Implicitly convert a lamba to a function ptr to create a class

Question

struct myclass
{
    myclass(void(*)()) {}
};

void test1(void(*)()) {}
void test2(myclass) { }
void cb() {}

int main()
{
    test1(cb);      // works
    test2(cb);      // works
    test1([](){});  // works

    test2([](){});  // does not work, why? there's no "explicit" keyword next to myclass.

}

Why this doesn't work?

The following obviously works but I don't want to use it.

     test2(myclass([]{}));

Notes: I do not want to accept a std::function<void()>> and I don't want to create a template<T> myclass(T f) {} either.

score 3 · Accepted Answer · edited May 23 '17 at 12:08

3

You can only have one user defined conversion - and lambda to function pointer counts as user defined. Instead use some sorcery to do the function pointer conversion explicitly:

test2(+[](){});

Note also that the parens in a nullary lambda are optional, so this also works:

test2(+[]{});

edited May 23 '17 at 12:08

Community

1
1

answered May 13 '16 at 20:40

Barry

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Yeah I already knew about the parens. Dunno why I had them there. I had no idea about the + trick tho. Thank you very much for that! – Gam May 13 '16 at 20:42
OMG! That's horrible. I can imagine using it, but there is going to be a 10 line comment beforehand explaining what's going on! – Martin Bonner supports Monica May 13 '16 at 20:55
1

@Martin Why would you need a 10 line comment? You see it once, look it up, and you know what it means. – Barry May 13 '16 at 21:24
Last time I checked, this doesn't work on MSVC (or maybe just 32-bit MSVC?) - but maybe they'll fix it one day... – T.C. May 13 '16 at 23:17
1

10 line comment: I'd like the explanation inline, also, I'd need the explanation again the *next* time I looked at the code. – Martin Bonner supports Monica May 14 '16 at 07:01
@Martin Barry knows it by heart, he won't understand why you need the 10 lines comment above it. – Johannes Schaub - litb May 16 '16 at 13:15

score 2 · Answer 2 · answered May 13 '16 at 20:33

2

You can only have one user-defined conversion in a conversion sequence. Try:

test2(static_cast<void(*)()>([](){}));

Or:

test2(static_cast<myclass>([](){}));

answered May 13 '16 at 20:33

Kerrek SB

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Upvoted for the user defined conversion answer. Simply accepted Barry's one since it was a bit more complete (it had the + "sorcery" in it), which is sick since I had absolutely no idea about it. – Gam May 13 '16 at 20:51

Implicitly convert a lamba to a function ptr to create a class

2 Answers2