[diff.cpp03] - C++23 → Trunk

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 ## C++ and ISO C++03 <a id="diff.cpp03">[[diff.cpp03]]</a>
 ### General <a id="diff.cpp03.general">[[diff.cpp03.general]]</a>
-Subclause [[diff.cpp03]] lists the differences between C++ and ISO C++03
-(ISO/IEC 14882:2003, *Programming Languages — C++*), in addition to
-those listed above, by the chapters of this document.
 ###  [[lex]]: lexical conventions <a id="diff.cpp03.lex">[[diff.cpp03.lex]]</a>
 **Change:** New kinds of *string-literal*s. **Rationale:** Required for
 new features. **Effect on original feature:** Valid C++03 code may fail
 to compile or produce different results in this revision of C++.
 Specifically, macros named `R`, `u8`, `u8R`, `u`, `uR`, `U`, `UR`, or
 `LR` will not be expanded when adjacent to a *string-literal* but will
-be interpreted as part of the *string-literal*. For example:
 ``` cpp
 #define u8 "abc"
 const char* s = u8"def";        // Previously "abcdef", now "def"
 ```
 **Change:** User-defined literal string support. **Rationale:** Required
 for new features. **Effect on original feature:** Valid C++03 code may
 fail to compile or produce different results in this revision of C++.
-For example:
 ``` cpp
 #define _x "there"
 "hello"_x           // #1
 ```
@@ -33,10 +38,12 @@ For example:
 Previously, \#1 would have consisted of two separate preprocessing
 tokens and the macro `_x` would have been expanded. In this revision of
 C++, \#1 consists of a single preprocessing token, so the macro is not
 expanded.
 **Change:** New keywords. **Rationale:** Required for new features.
 **Effect on original feature:** Added to [[lex.key]], the following
 identifiers are new keywords: `alignas`, `alignof`, `char16_t`,
 `char32_t`, `constexpr`, `decltype`, `noexcept`, `nullptr`,
 `static_assert`, and `thread_local`. Valid C++03 code using these
@@ -51,56 +58,133 @@ be represented by `long` could change from an unsigned integer type to
 **Change:** Only literals are integer null pointer constants.
 **Rationale:** Removing surprising interactions with templates and
 constant expressions. **Effect on original feature:** Valid C++03 code
 may fail to compile or produce different results in this revision of
-C++. For example:
 ``` cpp
 void f(void *);     // #1
 void f(...);        // #2
 template<int N> void g() {
   f(0*N);           // calls #2; used to call #1
 }
 ```
 **Change:** Specify rounding for results of integer `/` and `%`.
 **Rationale:** Increase portability, C99 compatibility. **Effect on
 original feature:** Valid C++03 code that uses integer division rounds
 the result toward 0 or toward negative infinity, whereas this revision
 of C++ always rounds the result toward 0.
 **Change:** `&&` is valid in a *type-name*. **Rationale:** Required for
 new features. **Effect on original feature:** Valid C++03 code may fail
-to compile or produce different results in this revision of C++. For
-example:
 ``` cpp
 bool b1 = new int && false;             // previously false, now ill-formed
 struct S { operator int(); };
 bool b2 = &S::operator int && false;    // previously false, now ill-formed
 ```
-###  [[dcl.dcl]]: declarations <a id="diff.cpp03.dcl.dcl">[[diff.cpp03.dcl.dcl]]</a>
 **Change:** Remove `auto` as a storage class specifier. **Rationale:**
 New feature. **Effect on original feature:** Valid C++03 code that uses
 the keyword `auto` as a storage class specifier may be invalid in this
 revision of C++. In this revision of C++, `auto` indicates that the type
 of a variable is to be deduced from its initializer expression.
 **Change:** Narrowing restrictions in aggregate initializers.
 **Rationale:** Catches bugs. **Effect on original feature:** Valid C++03
-code may fail to compile in this revision of C++. For example:
 ``` cpp
 int x[] = { 2.0 };
 ```
 This code is valid in C++03 but invalid in this revision of C++ because
 `double` to `int` is a narrowing conversion.
 ###  [[class]]: classes <a id="diff.cpp03.class">[[diff.cpp03.class]]</a>
 **Change:** Implicitly-declared special member functions are defined as
 deleted when the implicit definition would have been ill-formed.
 **Rationale:** Improves template argument deduction failure. **Effect on
@@ -128,65 +212,65 @@ ill-formed in this revision of C++.
 right angle brackets. **Rationale:** Considered a persistent but minor
 annoyance. Template aliases representing non-class types would
 exacerbate whitespace issues. **Effect on original feature:** Change to
 semantics of well-defined expression. A valid C++03 expression
 containing a right angle bracket (“`>`”) followed immediately by another
-right angle bracket may now be treated as closing two templates. For
-example:
 ``` cpp
 template <class T> struct X { };
 template <int N> struct Y { };
 X< Y< 1 >> 2 > > x;
 ```
 This code is valid in C++03 because “`>>`” is a right-shift operator,
 but invalid in this revision of C++ because “`>>`” closes two templates.
