What is C++20's string literal operator template?












7















What is C++20's string literal operator template? Cppreference's example in this respect is quite concise and not very clear to me:



struct A { A(const char *); auto operator<=>(const A&) const = default; };



template<A a> A operator ""_a();


In trying to understand what this feature is I've just learned that you can have numeric literal operator templates in C++, which make each digit of numerical constant be passed as a non-type argument to a template (cf. a better explanation here). Currently, literal operator templates do not work with character literals, though there are compilers extensions enabling that. I don't think C++20's string literal operator templates have anything to do with that as I've learned that proposals to extend literal operator templates to work with character literals were voted down in the commitee?






c++ c++20 user-defined-literals







share|improve this question























  • Does the proposal answer your question?

    – Lightness Races in Orbit
    1 hour ago













  • @LightnessRacesinOrbit That's the version that got voted down.

    – cpplearner
    50 mins ago











  • Okay, how about this one?

    – Lightness Races in Orbit
    49 mins ago
















7















What is C++20's string literal operator template? Cppreference's example in this respect is quite concise and not very clear to me:



struct A { A(const char *); auto operator<=>(const A&) const = default; };



template<A a> A operator ""_a();


In trying to understand what this feature is I've just learned that you can have numeric literal operator templates in C++, which make each digit of numerical constant be passed as a non-type argument to a template (cf. a better explanation here). Currently, literal operator templates do not work with character literals, though there are compilers extensions enabling that. I don't think C++20's string literal operator templates have anything to do with that as I've learned that proposals to extend literal operator templates to work with character literals were voted down in the commitee?






c++ c++20 user-defined-literals







share|improve this question























  • Does the proposal answer your question?

    – Lightness Races in Orbit
    1 hour ago













  • @LightnessRacesinOrbit That's the version that got voted down.

    – cpplearner
    50 mins ago











  • Okay, how about this one?

    – Lightness Races in Orbit
    49 mins ago














7












7








7


1






What is C++20's string literal operator template? Cppreference's example in this respect is quite concise and not very clear to me:



struct A { A(const char *); auto operator<=>(const A&) const = default; };



template<A a> A operator ""_a();


In trying to understand what this feature is I've just learned that you can have numeric literal operator templates in C++, which make each digit of numerical constant be passed as a non-type argument to a template (cf. a better explanation here). Currently, literal operator templates do not work with character literals, though there are compilers extensions enabling that. I don't think C++20's string literal operator templates have anything to do with that as I've learned that proposals to extend literal operator templates to work with character literals were voted down in the commitee?






c++ c++20 user-defined-literals







share|improve this question














What is C++20's string literal operator template? Cppreference's example in this respect is quite concise and not very clear to me:



struct A { A(const char *); auto operator<=>(const A&) const = default; };



template<A a> A operator ""_a();


In trying to understand what this feature is I've just learned that you can have numeric literal operator templates in C++, which make each digit of numerical constant be passed as a non-type argument to a template (cf. a better explanation here). Currently, literal operator templates do not work with character literals, though there are compilers extensions enabling that. I don't think C++20's string literal operator templates have anything to do with that as I've learned that proposals to extend literal operator templates to work with character literals were voted down in the commitee?






c++ c++20 user-defined-literals




c++ c++20 user-defined-literals






share|improve this question













share|improve this question











share|improve this question




share|improve this question










asked 1 hour ago









lukeglukeg

1,6991825




1,6991825













  • Does the proposal answer your question?

    – Lightness Races in Orbit
    1 hour ago













  • @LightnessRacesinOrbit That's the version that got voted down.

    – cpplearner
    50 mins ago











  • Okay, how about this one?

    – Lightness Races in Orbit
    49 mins ago



















  • Does the proposal answer your question?

    – Lightness Races in Orbit
    1 hour ago













  • @LightnessRacesinOrbit That's the version that got voted down.

    – cpplearner
    50 mins ago











  • Okay, how about this one?

    – Lightness Races in Orbit
    49 mins ago

















Does the proposal answer your question?

– Lightness Races in Orbit
1 hour ago







Does the proposal answer your question?

– Lightness Races in Orbit
1 hour ago















@LightnessRacesinOrbit That's the version that got voted down.

