python-like kwargs in C++

When dealing with functions that have lots of optional parameters, or at least for which resonable defaults are readily available, it’s often a bit of a frustration in C++. Generally defaults are specified during a function declaration as in:

double BinarySearch(std::function<double(double> fn,
                    int max_depth = 16, double epsilon = 1e-9,
                    double lower_bound = 0, double upper_bound = 100);

And then if we call BinarySearch with only one parameter then the call will use the default values for the rest. But what if I want to specify custom bound, and use the defaults for the other parameters? Admittedly, this is a somewhat contrived example since bounds are less likely to be optional then the others, and we could reorder them better going from most-likely-to-be-specified to least, but it’s easy to see how something more flexible would be desirable.

Consider then the following two code snippets. Which is more readable?

First snippet:

double solution = BinarySearch(fn, 0, 100);

Second snippet:

double solution = BinarySearch(fn, lower_bound = 0, upper_bound = 100);

I really like the way that optional arguments work in python with kwargs. I’d love to have that same kind of functionality in C++. kwargs.h implements one mechanism of achieving this.

How does it work

kwargs takes advantage of variadic templates in C++ to build up a single data structure which contains all of the optional parameters (I’ll call this a “parameter pack”). Each optional parameter of type T is stored in a structure of type Arg<tag,T> where tag is a unique numeric identifier associated with a particular optional argument key.

The parameter pack data structure derives from Arg<tag,T> for each (tag,T) pair that shows up in the list of optional arguments.

Overloading of the assignment (=) operator gives us an opportunity for building the (tag,T) pairs within the parameter list of the function call.

Get it

See the source and doxygen on github.

Example

I’ll conclude with an example usage:

#include <iostream>
#include "kwargs.h"
 
// these are tags which will uniquely identify the arguments in a parameter
// pack
enum Keys {
  c_tag,
  d_tag
};
 
// global symbols used as keys in list of kwargs
kw::Key<c_tag> c_key;
kw::Key<d_tag> d_key;
 
// a function taking kwargs parameter pack
template <typename... Args>
void foo(int a, int b, Args... kwargs) {
  // first, we construct the parameter pack from the parameter pack
  kw::ParamPack<Args...> params(kwargs...);
 
  std::cout << "foo:\n--------"
            << "\na: " << a
            << "\nb: " << b
  // We can attempt to retrieve a key while providing a default fallback value.
  // If c_key is in kwargs then this will return the value associated with
  // that key, and will have the correct type. Note that the type of the default
  // parameter in this case is const char*.
            << "\nc: " << kw::Get(params,c_key,"null");
  // We can also do stuff conditionally based on whether or not arg exists in
  // the param pack. We still need to provide a default value, since we need to
  // know the return type of the Get function when the key is not in kwargs.
  // In this case, the default value wont ever be used at runtime.
  if( kw::ContainsTag<c_tag,Args...>::result ) {
    std::cout << "\nd: " << kw::Get(params,d_key,0);
  }
 
  std::cout << "\n\n";
}
 
int main( int argc, char** argv )
{
  foo(1, 2);
  foo(1, 2, c_key=3);
  foo(1, 2, c_key=3, d_key=4);
  foo(1, 2, d_key=4);
}

This entry was posted on June 16, 2014, 3:18 am and is filed under C++ Discoveries and Notes. You can follow any responses to this entry through RSS 2.0. You can leave a response, or trackback from your own site.

Brain Dump