Overload resolution looking into namespaces












34















The following code fails as expected, because no overload of get is found. Using std::getwould solve the problem.



#include <array>

int main()
{
std::array<int, 2> ar{2,3};
auto r = get<0>(ar);//fails, get was not declared in this scope
}


However, introducing a templated version of get, even though it's not matching the function call, somehow makes the compiler use the std::get version:



#include <array>

template <typename T>
void get(){};

int main()
{
std::array<int, 2> ar{2,3};

auto r = get<0>(ar);//returns 2
}


I can't find any part of the standard that explains this. Is this a bug in all 3 compilers I tested (probably not), or am I missing something?



This behaviour was tested in




  • MSVC 15.9.2

  • Clang 8.0.0

  • GCC 9.0.0 (still an experimental version)


EDIT:
I am aware of ADL. But if ADL makes the second code work, why does it not in the first part?










share|improve this question

























  • It is found by ADL. Seems that ADL is prevented without providing that declaration, .

    – felix
    Nov 20 '18 at 13:11






  • 1





    I was suspecting ADL. However, it does not explain why ADL doesn't work without the fake-template

    – LcdDrm
    Nov 20 '18 at 13:13











  • Alternatively, just don't give your functions names that are already within the standard library.

    – jfh
    Nov 20 '18 at 16:26






  • 1





    @jfh Bad advice. Don’t try to work around something that namespaces already fix for you.

    – Konrad Rudolph
    Nov 20 '18 at 16:51






  • 1





    @LcdDrm As an exercise for myself I wrote the above functionality that you describe. I can share it with you if you like.

    – jfh
    Nov 24 '18 at 23:09
















34















The following code fails as expected, because no overload of get is found. Using std::getwould solve the problem.



#include <array>

int main()
{
std::array<int, 2> ar{2,3};
auto r = get<0>(ar);//fails, get was not declared in this scope
}


However, introducing a templated version of get, even though it's not matching the function call, somehow makes the compiler use the std::get version:



#include <array>

template <typename T>
void get(){};

int main()
{
std::array<int, 2> ar{2,3};

auto r = get<0>(ar);//returns 2
}


I can't find any part of the standard that explains this. Is this a bug in all 3 compilers I tested (probably not), or am I missing something?



This behaviour was tested in




  • MSVC 15.9.2

  • Clang 8.0.0

  • GCC 9.0.0 (still an experimental version)


EDIT:
I am aware of ADL. But if ADL makes the second code work, why does it not in the first part?










share|improve this question

























  • It is found by ADL. Seems that ADL is prevented without providing that declaration, .

    – felix
    Nov 20 '18 at 13:11






  • 1





    I was suspecting ADL. However, it does not explain why ADL doesn't work without the fake-template

    – LcdDrm
    Nov 20 '18 at 13:13











  • Alternatively, just don't give your functions names that are already within the standard library.

    – jfh
    Nov 20 '18 at 16:26






  • 1





    @jfh Bad advice. Don’t try to work around something that namespaces already fix for you.

    – Konrad Rudolph
    Nov 20 '18 at 16:51






  • 1





    @LcdDrm As an exercise for myself I wrote the above functionality that you describe. I can share it with you if you like.

    – jfh
    Nov 24 '18 at 23:09














34












34








34


5






The following code fails as expected, because no overload of get is found. Using std::getwould solve the problem.



#include <array>

int main()
{
std::array<int, 2> ar{2,3};
auto r = get<0>(ar);//fails, get was not declared in this scope
}


However, introducing a templated version of get, even though it's not matching the function call, somehow makes the compiler use the std::get version:



#include <array>

template <typename T>
void get(){};

int main()
{
std::array<int, 2> ar{2,3};

auto r = get<0>(ar);//returns 2
}


I can't find any part of the standard that explains this. Is this a bug in all 3 compilers I tested (probably not), or am I missing something?



This behaviour was tested in




  • MSVC 15.9.2

  • Clang 8.0.0

  • GCC 9.0.0 (still an experimental version)


EDIT:
I am aware of ADL. But if ADL makes the second code work, why does it not in the first part?










share|improve this question
















The following code fails as expected, because no overload of get is found. Using std::getwould solve the problem.



