6 ms·

template <typename U> using func_type = decltype(U{}.func()); I admit I'm not following all the new stuff that is added to C++. What are that two lines supp

by acqq 8y ago

template <typename U> using func_type = decltype(U{}.func()); I admit I'm not following all the new stuff that is added to C++. What are that two lines supposed to mean? I don't know what the U{}.func() and using x = decltype() constructs should mean or their names. The names of these and links highly appreciated. Thanks. Moreover, regarding the "bug": if "U" is actually special as an identifier (e.g. some predefined macro or something) it's a pity that the author hasn't investigated a little more. Even not knowing the meaning of the lines I've quoted, the symptoms look somewhat familiar to me with these "special behaviors" of identifiers.

jchw 8y ago
Didn't you read the rest of the article? They tested with arguments with different names. Further they showed that T was accessible from within the other template when it didn't have a template parameter T, making this even more bizarre. Also, just to be completely sure, I tested it myself. Using X: https://godbolt.org/g/B9YEgZ https://godbolt.org/g/B9YEgZ Showing the template parameter leaking with neither T nor U: https://godbolt.org/g/CKe2wG https://godbolt.org/g/CKe2wG
- dgellow 8y ago
  I know that’s a detail, but if I’m not mistaking the author is only one person, not sure why you wrote your comment in plural form (that personally slightly confused me).
  - jchw 8y ago
    I am a purveyor of singular they, lacking any other good gender-neutral way to refer to a singular person in English.
    - deleted 8y ago
      [deleted]
    - jacquesm 8y ago
      But in this case 'he' is a 'he' and that's clearly listed at the top of the article. So no need to be 'gender-neutral'.
      - jchw 8y ago
        I know I don't have to, but I do anyways; personal choice. First time anyone's really even pointed it out.
        dgellow 8y ago
        I’m not a native English speaker myself, and I don’t live in a place with a majority of native speakers, so maybe that’s why. I got confused reading your comment as I thought that you were talking about something that I missed from the article (a link, or something else), thus I shared this feedback. Though I’m not sure why people downvoted my comment. I haven’t expressed anything against the use of neutral pronouns (though that’s unexpected when the author is clearly a man). And I clearly specified that was a detail, not something to give importance to.
        schrodinger 8y ago
        FWIW as a native English speaker I didn’t even notice that they (hah) used they. It sounds natural when you don’t know (or don’t care to emphasize) the gender of the singular person in question. Just trying to be helpful for the next time you notice that construct :)
- jchw 8y ago
  Sheesh, I'm not exactly obsessed with internet points but I am kinda curious why I got so downvoted. I wasn't really trying to be condescending, but the article kinda does answer this concern (regarding U being reserved) in a couple ways.
  - mdip 8y ago
    At risk of being downvoted, myself, as this is off-topic, but since there is no 'meta' and you're clearly looking to understand what happened, I suspect the issue originates in the first statement made from the original comment, which was posed in the form of a question: Didn't you read the rest of the article? My rule of thumb is that if a question can have ", moron?"[0] appended to it and both the meaning and tone remain unchanged, then it's going to be taken as condescending. You'll probably find that people who are particularly sensitive to keeping Hacker News a "civil place" (among which I count myself) will read that sentence and skip the rest of the comment before hitting the downvote button. Had it been left off, the rest of your comment offered clarity to anyone who simply skimmed the article or otherwise wasn't understanding what was being presented and might have missed that point. Removing the sentence also doesn't affect the information being conveyed[1]. While I'll take you at your word that it wasn't meant as condescending, a variant of Hanlon's Razor[2] applies which I had shared with me as "Where there are gaps of understanding, people tend to jump to the worst possible conclusion." and this serves as a pretty good example. You assumed the reader didn't bother to read the article, when it was