 **Change:** Allow dependent calls of functions with internal linkage.
 **Rationale:** Overly constrained, simplify overload resolution rules.
 **Effect on original feature:** A valid C++03 program can get a
 different result in this revision of C++.
 ###  [[library]]: library introduction <a id="diff.cpp03.library">[[diff.cpp03.library]]</a>
-**Affected:** [[library]] – [[thread]] **Change:** New reserved
 identifiers. **Rationale:** Required by new features. **Effect on
 original feature:** Valid C++03 code that uses any identifiers added to
 the C++ standard library by later revisions of C++ may fail to compile
 or produce different results in this revision of C++. A comprehensive
 list of identifiers used by the C++ standard library can be found in the
 Index of Library Names in this document.
 **Change:** New headers. **Rationale:** New functionality. **Effect on
 original feature:** The following C++ headers are new: `<array>`,
-`<atomic>`, `<chrono>`, , `<condition_variable>`, `<forward_list>`,
-`<future>`, `<initializer_list>`, `<mutex>`, `<random>`, `<ratio>`,
-`<regex>`, `<scoped_allocator>`, `<system_error>`, `<thread>`,
-`<tuple>`, `<typeindex>`, `<type_traits>`, `<unordered_map>`, and
-`<unordered_set>`. In addition the following C compatibility headers are
-new: `<cfenv>`, `<cinttypes>`, `<cstdint>`, and `<cuchar>`. Valid C++03
-code that `#include`s headers with these names may be invalid in this
-revision of C++.
-**Effect on original feature:** Function `swap` moved to a different
-header **Rationale:** Remove dependency on `<algorithm>` for `swap`.
-**Effect on original feature:** Valid C++03 code that has been compiled
-expecting swap to be in `<algorithm>` may have to instead include
-`<utility>`.
 **Change:** New reserved namespace. **Rationale:** New functionality.
 **Effect on original feature:** The global namespace `posix` is now
 reserved for standardization. Valid C++03 code that uses a top-level
 namespace `posix` may be invalid in this revision of C++.
 **Change:** Additional restrictions on macro names. **Rationale:** Avoid
 hard to diagnose or non-portable constructs. **Effect on original
 feature:** Names of attribute identifiers may not be used as macro
-names. Valid C++03 code that defines `override`, `final`,
-`carries_dependency`, or `noreturn` as macros is invalid in this
-revision of C++.
 ###  [[support]]: language support library <a id="diff.cpp03.language.support">[[diff.cpp03.language.support]]</a>
 **Change:** `operator new` may throw exceptions other than
 `std::bad_alloc`. **Rationale:** Consistent application of `noexcept`.
@@ -309,11 +393,11 @@ that uses implementation-specific knowledge about the binary
 representation of the required template specializations of
 `std::complex` may not be compatible with this revision of C++.
 ###  [[localization]]: localization library <a id="diff.cpp03.locale">[[diff.cpp03.locale]]</a>
-**Change:** The `num_get` facet recognizes hexadecimal floating point
 values. **Rationale:** Required by new feature. **Effect on original
 feature:** Valid C++03 code may have different behavior in this revision
 of C++.
 ###  [[input.output]]: input/output library <a id="diff.cpp03.input.output">[[diff.cpp03.input.output]]</a>
@@ -340,17 +424,20 @@ may execute differently in this revision of C++.
 **Change:** Flag types in `std::ios_base` are now bitmasks with values
 defined as constexpr static members. **Rationale:** Required for new
 features. **Effect on original feature:** Valid C++03 code that relies
 on `std::ios_base` flag types being represented as `std::bitset` or as
-an integer type may fail to compile with this revision of C++. For
-example:
 ``` cpp
 #include <iostream>
 int main() {
   int flag = std::ios_base::hex;
   std::cout.setf(flag);         // error: setf does not take argument of type int
 }
 ```

 ## C++ and ISO C++03 <a id="diff.cpp03">[[diff.cpp03]]</a>
 ### General <a id="diff.cpp03.general">[[diff.cpp03.general]]</a>
+Subclause [[diff.cpp03]] lists the differences between C++ and ISO
+C++03, in addition to those listed above, by the chapters of this
+document.
 ###  [[lex]]: lexical conventions <a id="diff.cpp03.lex">[[diff.cpp03.lex]]</a>
 **Change:** New kinds of *string-literal*s. **Rationale:** Required for
 new features. **Effect on original feature:** Valid C++03 code may fail
 to compile or produce different results in this revision of C++.
 Specifically, macros named `R`, `u8`, `u8R`, `u`, `uR`, `U`, `UR`, or
 `LR` will not be expanded when adjacent to a *string-literal* but will
+be interpreted as part of the *string-literal*.