#include <array>

int main()
{
std::array<int, 2> ar{2,3};
auto r = get<0>(ar);//fails, get was not declared in this scope
}


However, introducing a templated version of get, even though it's not matching the function call, somehow makes the compiler use the std::get version:



#include <array>

template <typename T>
void get(){};

int main()
{
std::array<int, 2> ar{2,3};

auto r = get<0>(ar);//returns 2
}


I can't find any part of the standard that explains this. Is this a bug in all 3 compilers I tested (probably not), or am I missing something?



This behaviour was tested in




  • MSVC 15.9.2

  • Clang 8.0.0

  • GCC 9.0.0 (still an experimental version)


EDIT:
I am aware of ADL. But if ADL makes the second code work, why does it not in the first part?







c++ overload-resolution






share|improve this question















share|improve this question













share|improve this question




share|improve this question








edited Nov 20 '18 at 14:03







LcdDrm

















asked Nov 20 '18 at 12:55









LcdDrmLcdDrm

582411




582411













  • It is found by ADL. Seems that ADL is prevented without providing that declaration, .

    – felix
    Nov 20 '18 at 13:11






  • 1





    I was suspecting ADL. However, it does not explain why ADL doesn't work without the fake-template

    – LcdDrm
    Nov 20 '18 at 13:13











  • Alternatively, just don't give your functions names that are already within the standard library.

    – jfh
    Nov 20 '18 at 16:26






  • 1





    @jfh Bad advice. Don’t try to work around something that namespaces already fix for you.

    – Konrad Rudolph
    Nov 20 '18 at 16:51






  • 1





    @LcdDrm As an exercise for myself I wrote the above functionality that you describe. I can share it with you if you like.

    – jfh
    Nov 24 '18 at 23:09



















  • It is found by ADL. Seems that ADL is prevented without providing that declaration, .

    – felix
    Nov 20 '18 at 13:11






  • 1





    I was suspecting ADL. However, it does not explain why ADL doesn't work without the fake-template

    – LcdDrm
    Nov 20 '18 at 13:13











  • Alternatively, just don't give your functions names that are already within the standard library.

    – jfh
    Nov 20 '18 at 16:26






  • 1





    @jfh Bad advice. Don’t try to work around something that namespaces already fix for you.

    – Konrad Rudolph
    Nov 20 '18 at 16:51






  • 1





    @LcdDrm As an exercise for myself I wrote the above functionality that you describe. I can share it with you if you like.

    – jfh
    Nov 24 '18 at 23:09

















It is found by ADL. Seems that ADL is prevented without providing that declaration, .

– felix
Nov 20 '18 at 13:11





It is found by ADL. Seems that ADL is prevented without providing that declaration, .

– felix
Nov 20 '18 at 13:11




1




1





I was suspecting ADL. However, it does not explain why ADL doesn't work without the fake-template

– LcdDrm
Nov 20 '18 at 13:13





I was suspecting ADL. However, it does not explain why ADL doesn't work without the fake-template

– LcdDrm
Nov 20 '18 at 13:13













Alternatively, just don't give your functions names that are already within the standard library.

– jfh
Nov 20 '18 at 16:26





Alternatively, just don't give your functions names that are already within the standard library.

– jfh
Nov 20 '18 at 16:26




1




1





@jfh Bad advice. Don’t try to work around something that namespaces already fix for you.

– Konrad Rudolph
Nov 20 '18 at 16:51





@jfh Bad advice. Don’t try to work around something that namespaces already fix for you.

– Konrad Rudolph
Nov 20 '18 at 16:51




1




1





@LcdDrm As an exercise for myself I wrote the above functionality that you describe. I can share it with you if you like.

– jfh
Nov 24 '18 at 23:09





@LcdDrm As an exercise for myself I wrote the above functionality that you describe. I can share it with you if you like.

– jfh
Nov 24 '18 at 23:09












2 Answers
2






active

oldest

votes


















20














ADL is not used when explicit template arguments are involved unless you introduce a template function declaration at the call point. You're using an unqualified form of get using a non-type template argument 0, so you need to introduce a template function declaration or use the qualified version of get as std::get<0>(ar).



In standardese [temp.arg.explicit]/8: (emphasis mine)




[ Note: For simple function names, argument dependent lookup (6.4.2) applies even when the function name is not visible within the scope of the call. This is because the call still has the syntactic form of a function call (6.4.1). But when a function template with explicit template arguments is used, the call does not have the correct syntactic form unless there is a function template with that name visible at the point of the call. If no such name is visible, the call is not syntactically well-formed and argument-dependent lookup does not apply. If some such name is visible, argument dependent lookup applies and additional function templates may be found in other namespaces.