equally likely that they simply didn't understand what was being presented -- a fair estimate since this is C++ templates, which is not something everyone encounters in their day-to-day. And folks who are able to downvote likely did so because they assumed you were being condescending, when it's equally likely that you may have simply written the comment hastily or meant the question in a less-sarcastically sounding manner -- a fair estimate since verbal queues are missing from written text and most of us type near the speed that we think. [0] Or "dumba$$" or "?, God!" (followed by a gesture of hands being thrown down in disgust) [1] I once had a great manager who, at a yearly feedback session, told me rather bluntly that I am too verbose and worked with me on how to convey a message by learning what is unnecessary for conveying it. As you can tell if you glance through my comment history, the lesson was entirely lost on me. While I'm much better at eliminating things that add negativity to my intent (often by adding more to my statements), I'm pretty terrible at identifying when brevity has higher value than precision and completeness. Though, when it really matters, I can pull it together if I take enough time... and on and on and on. [2] https://en.wikipedia.org/wiki/Hanlon%27s_razor https://en.wikipedia.org/wiki/Hanlon%27s_razor
tcbawo 8y ago
Here is a link that describes value initialization: http://en.cppreference.com/w/cpp/language/value_initialization http://en.cppreference.com/w/cpp/language/value_initializati... and aggregate initialization: http://en.cppreference.com/w/cpp/language/aggregate_initialization http://en.cppreference.com/w/cpp/language/aggregate_initiali... Either of which could apply to 'U{}'. decltype is just returning the result type of the contained expression, which is the return type of the method 'func' called on a default or value initialized instance of type U. A lot of times people will use std::declval<U>() instead which returns a reference to U w/o needing to call a valid constructor (https://en.cppreference.com/w/cpp/utility/declval https://en.cppreference.com/w/cpp/utility/declval). The templated using statement is creating a template type 'func_type<U>' which is similar to a typedef.
8xde0wcNwpslOw 8y ago
I can't guarantee I'm right, but AFAIK that's sort of a templated typedef, where the bound type is that of the return value of method "func" of class U. Short example: class Foo { int func(); }; // func_type<Foo> = int Also, for more info, see http://en.cppreference.com/w/cpp/language/type_alias http://en.cppreference.com/w/cpp/language/type_alias, https://en.cppreference.com/w/cpp/language/decltype https://en.cppreference.com/w/cpp/language/decltype.
- acqq 8y ago
  Thanks, in type_alias document I see the example: template<class T> using Vec = vector<T, Alloc<T>>; // type-id is vector<T, Alloc<T>> Vec<int> v; // Vec<int> is the same as vector<int, Alloc<int>> That explains to me the using construct. And tcbawo's answer tries to address the U{}.func() However "either of which could apply to U{}" ... I'd expect that it should be possible to say which of these is in this example? I also can't see how that can (or should?) match "aggregate_initialization."
pjmlp 8y ago
The expression is defining a type named func_type whose type value corresponds to the return type of the member function named func() in the given type.
beached_whale 8y ago
Using allows for type aliases like typedef but with a more readable syntax. Decltype will take the type of the expression within. So the U{} constructs a value of type U and then the func() is called. So func type will either be complete when the struct has a func member function or not. The void_t is always void but only if the template arguments are complete. So when func exists it will be a true type else a false type. True and false type are integral constants that have a constant value member
- acqq 8y ago
  Thank you, this is my favorite explanation.
Matthias247 8y ago
U is a class name, which is given as a template parameter. The braces are the new uniform initialization syntax, which basically means U{} constructs an object of type U. U{}.func() then calls the member function with the name "func" on the constructed object. decltype() returns the type of the contained expression. So I think it should return the return type of the member function. Then "using" assigns that type to an alias, as some typedef.

