Comments on: Defaulted: A Helper to Work Around the Constraints of C++ Default Parameters

By: Jonathan Boccara

Jonathan Boccara — Fri, 17 Aug 2018 15:20:00 +0000

In reply to Vincent Zalzal. Interesting solution indeed, thanks for sharing!

By: Kuba Ober

Kuba Ober — Wed, 15 Aug 2018 22:38:00 +0000

In reply to Kaushik Basu. That's what source code rewriting is for. Take Eli Benderski's tooling_sample.cpp, have it detect the calls and rewrite them the way you want them to be. Boom and done.

By: jft

jft — Wed, 15 Aug 2018 10:10:00 +0000

As named parameter passing has been rejected for good reasons, why not simply be able to leave out any required default argument?

If we have
void f(int x, int y = 42, int z)
…

then why not simply
f(1, , 3);

to imply that the missing argument should take the default one? If no default one specified, then report an error.

By: Kaushik Basu

Kaushik Basu — Tue, 14 Aug 2018 11:54:00 +0000

One drawback I see is if I have to change an existing function which is already called from multiple places to have an additional parameter for a new requirement, then I have to change all existing calls to use the “defaultValue”.
For large code bases, this can be a problem.

By: Vincent Zalzal

Vincent Zalzal — Tue, 14 Aug 2018 02:58:00 +0000

There is another alternative to handle multiple default parameters: not encoding the default value in the Defaulted type, and letting the callee handle the default value. In that case, std::optional can be used instead of Defaulted.


void g(int x, std::optional y /* = 45.6 */, int z)
{
    std::cout << "x = " << x << 'n'
              << "y = " << y.value_or(45.6) << 'n'
              << "z = " << z << 'n';
}

g(0, std::nullopt, 1);
g(0, {}, 1);

It has the advantage of being able to support doubles or aggregates that a template parameter can't. It has the drawback that the default value is not documented in the interface anymore, which I suspect you won't like much. Also, optional references are not supported (yet?), so it is not a good option for slow-to-copy types.

By: Vincent Zalzal

Vincent Zalzal — Tue, 14 Aug 2018 02:37:00 +0000

A neat trick that will probably become available in C++2a is to use designated initializers to handle named parameters with default values. This has been featured in a recent video of Jason Turner : https://youtu.be/44rs_hX1dxE