EDIT:



As @Yakk - Adam Nevraumont has pointed out in the comment, without the presence of the template function declaration, the expression get<0>(ar) will be parsed as (get<0)>(ar), i.e as a serie of comparison expressions instead of a function call.






share|improve this answer


























  • Well that makes sense. Although in GCC 9.0.0, a void get(); is enough to make ADL work again. If I interpret that snipped of the standard correctly, this is a compiler bug?

    – LcdDrm
    Nov 20 '18 at 13:34













  • That version of GCC is still under development, its too early to say that

    – Jans
    Nov 20 '18 at 13:46






  • 5





    Good answer, but I'd mention what get<0>(ar) is parsed as without the template: (get<0)>(ar). This makes it more clear why the existence of the template get makes a difference.

    – Yakk - Adam Nevraumont
    Nov 20 '18 at 15:47





















4














Note that this changed in C++20 as a result of P0846R0. An unqualified name followed by a < token for which ordinary unqualified lookup either finds one or more functions or finds nothing is now assumed to name a template and the < is parsed accordingly.






share|improve this answer























    Your Answer






    StackExchange.ifUsing("editor", function () {
    StackExchange.using("externalEditor", function () {
    StackExchange.using("snippets", function () {
    StackExchange.snippets.init();
    });
    });
    }, "code-snippets");

    StackExchange.ready(function() {
    var channelOptions = {
    tags: "".split(" "),
    id: "1"
    };
    initTagRenderer("".split(" "), "".split(" "), channelOptions);

    StackExchange.using("externalEditor", function() {
    // Have to fire editor after snippets, if snippets enabled
    if (StackExchange.settings.snippets.snippetsEnabled) {
    StackExchange.using("snippets", function() {
    createEditor();
    });
    }
    else {
    createEditor();
    }
    });

    function createEditor() {
    StackExchange.prepareEditor({
    heartbeatType: 'answer',
    autoActivateHeartbeat: false,
    convertImagesToLinks: true,
    noModals: true,
    showLowRepImageUploadWarning: true,
    reputationToPostImages: 10,
    bindNavPrevention: true,
    postfix: "",
    imageUploader: {
    brandingHtml: "Powered by u003ca class="icon-imgur-white" href="https://imgur.com/"u003eu003c/au003e",
    contentPolicyHtml: "User contributions licensed under u003ca href="https://creativecommons.org/licenses/by-sa/3.0/"u003ecc by-sa 3.0 with attribution requiredu003c/au003e u003ca href="https://stackoverflow.com/legal/content-policy"u003e(content policy)u003c/au003e",
    allowUrls: true
    },
    onDemand: true,
    discardSelector: ".discard-answer"
    ,immediatelyShowMarkdownHelp:true
    });


    }
    });














    draft saved

    draft discarded


















    StackExchange.ready(
    function () {
    StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fstackoverflow.com%2fquestions%2f53393467%2foverload-resolution-looking-into-namespaces%23new-answer', 'question_page');
    }
    );

    Post as a guest















    Required, but never shown

























    2 Answers
    2






    active

    oldest

    votes








    2 Answers
    2






    active

    oldest

    votes









    active

    oldest

    votes






    active

    oldest

    votes









    20














    ADL is not used when explicit template arguments are involved unless you introduce a template function declaration at the call point. You're using an unqualified form of get using a non-type template argument 0, so you need to introduce a template function declaration or use the qualified version of get as std::get<0>(ar).



    In standardese [temp.arg.explicit]/8: (emphasis mine)




    [ Note: For simple function names, argument dependent lookup (6.4.2) applies even when the function name is not visible within the scope of the call. This is because the call still has the syntactic form of a function call (6.4.1). But when a function template with explicit template arguments is used, the call does not have the correct syntactic form unless there is a function template with that name visible at the point of the call. If no such name is visible, the call is not syntactically well-formed and argument-dependent lookup does not apply. If some such name is visible, argument dependent lookup applies and additional function templates may be found in other namespaces.




    EDIT:



    As @Yakk - Adam Nevraumont has pointed out in the comment, without the presence of the template function declaration, the expression get<0>(ar) will be parsed as (get<0)>(ar), i.e as a serie of comparison expressions instead of a function call.






    share|improve this answer


























    • Well that makes sense. Although in GCC 9.0.0, a void get(); is enough to make ADL work again. If I interpret that snipped of the standard correctly, this is a compiler bug?