– cpplearner
50 mins ago





@LightnessRacesinOrbit That's the version that got voted down.

– cpplearner
50 mins ago













Okay, how about this one?

– Lightness Races in Orbit
49 mins ago





Okay, how about this one?

– Lightness Races in Orbit
49 mins ago












1 Answer
1






active

oldest

votes


















6














There were two separate proposals:




  • Allowing string literals as non-type template parameters (P0424)

  • Allowing class types as non-type template parameters (P0732)


The first proposal was partially merged into the second. String literals still are not valid arguments as non-type template parameters, but they are valid arguments into class types. The example from [temp.arg.nontype]/4 might help:




template<class T, T p> class X {

  /* ... */

};



X<const char*, "Studebaker"> x; // error: string literal as template-argument



const char p = "Vivisectionist";

X<const char*, p> y;            // OK



struct A {

  constexpr A(const char*) {}

  friend auto operator<=>(const A&, const A&) = default;

};



X<A, "Pyrophoricity"> z;        // OK, string literal is a constructor argument to A



However, the part of the first proposal which extended the literal operators was what was merged into the second, [lex.ext]/5:




If S contains a literal operator template with a non-type template parameter for which str is a well-formed template-argument, the literal L is treated as a call of the form
operator "" X<str>()




So using this:



struct A { A(const char *); auto operator<=>(const A&) const = default; };     

template<A a> A operator ""_a() { return a; }


We can write "Hello"_a, which will be interpreted as calling operator "" _a<A("Hello")>.




Note that these rules are slightly in flux, as the defaulted <=> requirement will be changing to a defaulted == requirement as per P1185.






share|improve this answer
























  • Thanks for the explanation. As a sidenote, I must say that class-type being non-type parameter is one of many signs of this language's complexity.

    – lukeg
    24 mins ago






  • 1





    @lukeg I don't understand the comment. We've had non-type parameters for a long time. In array<int, 10>, 10 is a non-type template argument for the non-type template parameter N. It's just that the kinds of types you could use for those parameters was very, very limited. Non-type just refers to the fact that the parameter is not a type - it's a value. A better name would value template parameter.

    – Barry
    11 mins ago













  • @lukeg: It's not an increase in the language's complexity; it's an increase in the language's complexity that you care about. Before, you probably never gave a thought to non-type template parameters or what types you could use with them. Now you do, so it seems more complex. But it isn't; the complexity was always there. There is simply a dimension of template parameters that used to be highly restricted, and now it isn't.

    – Nicol Bolas
    6 mins ago













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%2f54278201%2fwhat-is-c20s-string-literal-operator-template%23new-answer', 'question_page');
}
);

Post as a guest















Required, but never shown

























1 Answer
1






active

oldest

votes








1 Answer
1






active

oldest

votes









active

oldest

votes






active

oldest

votes









6














There were two separate proposals:




  • Allowing string literals as non-type template parameters (P0424)

  • Allowing class types as non-type template parameters (P0732)


The first proposal was partially merged into the second. String literals still are not valid arguments as non-type template parameters, but they are valid arguments into class types. The example from [temp.arg.nontype]/4 might help:




template<class T, T p> class X {

  /* ... */

};



X<const char*, "Studebaker"> x; // error: string literal as template-argument



const char p = "Vivisectionist";

X<const char*, p> y;            // OK



struct A {

  constexpr A(const char*) {}

  friend auto operator<=>(const A&, const A&) = default;

};



X<A, "Pyrophoricity"> z;        // OK, string literal is a constructor argument to A



However, the part of the first proposal which extended the literal operators was what was merged into the second, [lex.ext]/5:




If S contains a literal operator template with a non-type template parameter for which str is a well-formed template-argument, the literal L is treated as a call of the form
operator "" X<str>()




So using this:



struct A { A(const char *); auto operator<=>(const A&) const = default; };     

template<A a> A operator ""_a() { return a; }


We can write "Hello"_a, which will be interpreted as calling operator "" _a<A("Hello")>.




Note that these rules are slightly in flux, as the defaulted <=> requirement will be changing to a defaulted == requirement as per P1185.






share|improve this answer
























  • Thanks for the explanation. As a sidenote, I must say that class-type being non-type parameter is one of many signs of this language's complexity.

