has extra (defaulted) parameters.
so you cannot create the std::function<Ts...> with Ts... deduced from parameters.
To bypass that issue, you might use lambda instead:
run_test(
"erode 3x3",
100,
[&](){ cv::erode(im_in, out, cv::Mat::ones(3,3,CV_8U)); }
);

Answer 2

the error expected primary-expression before ')' is because you left a , after your last parameter
parameter pack must always be the last parameters of a functions
you should probably try this
template<typename Callable, typename ...Ts>
void run_test(std::string test_name, int num_repeats, Callable func, Ts&& ... fargs)
{
std::cout << std::endl << "Starting " << test_name << std::endl;
start_timing(test_name);
for(int i=0; i<num_repeats; i++)
{
func(std::forward<Ts>(fargs)...);
}
end_timing(num_repeats);
}