[diff.cpp14] - C++23 → Trunk

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@@ -1,12 +1,12 @@
 ## C++ and ISO C++14 <a id="diff.cpp14">[[diff.cpp14]]</a>
 ### General <a id="diff.cpp14.general">[[diff.cpp14.general]]</a>
-Subclause [[diff.cpp14]] lists the differences between C++ and ISO C++14
-(ISO/IEC 14882:2014, *Programming Languages — C++*), in addition to
-those listed above, by the chapters of this document.
 ###  [[lex]]: lexical conventions <a id="diff.cpp14.lex">[[diff.cpp14.lex]]</a>
 **Change:** Removal of trigraph support as a required feature.
 **Rationale:** Prevents accidental uses of trigraphs in non-raw string
@@ -21,17 +21,21 @@ characters to the translation character set.
 **Rationale:** Necessary to enable
 *hexadecimal-floating-point-literal*s. **Effect on original feature:**
 Valid C++14 code may fail to compile or produce different results in
 this revision of C++. Specifically, character sequences like `0p+0` and
 `0e1_p+0` are three separate tokens each in C++14, but one single token
-in this revision of C++. For example:
 ``` cpp
 #define F(a) b ## a
 int b0p = F(0p+0);  // ill-formed; equivalent to ``int b0p = b0p + 0;'' in C++14{}
 ```
 ###  [[expr]]: expressions <a id="diff.cpp14.expr">[[diff.cpp14.expr]]</a>
 **Change:** Remove increment operator with `bool` operand.
 **Rationale:** Obsolete feature with occasionally surprising semantics.
 **Effect on original feature:** A valid C++14 expression utilizing the
@@ -44,11 +48,11 @@ feature:** In C++14 code that uses a *new-expression* to allocate an
 object with an over-aligned class type, where that class has no
 allocation functions of its own, `::operator new(std::size_t)` is used
 to allocate the memory. In this revision of C++,
 `::operator new(std::size_t, std::align_val_t)` is used instead.
-###  [[dcl.dcl]]: declarations <a id="diff.cpp14.dcl.dcl">[[diff.cpp14.dcl.dcl]]</a>
 **Change:** Removal of `register` *storage-class-specifier*.
 **Rationale:** Enable repurposing of deprecated keyword in future
 revisions of C++. **Effect on original feature:** A valid C++14
 declaration utilizing the `register` *storage-class-specifier* is
@@ -56,36 +60,46 @@ ill-formed in this revision of C++. The specifier can simply be removed
 to retain the original meaning.
 **Change:** `auto` deduction from *braced-init-list*. **Rationale:**
 More intuitive deduction behavior. **Effect on original feature:** Valid
 C++14 code may fail to compile or may change meaning in this revision of
-C++. For example:
 ``` cpp
 auto x1{1};         // was std::initializer_list<int>, now int
 auto x2{1, 2};      // was std::initializer_list<int>, now ill-formed
 ```
 **Change:** Make exception specifications be part of the type system.
 **Rationale:** Improve type-safety. **Effect on original feature:**
 Valid C++14 code may fail to compile or change meaning in this revision
-of C++. For example:
 ``` cpp
 void g1() noexcept;
 void g2();
 template<class T> int f(T *, T *);
 int x = f(g1, g2);              // ill-formed; previously well-formed
 ```
 **Change:** Definition of an aggregate is extended to apply to
 user-defined types with base classes. **Rationale:** To increase
 convenience of aggregate initialization. **Effect on original feature:**
 Valid C++14 code may fail to compile or produce different results in
 this revision of C++; initialization from an empty initializer list will
 perform aggregate initialization instead of invoking a default
-constructor for the affected types. For example:
 ``` cpp
 struct derived;
 struct base {
   friend struct derived;
@@ -96,20 +110,24 @@ struct derived : base {};
 derived d1{};       // error; the code was well-formed in C++14{}
 derived d2;         // still OK
 ```
 ###  [[class]]: classes <a id="diff.cpp14.class">[[diff.cpp14.class]]</a>
 **Change:** Inheriting a constructor no longer injects a constructor
 into the derived class. **Rationale:** Better interaction with other
 language features. **Effect on original feature:** Valid C++14 code that
 uses inheriting constructors may not be valid or may have different
 semantics. A *using-declaration* that names a constructor now makes the
 corresponding base class constructors visible to initializations of the
 derived class rather than declaring additional derived class
-constructors. For example:
 ``` cpp
 struct A {
   template<typename T> A(T, typename T::type = 0);
   A(int);
@@ -121,29 +139,34 @@ struct B : A {
 B b(42L);           // now calls B(int), used to call B<long>(long),
                     // which called A(int) due to substitution failure
                     // in A<long>(long).
 ```
 ###  [[temp]]: templates <a id="diff.cpp14.temp">[[diff.cpp14.temp]]</a>
-**Change:** Allowance to deduce from the type of a non-type template
 argument. **Rationale:** In combination with the ability to declare
-non-type template arguments with placeholder types, allows partial
-specializations to decompose from the type deduced for the non-type
 template argument. **Effect on original feature:** Valid C++14 code may
 fail to compile or produce different results in this revision of C++.