+[*Example 1*:
 ``` cpp
 #define u8 "abc"
 const char* s = u8"def";        // Previously "abcdef", now "def"
 ```
+— *end example*]
 **Change:** User-defined literal string support. **Rationale:** Required
 for new features. **Effect on original feature:** Valid C++03 code may
 fail to compile or produce different results in this revision of C++.
+[*Example 2*:
 ``` cpp
 #define _x "there"
 "hello"_x           // #1
 ```
 Previously, \#1 would have consisted of two separate preprocessing
 tokens and the macro `_x` would have been expanded. In this revision of
 C++, \#1 consists of a single preprocessing token, so the macro is not
 expanded.
+— *end example*]
 **Change:** New keywords. **Rationale:** Required for new features.
 **Effect on original feature:** Added to [[lex.key]], the following
 identifiers are new keywords: `alignas`, `alignof`, `char16_t`,
 `char32_t`, `constexpr`, `decltype`, `noexcept`, `nullptr`,
 `static_assert`, and `thread_local`. Valid C++03 code using these
 **Change:** Only literals are integer null pointer constants.
 **Rationale:** Removing surprising interactions with templates and
 constant expressions. **Effect on original feature:** Valid C++03 code
 may fail to compile or produce different results in this revision of
+C++.
+[*Example 1*:
 ``` cpp
 void f(void *);     // #1
 void f(...);        // #2
 template<int N> void g() {
   f(0*N);           // calls #2; used to call #1
 }
 ```
+— *end example*]
+**Change:** Evaluation of operands in `typeid`. **Rationale:** Introduce
+additional expression value categories. **Effect on original feature:**
+Valid C++03 code that uses xvalues as operands for `typeid` may change
+behavior in this revision of C++.
+[*Example 2*:
+``` cpp
+void f() {
+  struct B {
+    B() {}
+    virtual ~B() { }
+  };
+  struct C { B b; };
+  typeid(C().b);    // unevaluated in C++03{}, evaluated in C++11{}
+}
+```
+— *end example*]
 **Change:** Specify rounding for results of integer `/` and `%`.
 **Rationale:** Increase portability, C99 compatibility. **Effect on
 original feature:** Valid C++03 code that uses integer division rounds
 the result toward 0 or toward negative infinity, whereas this revision
 of C++ always rounds the result toward 0.
 **Change:** `&&` is valid in a *type-name*. **Rationale:** Required for
 new features. **Effect on original feature:** Valid C++03 code may fail
+to compile or produce different results in this revision of C++.
+[*Example 3*:
 ``` cpp
 bool b1 = new int && false;             // previously false, now ill-formed
 struct S { operator int(); };
 bool b2 = &S::operator int && false;    // previously false, now ill-formed
 ```
+— *end example*]
+**Change:** Fewer copies in the conditional operator. **Rationale:**
+Introduce additional expression value categories. **Effect on original
+feature:** Valid C++03 code that uses xvalues as operands for the
+conditional operator may change behavior in this revision of C++.
+[*Example 4*:
+``` cpp
+void f() {
+  struct B {
+    B() {}
+    B(const B&) { }
+  };
+  struct D : B {};
+  struct BB { B b; };
+  struct DD { D d; };
+  true ? BB().b : DD().d;       // additional copy in C++03{}, no copy or move in C++11{}
+}
+```
+— *end example*]
+###  [[dcl]]: declarations <a id="diff.cpp03.dcl.dcl">[[diff.cpp03.dcl.dcl]]</a>
 **Change:** Remove `auto` as a storage class specifier. **Rationale:**
 New feature. **Effect on original feature:** Valid C++03 code that uses
 the keyword `auto` as a storage class specifier may be invalid in this
 revision of C++. In this revision of C++, `auto` indicates that the type
 of a variable is to be deduced from its initializer expression.
 **Change:** Narrowing restrictions in aggregate initializers.
 **Rationale:** Catches bugs. **Effect on original feature:** Valid C++03
+code may fail to compile in this revision of C++.
+[*Example 1*:
 ``` cpp
 int x[] = { 2.0 };
 ```
 This code is valid in C++03 but invalid in this revision of C++ because
 `double` to `int` is a narrowing conversion.
+— *end example*]
+**Change:** Names declared in an anonymous namespace changed from
+external linkage to internal linkage; language linkage applies to names
+with external linkage only. **Rationale:** Alignment with user
+expectations. **Effect on original feature:** Valid C++03 code may
+violate the one-definition rule [[basic.def.odr]] in this revision of
+C++.