    – lukeg
    24 mins ago






  • 1





    @lukeg I don't understand the comment. We've had non-type parameters for a long time. In array<int, 10>, 10 is a non-type template argument for the non-type template parameter N. It's just that the kinds of types you could use for those parameters was very, very limited. Non-type just refers to the fact that the parameter is not a type - it's a value. A better name would value template parameter.

    – Barry
    11 mins ago













  • @lukeg: It's not an increase in the language's complexity; it's an increase in the language's complexity that you care about. Before, you probably never gave a thought to non-type template parameters or what types you could use with them. Now you do, so it seems more complex. But it isn't; the complexity was always there. There is simply a dimension of template parameters that used to be highly restricted, and now it isn't.

    – Nicol Bolas
    6 mins ago


















6














There were two separate proposals:




  • Allowing string literals as non-type template parameters (P0424)

  • Allowing class types as non-type template parameters (P0732)


The first proposal was partially merged into the second. String literals still are not valid arguments as non-type template parameters, but they are valid arguments into class types. The example from [temp.arg.nontype]/4 might help:




template<class T, T p> class X {

  /* ... */

};



X<const char*, "Studebaker"> x; // error: string literal as template-argument



const char p = "Vivisectionist";

X<const char*, p> y;            // OK



struct A {

  constexpr A(const char*) {}

  friend auto operator<=>(const A&, const A&) = default;

};



X<A, "Pyrophoricity"> z;        // OK, string literal is a constructor argument to A



However, the part of the first proposal which extended the literal operators was what was merged into the second, [lex.ext]/5:




If S contains a literal operator template with a non-type template parameter for which str is a well-formed template-argument, the literal L is treated as a call of the form
operator "" X<str>()




So using this:



struct A { A(const char *); auto operator<=>(const A&) const = default; };     

template<A a> A operator ""_a() { return a; }


We can write "Hello"_a, which will be interpreted as calling operator "" _a<A("Hello")>.




Note that these rules are slightly in flux, as the defaulted <=> requirement will be changing to a defaulted == requirement as per P1185.






share|improve this answer
























  • Thanks for the explanation. As a sidenote, I must say that class-type being non-type parameter is one of many signs of this language's complexity.

    – lukeg
    24 mins ago






  • 1





    @lukeg I don't understand the comment. We've had non-type parameters for a long time. In array<int, 10>, 10 is a non-type template argument for the non-type template parameter N. It's just that the kinds of types you could use for those parameters was very, very limited. Non-type just refers to the fact that the parameter is not a type - it's a value. A better name would value template parameter.

    – Barry
    11 mins ago













  • @lukeg: It's not an increase in the language's complexity; it's an increase in the language's complexity that you care about. Before, you probably never gave a thought to non-type template parameters or what types you could use with them. Now you do, so it seems more complex. But it isn't; the complexity was always there. There is simply a dimension of template parameters that used to be highly restricted, and now it isn't.

    – Nicol Bolas
    6 mins ago
















6












6








6







There were two separate proposals:




  • Allowing string literals as non-type template parameters (P0424)

  • Allowing class types as non-type template parameters (P0732)


The first proposal was partially merged into the second. String literals still are not valid arguments as non-type template parameters, but they are valid arguments into class types. The example from [temp.arg.nontype]/4 might help:




template<class T, T p> class X {

  /* ... */

};



X<const char*, "Studebaker"> x; // error: string literal as template-argument



const char p = "Vivisectionist";

X<const char*, p> y;            // OK



struct A {

  constexpr A(const char*) {}

  friend auto operator<=>(const A&, const A&) = default;

};



X<A, "Pyrophoricity"> z;        // OK, string literal is a constructor argument to A



However, the part of the first proposal which extended the literal operators was what was merged into the second, [lex.ext]/5:




If S contains a literal operator template with a non-type template parameter for which str is a well-formed template-argument, the literal L is treated as a call of the form
operator "" X<str>()




So using this:



struct A { A(const char *); auto operator<=>(const A&) const = default; };     

template<A a> A operator ""_a() { return a; }


We can write "Hello"_a, which will be interpreted as calling operator "" _a<A("Hello")>.




