Elegant way to pass multiple arguments to a function

I've got a function which looks like this:

bool generate_script (bool net, bool tv, bool phone,
                        std::string clientsID,
                        std::string password,
                        int index, std::string number, 
                        std::string Iport, std::string sernoID,
                        std::string VoiP_number, std::string  VoiP_pass,
                        std::string target, int slot, int port, 
                        int onu, int extra, std::string IP, std::string MAC);

In my opinion it looks ugly. What is the proper way of handling this problem? Should I create few vectors with different data types (int, string and bool) and pass them as arguments to this function?

Solution 1:

If all these parameters are meaningfully related, pack them in a structure.

Solution 2:

Put them in a struct

Create a structure

struct GenerateScriptParams { /* ... */ };

and put all the parameters in there. You can actually provide default values for the initialization of the struct as well by implementing a default constructor or, in C++11, by providing default initialization of individual members. You can then change the values that are not supposed to be defaulted. This selective picking of non-default parameters is not possible for a function call with lots of parameters in C++.

Making the interface nice for the caller

Yet, the usage is a little ugly, since you have to create a temporary name object, then change the values that should not be default and then pass the object to the function:

GenerateScriptParams gsp;
gsp.net = true;
gsp.phone = false;
gps.extra = 10;
generate_script( gsp );

If you call that function in several different places, it makes sense to avoid this uglyness by providing mutating member functions that can be chained:

GenerateScriptParams & GenerateScriptParams::setNet  ( bool val );
GenerateScriptParams & GenerateScriptParams::setTV   ( bool val );
GenerateScriptParams & GenerateScriptParams::setPhone( bool val );
// ... //

Then calling code can write

generate_script( GenerateScriptParams()
    .setNet(true),
    .setPhone(false),
    .setExtra(10) );

without the above uglyness. This avoids the named object that is only used once.

Solution 3:

I personally do not believe that moving all the arguments in one struct will make the code much better. You just move dirt under the carpet. When you are going to deal with the creation of the struct you have the same problem.

The question is how much reusable this struct will be? If you end up with a 18 parameters for one function call something it is not quite right in your design. After further analysis you may discover that those parameters can be group in several different classes and those classes could be aggregated to one single object that will be the input of your function. You may want also prefer classes to struct in order to protect your data.

EDIT

I will give you a small example to describe why several classes are better than one monolithic struct. Let's start counting the tests that you need to write to cover the function above. There are 18 parameters as input (3 boolean). So we are going to need at least 15 tests only to validate the input (assuming the values are not interconnected).

The overall number of tests is impossible to be calculated without the implementation, but we can have an idea of the magnitude. Let take the lower bound all the input can be treat as boolean the number of possible combination are 2^18 so around 262000 tests.

Now, what happen if we split the input in several objects?

First of all, the code to validate the input is moved away from the function to the body of every single object (and it can be reused).

But more importantly the number of tests will collapse, let say in group of four (4,4,4 and 4 params per object) the total number of tests is only:

2^4 + 2^4 + 2^4 + 2^4 + 2^4 = 80

The fifth attributes is due to the permutation of the objects with themselves.

So, what is more cost demanding? Write thousand of tests or few more classes?

Obviously, this is a crude simplification, however, it will underlying the core of the problem. A clutter interface is not just matter of style or an inconvenient for the developer it is a true impediment to produce quality code.

This is the most important lesson I ever learnt in my career as a professional developer: "Big classes and fat interfaces are evil". That's just my heuristic version of the single responsibility principle (I have notice that the SRP can be tricky to get it right, what it seems reasonable to be single responsibility it can be not quite the same after a hour coding, so I used some heuristic rule to help me to revaulate my initial choices).

Solution 4:

Or you could use a fluent interface. It would look like this:

script my_script(mandatory, parameters);
my_script.net(true).tv(false).phone(true);

This is applicable if you have default values for your specified parameters or it is allowed to have a partially constructed script.

Solution 5:

Ignoring the possibility or desirability of changing the function or program in some way as to reduce the number of parameters...

I have seen coding standards that specify how long parameter lists should be formatted, for cases where refactoring is not possible. One such example is using double indentations and one parameter per line (Not for all functions - only for those that have multiple-lines of parameters).

E.g.

bool generate_script (
        bool net,
        bool tv,
        bool phone,
        std::string clientsID,
        std::string password,
        int index,
        std::string number,
        std::string Iport,
        std::string sernoID,
        std::string VoiP_number,
        std::string  VoiP_pass,
        std::string target,
        int slot,
        int port,
        int onu,
        int extra,
        std::string IP,
        std::string MAC);

The point here is to create a consistent layout and look for all functions with a large number of parameters.