C++ template constructor default arguments

Question

In a very simple way. We have following code:

struct templateTest{
    struct {}m_none;
    template <typename T1, typename T2, typename T3, typename T4>
    templateTest(T1 arg1, T2 arg2=m_none, T3 arg3=m_none, T4 arg4=m_none){

    }
};

struct nonTemplate{
    nonTemplate(int arg1, int arg2=2, int arg3=3){}
};

int main()
{
    nonTemplate(5);
    nonTemplate(5,10);

    templateTest test2(5,10);
    templateTest test1(5);


    return 0;
}

What I would like to have is default constructor arguments in template class.

Default arguments work perfectly fine with nonTemplate class, but it's not that way with templateTest type. Error I have are following:

C2660: 'templateTest::templateTest' : function does not take 2 arguments

for constructor called with 2 arguments

And for constructor called with one argument compiler wants to invoke copyConstructor and output is following:

C2664: 'templateTest::templateTest(const templateTest &)' : cannot convert argument 1 from 'int' to 'const templateTest &'

Is there any way to achieve what I want on old C++ compilers, like msvc 2011?

http://stackoverflow.com/questions/2447458/default-template-arguments-for-function-templates — Lingxi, Nov 27 '15 at 16:14

Yakk - Adam Nevraumont · Accepted Answer · 2015-11-27T16:15:41.020

5

struct none_t {};
template <class T1, class T2=none_t, class T3=none_t, class T4=none_t>
templateTest(T1 arg1, T2 arg2=none_t{}, T3 arg3=none_t{}, T4 arg4=none_t{}){

}

you cannot deduce the template type arguments from the default arguments. In essence, the template type arguments are deduced before default arguments are substituted.

live example.

edited Nov 27 '15 at 16:15

answered Nov 27 '15 at 16:14

Yakk - Adam Nevraumont

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Yeah I know in fact, that It works, but it does not compile on msvc 2012 as there I cannot have default template types on class's members. What then? – DawidPi Nov 27 '15 at 16:20
@DawidPi Overloading. – Angew is no longer proud of SO Nov 27 '15 at 16:25
@DawidPi create 4 overloads, with near-exact duplicates of the body and initializer list, each taking 1 2 3 and 4 arguments? Get a new compiler? – Yakk - Adam Nevraumont Nov 27 '15 at 16:25
That's the way I do this now in fact :) But I have 9 argument function, so you can just imagine how ugly code, becomes. Of course getting new compiler for whole company is impossible for me :) but still thanks :) – DawidPi Nov 27 '15 at 16:28
1

@DawidPi Boost has macros that they used to emulate variardics. You could also write a function object (not a template function, because you cannot pass those around, and not a function, because they cannot represent more than 1 overload) and create an adapter that makes it variardic (writing the above 9+ overloads, dispatching more/fewer `none_t`s to the function object depending on passed-in arguments) if you find yourself doing it more than once. – Yakk - Adam Nevraumont Nov 27 '15 at 18:23

C++ template constructor default arguments

1 Answers1