-For example:
 ``` cpp
 template <int N> struct A;
 template <typename T, T N> int foo(A<N> *) = delete;
 void foo(void *);
 void bar(A<0> *p) {
   foo(p);           // ill-formed; previously well-formed
 }
 ```
 ###  [[except]]: exception handling <a id="diff.cpp14.except">[[diff.cpp14.except]]</a>
 **Change:** Remove dynamic exception specifications. **Rationale:**
 Dynamic exception specifications were a deprecated feature that was
 complex and brittle in use. They interacted badly with the type system,
@@ -185,42 +208,51 @@ ill-formed and uses-allocator construction will not pass an allocator to
 **Change:** Different constraint on conversions from `unique_ptr`.
 **Rationale:** Adding array support to `shared_ptr`, via the syntax
 `shared_ptr<T[]>` and `shared_ptr<T[N]>`. **Effect on original
 feature:** Valid C++14 code may fail to compile or may change meaning in
-this revision of C++. For example:
 ``` cpp
 #include <memory>
 std::unique_ptr<int[]> arr(new int[1]);
 std::shared_ptr<int> ptr(std::move(arr));   // error: int(*)[] is not compatible with int*
 ```
 ###  [[strings]]: strings library <a id="diff.cpp14.string">[[diff.cpp14.string]]</a>
 **Change:** Non-const `.data()` member added. **Rationale:** The lack of
 a non-const `.data()` differed from the similar member of `std::vector`.
 This change regularizes behavior. **Effect on original feature:**
 Overloaded functions which have differing code paths for `char*` and
 `const char*` arguments will execute differently when called with a
-non-const string’s `.data()` member in this revision of C++. For
-example:
 ``` cpp
 int f(char *) = delete;
 int f(const char *);
 string s;
 int x = f(s.data());            // ill-formed; previously well-formed
 ```
 ###  [[containers]]: containers library <a id="diff.cpp14.containers">[[diff.cpp14.containers]]</a>
 **Change:** Requirements change: **Rationale:** Increase portability,
 clarification of associative container requirements. **Effect on
 original feature:** Valid C++14 code that attempts to use associative
 containers having a comparison object with non-const function call
-operator may fail to compile in this revision of C++. For example:
 ``` cpp
 #include <set>
 struct compare
@@ -235,10 +267,12 @@ int main() {
   const std::set<int, compare> s;
   s.find(0);
 }
 ```
 ###  [[depr]]: compatibility features <a id="diff.cpp14.depr">[[diff.cpp14.depr]]</a>
 **Change:** The class templates `auto_ptr`, `unary_function`, and
 `binary_function`, the function templates `random_shuffle`, and the
 function templates (and their return types) `ptr_fun`, `mem_fun`,

 ## C++ and ISO C++14 <a id="diff.cpp14">[[diff.cpp14]]</a>
 ### General <a id="diff.cpp14.general">[[diff.cpp14.general]]</a>
+Subclause [[diff.cpp14]] lists the differences between C++ and ISO
+C++14, in addition to those listed above, by the chapters of this
+document.
 ###  [[lex]]: lexical conventions <a id="diff.cpp14.lex">[[diff.cpp14.lex]]</a>
 **Change:** Removal of trigraph support as a required feature.
 **Rationale:** Prevents accidental uses of trigraphs in non-raw string
 **Rationale:** Necessary to enable
 *hexadecimal-floating-point-literal*s. **Effect on original feature:**
 Valid C++14 code may fail to compile or produce different results in
 this revision of C++. Specifically, character sequences like `0p+0` and
 `0e1_p+0` are three separate tokens each in C++14, but one single token
+in this revision of C++.
+[*Example 1*:
 ``` cpp
 #define F(a) b ## a
 int b0p = F(0p+0);  // ill-formed; equivalent to ``int b0p = b0p + 0;'' in C++14{}
 ```
+— *end example*]
 ###  [[expr]]: expressions <a id="diff.cpp14.expr">[[diff.cpp14.expr]]</a>
 **Change:** Remove increment operator with `bool` operand.
 **Rationale:** Obsolete feature with occasionally surprising semantics.
 **Effect on original feature:** A valid C++14 expression utilizing the
 object with an over-aligned class type, where that class has no
 allocation functions of its own, `::operator new(std::size_t)` is used
 to allocate the memory. In this revision of C++,
 `::operator new(std::size_t, std::align_val_t)` is used instead.
+###  [[dcl]]: declarations <a id="diff.cpp14.dcl.dcl">[[diff.cpp14.dcl.dcl]]</a>
 **Change:** Removal of `register` *storage-class-specifier*.
 **Rationale:** Enable repurposing of deprecated keyword in future
 revisions of C++. **Effect on original feature:** A valid C++14
 declaration utilizing the `register` *storage-class-specifier* is
 to retain the original meaning.
 **Change:** `auto` deduction from *braced-init-list*. **Rationale:**
 More intuitive deduction behavior. **Effect on original feature:** Valid
 C++14 code may fail to compile or may change meaning in this revision of
+C++.