+[*Example 2*:
+``` cpp
+namespace { extern "C" { extern int x; } }  // #1, previously external linkage and C language linkage,
+                                            // now internal linkage and C++{} language linkage
+namespace A { extern "C" int x = 42; }      // #2, external linkage and C language linkage
+int main(void) { return x; }
+```
+This code is valid in C++03, but `#2` is not a definition for `#1` in
+this revision of C++, violating the one-definition rule.
+— *end example*]
 ###  [[class]]: classes <a id="diff.cpp03.class">[[diff.cpp03.class]]</a>
 **Change:** Implicitly-declared special member functions are defined as
 deleted when the implicit definition would have been ill-formed.
 **Rationale:** Improves template argument deduction failure. **Effect on
 right angle brackets. **Rationale:** Considered a persistent but minor
 annoyance. Template aliases representing non-class types would
 exacerbate whitespace issues. **Effect on original feature:** Change to
 semantics of well-defined expression. A valid C++03 expression
 containing a right angle bracket (“`>`”) followed immediately by another
+right angle bracket may now be treated as closing two templates.
+[*Example 1*:
 ``` cpp
 template <class T> struct X { };
 template <int N> struct Y { };
 X< Y< 1 >> 2 > > x;
 ```
 This code is valid in C++03 because “`>>`” is a right-shift operator,
 but invalid in this revision of C++ because “`>>`” closes two templates.
+— *end example*]
 **Change:** Allow dependent calls of functions with internal linkage.
 **Rationale:** Overly constrained, simplify overload resolution rules.
 **Effect on original feature:** A valid C++03 program can get a
 different result in this revision of C++.
 ###  [[library]]: library introduction <a id="diff.cpp03.library">[[diff.cpp03.library]]</a>
+**Affected:** [[library]] – [[exec]] **Change:** New reserved
 identifiers. **Rationale:** Required by new features. **Effect on
 original feature:** Valid C++03 code that uses any identifiers added to
 the C++ standard library by later revisions of C++ may fail to compile
 or produce different results in this revision of C++. A comprehensive
 list of identifiers used by the C++ standard library can be found in the
 Index of Library Names in this document.
 **Change:** New headers. **Rationale:** New functionality. **Effect on
 original feature:** The following C++ headers are new: `<array>`,
+`<atomic>`, `<chrono>`, `<condition_variable>`, `<forward_list>`,
+`<future>`, , `<mutex>`, `<random>`, `<ratio>`, `<regex>`,
+`<scoped_allocator>`, `<system_error>`, `<thread>`, `<tuple>`, ,
+`<type_traits>`, `<unordered_map>`, and `<unordered_set>`. In addition
+the following C compatibility headers are new: `<cfenv>`, `<cinttypes>`,
+`<cstdint>`, and `<cuchar>`. Valid C++03 code that `#include`s headers
+with these names may be invalid in this revision of C++.
+**Change:** Function `swap` moved to a different header. **Rationale:**
+Remove dependency on `<algorithm>` for `swap`. **Effect on original
+feature:** Valid C++03 code that has been compiled expecting `swap` to
+be in `<algorithm>` may have to instead include `<utility>`.
 **Change:** New reserved namespace. **Rationale:** New functionality.
 **Effect on original feature:** The global namespace `posix` is now
 reserved for standardization. Valid C++03 code that uses a top-level
 namespace `posix` may be invalid in this revision of C++.
 **Change:** Additional restrictions on macro names. **Rationale:** Avoid
 hard to diagnose or non-portable constructs. **Effect on original
 feature:** Names of attribute identifiers may not be used as macro
+names. Valid C++03 code that defines `override`, `final`, or `noreturn`
+as macros is invalid in this revision of C++.
 ###  [[support]]: language support library <a id="diff.cpp03.language.support">[[diff.cpp03.language.support]]</a>
 **Change:** `operator new` may throw exceptions other than
 `std::bad_alloc`. **Rationale:** Consistent application of `noexcept`.
 representation of the required template specializations of
 `std::complex` may not be compatible with this revision of C++.
 ###  [[localization]]: localization library <a id="diff.cpp03.locale">[[diff.cpp03.locale]]</a>
+**Change:** The `num_get` facet recognizes hexadecimal floating-point
 values. **Rationale:** Required by new feature. **Effect on original
 feature:** Valid C++03 code may have different behavior in this revision
 of C++.
 ###  [[input.output]]: input/output library <a id="diff.cpp03.input.output">[[diff.cpp03.input.output]]</a>
 **Change:** Flag types in `std::ios_base` are now bitmasks with values
 defined as constexpr static members. **Rationale:** Required for new
 features. **Effect on original feature:** Valid C++03 code that relies
 on `std::ios_base` flag types being represented as `std::bitset` or as
+an integer type may fail to compile with this revision of C++.
+[*Example 1*:
 ``` cpp
 #include <iostream>
 int main() {
   int flag = std::ios_base::hex;
   std::cout.setf(flag);         // error: setf does not take argument of type int
 }
 ```
+— *end example*]

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