      – LcdDrm
      Nov 20 '18 at 13:34













    • That version of GCC is still under development, its too early to say that

      – Jans
      Nov 20 '18 at 13:46






    • 5





      Good answer, but I'd mention what get<0>(ar) is parsed as without the template: (get<0)>(ar). This makes it more clear why the existence of the template get makes a difference.

      – Yakk - Adam Nevraumont
      Nov 20 '18 at 15:47


















    20














    ADL is not used when explicit template arguments are involved unless you introduce a template function declaration at the call point. You're using an unqualified form of get using a non-type template argument 0, so you need to introduce a template function declaration or use the qualified version of get as std::get<0>(ar).



    In standardese [temp.arg.explicit]/8: (emphasis mine)




    [ Note: For simple function names, argument dependent lookup (6.4.2) applies even when the function name is not visible within the scope of the call. This is because the call still has the syntactic form of a function call (6.4.1). But when a function template with explicit template arguments is used, the call does not have the correct syntactic form unless there is a function template with that name visible at the point of the call. If no such name is visible, the call is not syntactically well-formed and argument-dependent lookup does not apply. If some such name is visible, argument dependent lookup applies and additional function templates may be found in other namespaces.




    EDIT:



    As @Yakk - Adam Nevraumont has pointed out in the comment, without the presence of the template function declaration, the expression get<0>(ar) will be parsed as (get<0)>(ar), i.e as a serie of comparison expressions instead of a function call.






    share|improve this answer


























    • Well that makes sense. Although in GCC 9.0.0, a void get(); is enough to make ADL work again. If I interpret that snipped of the standard correctly, this is a compiler bug?

      – LcdDrm
      Nov 20 '18 at 13:34













    • That version of GCC is still under development, its too early to say that

      – Jans
      Nov 20 '18 at 13:46






    • 5





      Good answer, but I'd mention what get<0>(ar) is parsed as without the template: (get<0)>(ar). This makes it more clear why the existence of the template get makes a difference.

      – Yakk - Adam Nevraumont
      Nov 20 '18 at 15:47
















    20












    20








    20







    ADL is not used when explicit template arguments are involved unless you introduce a template function declaration at the call point. You're using an unqualified form of get using a non-type template argument 0, so you need to introduce a template function declaration or use the qualified version of get as std::get<0>(ar).



    In standardese [temp.arg.explicit]/8: (emphasis mine)




    [ Note: For simple function names, argument dependent lookup (6.4.2) applies even when the function name is not visible within the scope of the call. This is because the call still has the syntactic form of a function call (6.4.1). But when a function template with explicit template arguments is used, the call does not have the correct syntactic form unless there is a function template with that name visible at the point of the call. If no such name is visible, the call is not syntactically well-formed and argument-dependent lookup does not apply. If some such name is visible, argument dependent lookup applies and additional function templates may be found in other namespaces.




    EDIT:



    As @Yakk - Adam Nevraumont has pointed out in the comment, without the presence of the template function declaration, the expression get<0>(ar) will be parsed as (get<0)>(ar), i.e as a serie of comparison expressions instead of a function call.






    share|improve this answer















    ADL is not used when explicit template arguments are involved unless you introduce a template function declaration at the call point. You're using an unqualified form of get using a non-type template argument 0, so you need to introduce a template function declaration or use the qualified version of get as std::get<0>(ar).



    In standardese [temp.arg.explicit]/8: (emphasis mine)




    [ Note: For simple function names, argument dependent lookup (6.4.2) applies even when the function name is not visible within the scope of the call. This is because the call still has the syntactic form of a function call (6.4.1). But when a function template with explicit template arguments is used, the call does not have the correct syntactic form unless there is a function template with that name visible at the point of the call. If no such name is visible, the call is not syntactically well-formed and argument-dependent lookup does not apply. If some such name is visible, argument dependent lookup applies and additional function templates may be found in other namespaces.




    EDIT:



    As @Yakk - Adam Nevraumont has pointed out in the comment, without the presence of the template function declaration, the expression get<0>(ar) will be parsed as (get<0)>(ar), i.e as a serie of comparison expressions instead of a function call.







    share|improve this answer














    share|improve this answer



    share|improve this answer








    edited Nov 21 '18 at 19:15

























    answered Nov 20 '18 at 13:14









    JansJans

    9,02422635




    9,02422635













    • Well that makes sense. Although in GCC 9.0.0, a void get(); is enough to make ADL work again. If I interpret that snipped of the standard correctly, this is a compiler bug?

