Mixing
What is C++20's string literal operator template?
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
asked Jan 20 at 15:54
lukeglukeg
1,81011027
add a comment |
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
asked Jan 20 at 15:54
lukeglukeg
1,81011027
Does the proposal answer your question?
– Lightness Races in Orbit
Jan 20 at 15:58
@LightnessRacesinOrbit That's the version that got voted down.
– cpplearner
Jan 20 at 16:08
Okay, how about this one?
– Lightness Races in Orbit
Jan 20 at 16:10
add a comment |
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
asked Jan 20 at 15:54
lukeglukeg
1,81011027
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
asked Jan 20 at 15:54
lukeglukeg
1,81011027
asked Jan 20 at 15:54
lukeglukeg
1,81011027
asked Jan 20 at 15:54
lukeglukeg
1,81011027
asked Jan 20 at 15:54
lukeglukeg
1,81011027
asked Jan 20 at 15:54
lukeglukeg
1,81011027
1,81011027
Does the proposal answer your question?
– Lightness Races in Orbit
Jan 20 at 15:58
@LightnessRacesinOrbit That's the version that got voted down.
– cpplearner
Jan 20 at 16:08
Okay, how about this one?
– Lightness Races in Orbit
Jan 20 at 16:10
add a comment |
Does the proposal answer your question?
– Lightness Races in Orbit
Jan 20 at 15:58
@LightnessRacesinOrbit That's the version that got voted down.
– cpplearner
Jan 20 at 16:08
Okay, how about this one?
– Lightness Races in Orbit
Jan 20 at 16:10
Does the proposal answer your question?
– Lightness Races in Orbit
Jan 20 at 15:58
Does the proposal answer your question?
– Lightness Races in Orbit
Jan 20 at 15:58
@LightnessRacesinOrbit That's the version that got voted down.
– cpplearner
Jan 20 at 16:08
@LightnessRacesinOrbit That's the version that got voted down.
– cpplearner
Jan 20 at 16:08
Okay, how about this one?
– Lightness Races in Orbit
Jan 20 at 16:10
Okay, how about this one?
– Lightness Races in Orbit
Jan 20 at 16:10
add a comment |
1 Answer
1
active
oldest
votes
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.
answered Jan 20 at 16:13
BarryBarry
183k21320588
add a comment |
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
});
}
});
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
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
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.
answered Jan 20 at 16:13
BarryBarry
183k21320588
add a comment |
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.
answered Jan 20 at 16:13
BarryBarry
183k21320588
add a comment |
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.
answered Jan 20 at 16:13
BarryBarry
183k21320588
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.
answered Jan 20 at 16:13
BarryBarry
183k21320588
answered Jan 20 at 16:13
BarryBarry
183k21320588
answered Jan 20 at 16:13
BarryBarry
183k21320588
answered Jan 20 at 16:13
BarryBarry
183k21320588
183k21320588
add a comment |
add a comment |
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.
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
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
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
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
Does the proposal answer your question?
– Lightness Races in Orbit
Jan 20 at 15:58
@LightnessRacesinOrbit That's the version that got voted down.
– cpplearner
Jan 20 at 16:08
Okay, how about this one?
– Lightness Races in Orbit
Jan 20 at 16:10