C++ elegant way of passing (a subset of) options to a function

Question

I have a function whose exact behaviour is controlled by a set of flags. Most of these are usually false. Having them all as parameters quickly becomes very messy.

I could create an enum for the flags:

enum ParseFlags { assignmentPossible = 1, commaDelimits = 2, isConst = 4 };
PExpression ParseExpr(unsigned int flags);

A bit better: now, when calling the function, I only have to specify the values I want to set, instead of a long list of bools.

However, I would like to know if there is a more elegant or more general way of passing a subset of options to a function. What if I have non-boolean options?

I'm looking for a C++ (Boost is ok) way to call a function like:

ParseExpr({ isConst: true; maxDepth: 5 });

where I can omit all values that I want to leave at their default.

What techniques and/or libraries exist for this?

Have you considered an overload that takes `std::map` or more generically `std::map`? — François Andrieux, Jun 08 '17 at 15:19
@πάνταῥεῖ `bool`s as flags in an `unsigned int` are basically a bitfield, or not? — Felix Dombek, Jun 08 '17 at 15:21
@FrançoisAndrieux I was a bit worried about the overhead, but it is a good idea. — Felix Dombek, Jun 08 '17 at 15:23
@DanielJour Interesting! But it has a rather clumsy feel to it :| — Felix Dombek, Jun 08 '17 at 15:24
@FelixDombek You could use `std::array, S>` if you are worried about allocations. You would just have to add `S` as a template parameter. — François Andrieux, Jun 08 '17 at 15:26
@Zereges I thought there must have been sth like this, but somehow overlooked it! The calling syntax is perfect, and the specification isn't much more work than any other way. This is probably the way to go! How about an answer for this? — Felix Dombek, Jun 08 '17 at 15:34
@FelixDombek I only know such library exists, I have never used it before so I won't be able to came up with proper example anyways :) — Zereges, Jun 08 '17 at 16:34
I'm working on something (IMO) pretty nice approach. Is the "set" of passed configuration parameters known at compile time? Or should something like this work: `auto flags = isConst{true}; if (condition) flags += maxDepth{5}; function(flags);` That's possible, but degrades performance (a bit). — Daniel Jour, Jun 08 '17 at 22:13

score 5 · Answer 1 · answered Jun 08 '17 at 15:37

5

Ripping from this library's design, you could have a parameter class which curries its set methods.

class ParseFlags
{
public:
    ParseFlags& assignable(bool b) {assignable_ = b; return *this;}
    ParseFlags& delimiter(char c) {delimiter_ = c; return *this;}
    ParseFlags& constness(bool b) {constness_ = b; return *this;}
private:
    bool assignable_ = false;
    char delimiter_ = ',';
    bool constness_ = false;
};

PExpression ParseExpr(const ParseFlags& = ParseFlags{});

Call it like

auto exp = ParseExpr(ParseFlags().constness(false).delimiter('.'));
auto exp2 = ParseExpr();

Which is almost as good as having named parameters.

answered Jun 08 '17 at 15:37

Passer By

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Currying is imho sth different, i.e creating a new function with one fixed argument by specifying one (of many) function arguments to a previously existing function. Otherwise, great answer and maybe the easiest technique. – Felix Dombek Jun 25 '17 at 23:46
@FelixDombek I agree, and I'd love to have named parameters in C++ too. I've searched high and low for something similar and this is the best I got. – Passer By Jul 02 '17 at 08:36

C++ elegant way of passing (a subset of) options to a function

1 Answers1