by acqq 8y ago

jchw 8y ago

Didn't you read the rest of the article? They tested with arguments with different names. Further they showed that T was accessible from within the other template when it didn't have a template parameter T, making this even more bizarre. Also, just to be completely sure, I tested it myself. Using X: https://godbolt.org/g/B9YEgZ https://godbolt.org/g/B9YEgZ Showing the template parameter leaking with neither T nor U: https://godbolt.org/g/CKe2wG https://godbolt.org/g/CKe2wG

dgellow 8y ago
I know that’s a detail, but if I’m not mistaking the author is only one person, not sure why you wrote your comment in plural form (that personally slightly confused me).
- jchw 8y ago
  I am a purveyor of singular they, lacking any other good gender-neutral way to refer to a singular person in English.
  - deleted 8y ago
    [deleted]
  - jacquesm 8y ago
    But in this case 'he' is a 'he' and that's clearly listed at the top of the article. So no need to be 'gender-neutral'.
    - jchw 8y ago
      I know I don't have to, but I do anyways; personal choice. First time anyone's really even pointed it out.
      - dgellow 8y ago
        I’m not a native English speaker myself, and I don’t live in a place with a majority of native speakers, so maybe that’s why. I got confused reading your comment as I thought that you were talking about something that I missed from the article (a link, or something else), thus I shared this feedback. Though I’m not sure why people downvoted my comment. I haven’t expressed anything against the use of neutral pronouns (though that’s unexpected when the author is clearly a man). And I clearly specified that was a detail, not something to give importance to.
        schrodinger 8y ago
        FWIW as a native English speaker I didn’t even notice that they (hah) used they. It sounds natural when you don’t know (or don’t care to emphasize) the gender of the singular person in question. Just trying to be helpful for the next time you notice that construct :)
jchw 8y ago
Sheesh, I'm not exactly obsessed with internet points but I am kinda curious why I got so downvoted. I wasn't really trying to be condescending, but the article kinda does answer this concern (regarding U being reserved) in a couple ways.
- mdip 8y ago
  At risk of being downvoted, myself, as this is off-topic, but since there is no 'meta' and you're clearly looking to understand what happened, I suspect the issue originates in the first statement made from the original comment, which was posed in the form of a question: Didn't you read the rest of the article? My rule of thumb is that if a question can have ", moron?"[0] appended to it and both the meaning and tone remain unchanged, then it's going to be taken as condescending. You'll probably find that people who are particularly sensitive to keeping Hacker News a "civil place" (among which I count myself) will read that sentence and skip the rest of the comment before hitting the downvote button. Had it been left off, the rest of your comment offered clarity to anyone who simply skimmed the article or otherwise wasn't understanding what was being presented and might have missed that point. Removing the sentence also doesn't affect the information being conveyed[1]. While I'll take you at your word that it wasn't meant as condescending, a variant of Hanlon's Razor[2] applies which I had shared with me as "Where there are gaps of understanding, people tend to jump to the worst possible conclusion." and this serves as a pretty good example. You assumed the reader didn't bother to read the article, when it was equally likely that they simply didn't understand what was being presented -- a fair estimate since this is C++ templates, which is not something everyone encounters in their day-to-day. And folks who are able to downvote likely did so because they assumed you were being condescending, when it's equally likely that you may have simply written the comment hastily or meant the question in a less-sarcastically sounding manner -- a fair estimate since verbal queues are missing from written text and most of us type near the speed that we think. [0] Or "dumba$$" or "?, God!" (followed by a gesture of hands being thrown down in disgust) [1] I once had a great manager who, at a yearly feedback session, told me rather bluntly that I am too verbose and worked with me on how to convey a message by learning what is unnecessary for conveying it. As you can tell if you glance through my comment history, the lesson was entirely lost on me. While I'm much better at eliminating things that add negativity to my intent (often by adding more to my statements), I'm pretty terrible at identifying when brevity has higher value than precision and completeness. Though, when it really matters, I can pull it together if I take enough time... and on and on and on. [2] https://en.wikipedia.org/wiki/Hanlon%27s_razor https://en.wikipedia.org/wiki/Hanlon%27s_razor

tcbawo 8y ago

Here is a link that describes value initialization: http://en.cppreference.com/w/cpp/language/value_initialization http://en.cppreference.com/w/cpp/language/value_initializati... and aggregate initialization: http://en.cppreference.com/w/cpp/language/aggregate_initialization http://en.cppreference.com/w/cpp/language/aggregate_initiali... Either of which could apply to 'U{}'. decltype is just returning the result type of the contained expression, which is the return type of the method 'func' called on a default or value initialized instance of type U. A lot of times people will use std::declval<U>() instead which returns a reference to U w/o needing to call a valid constructor (https://en.cppreference.com/w/cpp/utility/declval https://en.cppreference.com/w/cpp/utility/declval). The templated using statement is creating a template type 'func_type<U>' which is similar to a typedef.

8xde0wcNwpslOw 8y ago

I can't guarantee I'm right, but AFAIK that's sort of a templated typedef, where the bound type is that of the return value of method "func" of class U. Short example: class Foo { int func(); }; // func_type<Foo> = int Also, for more info, see http://en.cppreference.com/w/cpp/language/type_alias http://en.cppreference.com/w/cpp/language/type_alias, https://en.cppreference.com/w/cpp/language/decltype https://en.cppreference.com/w/cpp/language/decltype.

acqq 8y ago
Thanks, in type_alias document I see the example: template<class T> using Vec = vector<T, Alloc<T>>; // type-id is vector<T, Alloc<T>> Vec<int> v; // Vec<int> is the same as vector<int, Alloc<int>> That explains to me the using construct. And tcbawo's answer tries to address the U{}.func() However "either of which could apply to U{}" ... I'd expect that it should be possible to say which of these is in this example? I also can't see how that can (or should?) match "aggregate_initialization."

pjmlp 8y ago

The expression is defining a type named func_type whose type value corresponds to the return type of the member function named func() in the given type.

beached_whale 8y ago

Using allows for type aliases like typedef but with a more readable syntax. Decltype will take the type of the expression within. So the U{} constructs a value of type U and then the func() is called. So func type will either be complete when the struct has a func member function or not. The void_t is always void but only if the template arguments are complete. So when func exists it will be a true type else a false type. True and false type are integral constants that have a constant value member

acqq 8y ago
Thank you, this is my favorite explanation.

Matthias247 8y ago

U is a class name, which is given as a template parameter. The braces are the new uniform initialization syntax, which basically means U{} constructs an object of type U. U{}.func() then calls the member function with the name "func" on the constructed object. decltype() returns the type of the contained expression. So I think it should return the return type of the member function. Then "using" assigns that type to an alias, as some typedef.