Note that these rules are slightly in flux, as the defaulted <=> requirement will be changing to a defaulted == requirement as per P1185.






share|improve this answer













There were two separate proposals:




  • Allowing string literals as non-type template parameters (P0424)

  • Allowing class types as non-type template parameters (P0732)


The first proposal was partially merged into the second. String literals still are not valid arguments as non-type template parameters, but they are valid arguments into class types. The example from [temp.arg.nontype]/4 might help:




template<class T, T p> class X {

  /* ... */

};



X<const char*, "Studebaker"> x; // error: string literal as template-argument



const char p = "Vivisectionist";

X<const char*, p> y;            // OK



struct A {

  constexpr A(const char*) {}

  friend auto operator<=>(const A&, const A&) = default;

};



X<A, "Pyrophoricity"> z;        // OK, string literal is a constructor argument to A



However, the part of the first proposal which extended the literal operators was what was merged into the second, [lex.ext]/5:




If S contains a literal operator template with a non-type template parameter for which str is a well-formed template-argument, the literal L is treated as a call of the form
operator "" X<str>()




So using this:



struct A { A(const char *); auto operator<=>(const A&) const = default; };     

template<A a> A operator ""_a() { return a; }


We can write "Hello"_a, which will be interpreted as calling operator "" _a<A("Hello")>.




Note that these rules are slightly in flux, as the defaulted <=> requirement will be changing to a defaulted == requirement as per P1185.







share|improve this answer












share|improve this answer



share|improve this answer










answered 45 mins ago









BarryBarry

179k19310570




179k19310570













  • Thanks for the explanation. As a sidenote, I must say that class-type being non-type parameter is one of many signs of this language's complexity.

    – lukeg
    24 mins ago






  • 1





    @lukeg I don't understand the comment. We've had non-type parameters for a long time. In array<int, 10>, 10 is a non-type template argument for the non-type template parameter N. It's just that the kinds of types you could use for those parameters was very, very limited. Non-type just refers to the fact that the parameter is not a type - it's a value. A better name would value template parameter.

    – Barry
    11 mins ago













  • @lukeg: It's not an increase in the language's complexity; it's an increase in the language's complexity that you care about. Before, you probably never gave a thought to non-type template parameters or what types you could use with them. Now you do, so it seems more complex. But it isn't; the complexity was always there. There is simply a dimension of template parameters that used to be highly restricted, and now it isn't.

    – Nicol Bolas
    6 mins ago





















  • Thanks for the explanation. As a sidenote, I must say that class-type being non-type parameter is one of many signs of this language's complexity.

    – lukeg
    24 mins ago






  • 1





    @lukeg I don't understand the comment. We've had non-type parameters for a long time. In array<int, 10>, 10 is a non-type template argument for the non-type template parameter N. It's just that the kinds of types you could use for those parameters was very, very limited. Non-type just refers to the fact that the parameter is not a type - it's a value. A better name would value template parameter.

    – Barry
    11 mins ago













  • @lukeg: It's not an increase in the language's complexity; it's an increase in the language's complexity that you care about. Before, you probably never gave a thought to non-type template parameters or what types you could use with them. Now you do, so it seems more complex. But it isn't; the complexity was always there. There is simply a dimension of template parameters that used to be highly restricted, and now it isn't.

    – Nicol Bolas
    6 mins ago



















Thanks for the explanation. As a sidenote, I must say that class-type being non-type parameter is one of many signs of this language's complexity.

– lukeg
24 mins ago





Thanks for the explanation. As a sidenote, I must say that class-type being non-type parameter is one of many signs of this language's complexity.

– lukeg
24 mins ago




1




1





@lukeg I don't understand the comment. We've had non-type parameters for a long time. In array<int, 10>, 10 is a non-type template argument for the non-type template parameter N. It's just that the kinds of types you could use for those parameters was very, very limited. Non-type just refers to the fact that the parameter is not a type - it's a value. A better name would value template parameter.

– Barry
11 mins ago







@lukeg I don't understand the comment. We've had non-type parameters for a long time. In array<int, 10>, 10 is a non-type template argument for the non-type template parameter N. It's just that the kinds of types you could use for those parameters was very, very limited. Non-type just refers to the fact that the parameter is not a type - it's a value. A better name would value template parameter.