      – LcdDrm
      Nov 20 '18 at 13:34













    • That version of GCC is still under development, its too early to say that

      – Jans
      Nov 20 '18 at 13:46






    • 5





      Good answer, but I'd mention what get<0>(ar) is parsed as without the template: (get<0)>(ar). This makes it more clear why the existence of the template get makes a difference.

      – Yakk - Adam Nevraumont
      Nov 20 '18 at 15:47





















    • Well that makes sense. Although in GCC 9.0.0, a void get(); is enough to make ADL work again. If I interpret that snipped of the standard correctly, this is a compiler bug?

      – LcdDrm
      Nov 20 '18 at 13:34













    • That version of GCC is still under development, its too early to say that

      – Jans
      Nov 20 '18 at 13:46






    • 5





      Good answer, but I'd mention what get<0>(ar) is parsed as without the template: (get<0)>(ar). This makes it more clear why the existence of the template get makes a difference.

      – Yakk - Adam Nevraumont
      Nov 20 '18 at 15:47



















    Well that makes sense. Although in GCC 9.0.0, a void get(); is enough to make ADL work again. If I interpret that snipped of the standard correctly, this is a compiler bug?

    – LcdDrm
    Nov 20 '18 at 13:34







    Well that makes sense. Although in GCC 9.0.0, a void get(); is enough to make ADL work again. If I interpret that snipped of the standard correctly, this is a compiler bug?

    – LcdDrm
    Nov 20 '18 at 13:34















    That version of GCC is still under development, its too early to say that

    – Jans
    Nov 20 '18 at 13:46





    That version of GCC is still under development, its too early to say that

    – Jans
    Nov 20 '18 at 13:46




    5




    5





    Good answer, but I'd mention what get<0>(ar) is parsed as without the template: (get<0)>(ar). This makes it more clear why the existence of the template get makes a difference.

    – Yakk - Adam Nevraumont
    Nov 20 '18 at 15:47







    Good answer, but I'd mention what get<0>(ar) is parsed as without the template: (get<0)>(ar). This makes it more clear why the existence of the template get makes a difference.

    – Yakk - Adam Nevraumont
    Nov 20 '18 at 15:47















    4














    Note that this changed in C++20 as a result of P0846R0. An unqualified name followed by a < token for which ordinary unqualified lookup either finds one or more functions or finds nothing is now assumed to name a template and the < is parsed accordingly.






    share|improve this answer




























      4














      Note that this changed in C++20 as a result of P0846R0. An unqualified name followed by a < token for which ordinary unqualified lookup either finds one or more functions or finds nothing is now assumed to name a template and the < is parsed accordingly.






      share|improve this answer


























        4












        4








        4







        Note that this changed in C++20 as a result of P0846R0. An unqualified name followed by a < token for which ordinary unqualified lookup either finds one or more functions or finds nothing is now assumed to name a template and the < is parsed accordingly.






        share|improve this answer













        Note that this changed in C++20 as a result of P0846R0. An unqualified name followed by a < token for which ordinary unqualified lookup either finds one or more functions or finds nothing is now assumed to name a template and the < is parsed accordingly.







        share|improve this answer












        share|improve this answer



        share|improve this answer










        answered Nov 20 '18 at 18:54









        T.C.T.C.

        107k14220326




        107k14220326






























            draft saved

            draft discarded




















































            Thanks for contributing an answer to Stack Overflow!


            • Please be sure to answer the question. Provide details and share your research!

            But avoid



            • Asking for help, clarification, or responding to other answers.

            • Making statements based on opinion; back them up with references or personal experience.


            To learn more, see our tips on writing great answers.




            draft saved


            draft discarded














            StackExchange.ready(
            function () {
            StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fstackoverflow.com%2fquestions%2f53393467%2foverload-resolution-looking-into-namespaces%23new-answer', 'question_page');
            }
            );

            Post as a guest















            Required, but never shown





















































            Required, but never shown














            Required, but never shown












            Required, but never shown







            Required, but never shown

































            Required, but never shown














            Required, but never shown












            Required, but never shown







            Required, but never shown







            這個網誌中的熱門文章

            Xamarin.form Move up view when keyboard appear

            Post-Redirect-Get with Spring WebFlux and Thymeleaf

            Anylogic : not able to use stopDelay()