+[*Example 1*:
 ``` cpp
 auto x1{1};         // was std::initializer_list<int>, now int
 auto x2{1, 2};      // was std::initializer_list<int>, now ill-formed
 ```
+— *end example*]
 **Change:** Make exception specifications be part of the type system.
 **Rationale:** Improve type-safety. **Effect on original feature:**
 Valid C++14 code may fail to compile or change meaning in this revision
+of C++.
+[*Example 2*:
 ``` cpp
 void g1() noexcept;
 void g2();
 template<class T> int f(T *, T *);
 int x = f(g1, g2);              // ill-formed; previously well-formed
 ```
+— *end example*]
 **Change:** Definition of an aggregate is extended to apply to
 user-defined types with base classes. **Rationale:** To increase
 convenience of aggregate initialization. **Effect on original feature:**
 Valid C++14 code may fail to compile or produce different results in
 this revision of C++; initialization from an empty initializer list will
 perform aggregate initialization instead of invoking a default
+constructor for the affected types.
+[*Example 3*:
 ``` cpp
 struct derived;
 struct base {
   friend struct derived;
 derived d1{};       // error; the code was well-formed in C++14{}
 derived d2;         // still OK
 ```
+— *end example*]
 ###  [[class]]: classes <a id="diff.cpp14.class">[[diff.cpp14.class]]</a>
 **Change:** Inheriting a constructor no longer injects a constructor
 into the derived class. **Rationale:** Better interaction with other
 language features. **Effect on original feature:** Valid C++14 code that
 uses inheriting constructors may not be valid or may have different
 semantics. A *using-declaration* that names a constructor now makes the
 corresponding base class constructors visible to initializations of the
 derived class rather than declaring additional derived class
+constructors.
+[*Example 1*:
 ``` cpp
 struct A {
   template<typename T> A(T, typename T::type = 0);
   A(int);
 B b(42L);           // now calls B(int), used to call B<long>(long),
                     // which called A(int) due to substitution failure
                     // in A<long>(long).
 ```
+— *end example*]
 ###  [[temp]]: templates <a id="diff.cpp14.temp">[[diff.cpp14.temp]]</a>
+**Change:** Allowance to deduce from the type of a constant template
 argument. **Rationale:** In combination with the ability to declare
+constant template arguments with placeholder types, allows partial
+specializations to decompose from the type deduced for the constant
 template argument. **Effect on original feature:** Valid C++14 code may
 fail to compile or produce different results in this revision of C++.
+[*Example 1*:
 ``` cpp
 template <int N> struct A;
 template <typename T, T N> int foo(A<N> *) = delete;
 void foo(void *);
 void bar(A<0> *p) {
   foo(p);           // ill-formed; previously well-formed
 }
 ```
+— *end example*]
 ###  [[except]]: exception handling <a id="diff.cpp14.except">[[diff.cpp14.except]]</a>
 **Change:** Remove dynamic exception specifications. **Rationale:**
 Dynamic exception specifications were a deprecated feature that was
 complex and brittle in use. They interacted badly with the type system,
 **Change:** Different constraint on conversions from `unique_ptr`.
 **Rationale:** Adding array support to `shared_ptr`, via the syntax
 `shared_ptr<T[]>` and `shared_ptr<T[N]>`. **Effect on original
 feature:** Valid C++14 code may fail to compile or may change meaning in
+this revision of C++.
+[*Example 1*:
 ``` cpp
 #include <memory>
 std::unique_ptr<int[]> arr(new int[1]);
 std::shared_ptr<int> ptr(std::move(arr));   // error: int(*)[] is not compatible with int*
 ```
+— *end example*]
 ###  [[strings]]: strings library <a id="diff.cpp14.string">[[diff.cpp14.string]]</a>
 **Change:** Non-const `.data()` member added. **Rationale:** The lack of
 a non-const `.data()` differed from the similar member of `std::vector`.
 This change regularizes behavior. **Effect on original feature:**
 Overloaded functions which have differing code paths for `char*` and
 `const char*` arguments will execute differently when called with a
+non-const string’s `.data()` member in this revision of C++.
+[*Example 1*:
 ``` cpp
 int f(char *) = delete;
 int f(const char *);
 string s;
 int x = f(s.data());            // ill-formed; previously well-formed
 ```
+— *end example*]
 ###  [[containers]]: containers library <a id="diff.cpp14.containers">[[diff.cpp14.containers]]</a>
 **Change:** Requirements change: **Rationale:** Increase portability,
 clarification of associative container requirements. **Effect on
 original feature:** Valid C++14 code that attempts to use associative
 containers having a comparison object with non-const function call
+operator may fail to compile in this revision of C++.
+[*Example 1*:
 ``` cpp
 #include <set>
 struct compare
   const std::set<int, compare> s;
   s.find(0);
 }
 ```
+— *end example*]
 ###  [[depr]]: compatibility features <a id="diff.cpp14.depr">[[diff.cpp14.depr]]</a>
 **Change:** The class templates `auto_ptr`, `unary_function`, and
 `binary_function`, the function templates `random_shuffle`, and the
 function templates (and their return types) `ptr_fun`, `mem_fun`,

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