– Barry
11 mins ago















@lukeg: It's not an increase in the language's complexity; it's an increase in the language's complexity that you care about. Before, you probably never gave a thought to non-type template parameters or what types you could use with them. Now you do, so it seems more complex. But it isn't; the complexity was always there. There is simply a dimension of template parameters that used to be highly restricted, and now it isn't.

– Nicol Bolas
6 mins ago







@lukeg: It's not an increase in the language's complexity; it's an increase in the language's complexity that you care about. Before, you probably never gave a thought to non-type template parameters or what types you could use with them. Now you do, so it seems more complex. But it isn't; the complexity was always there. There is simply a dimension of template parameters that used to be highly restricted, and now it isn't.

– Nicol Bolas
6 mins ago




















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%2f54278201%2fwhat-is-c20s-string-literal-operator-template%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







-

Popular posts from this blog

How to label and detect the document text images

Image
1 $begingroup$ This is what I mean as document text image: I want to label the texts in image as separate blocks and my model should detect these labels as classes. NOTE: This is how the end result should be like: The labels like Block 1, Block 2, Block 3,.. should be Logo, Title, Date,.. Others, etc. Work done: First approach : I tried to implement this method via Object Detection, it didn't work. It didn't even detect any text. Second approach : Then I tried it using PixelLink. As this model is build for scene text detection, it detected each and every text in the image. But this method can detect multiple lines of text if the threshold values are increased. But I have no idea how do I add labels to the text blocks. PIXEL_CLS_WEIGHT_all_ones = 'PIXEL_CLS_WEIGHT_all_ones' PIXEL_C...
Read more

Vallis Paradisi

Image
Coordinata: .mw-parser-output .geo-default,.mw-parser-output .geo-dms,.mw-parser-output .geo-dec{display:inline}.mw-parser-output .geo-nondefault,.mw-parser-output .geo-multi-punct{display:none}.mw-parser-output .longitude,.mw-parser-output .latitude{white-space:nowrap} 33°03′S 71°37′W  /  33.05°S 71.617°W  / -33.05; -71.617 Vallis Paradisi Res apud Vicidata repertae: Civitas : Chilia Numerus incolarum : 296 655 Zona horaria : UTC-4 Situs interretialis Nomen officiale : Valparaíso Gubernium Consilium : municipal council of Valparaíso Geographia Superficies : 401 600 000 Metrum quadratum Coniunctiones urbium Urbes gemellae : Ovetum , Badalona , Santa Fe , Ora longa , Sciamhaevum , Quantunia , Callao , Bat Yam , Melaka , Novorossiysk , Busan , Corduba , Odessa , Rosario , Malacca , Barcino , Soteropolis , Franciscopolis (California) , Guanajuato, Silao Tabula aut despectus Despectus in portum sinumque et urbem Vallis Paradisi Vallis Para...
Read more

Tabula Rosettana

Image
Tabula multilinguis Rosettana in Museo Britannico ostenditur. Tabula Rosettana, [1] etiam titulo OGIS 90 agnita, est stela decreto de rebus sacris in Aegypto anno 196 a.C.n. lato inscripta. Tabula iuxta Rosettam Aegypti, urbem in delta Nili et ad oram maris Mediterranei iacentem, anno 1799 a milite Francico reperta est. Inventio stelae, linguis duabus scripturisque tribus inscriptae, eruditis Instituti Aegypti statim nuntiata est; ibi enim iussu imperatoris Napoleonis eruditi omnium scientiarum (sub aegide Commissionis Scientiarum et Artium) properaverant cum expeditione Francica. Qua a Britannis mox debellata, tabula Rosettana Londinium missa hodie apud Museum Britannicum iacet. Textus Graecus cito lectus interpretationi textuum Aegyptiorum (in formis hieroglyphica et demotica expressorum) gradatim adiuvit. Denique textum plene interpretatus est Ioannes Franciscus Champollion. Ab opere eruditorum cumulativo coepit hodiernus scripturae hieroglyphicae linguaeque Aegyptiae a...
Read more