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cppdraft/diff/basic.md

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+[diff.basic]
+
+# Annex C (informative) Compatibility [[diff]](./#diff)
+
+## C.7 C++ and C [[diff.iso]](diff.iso#diff.basic)
+
+### C.7.3 [[basic]](basic "6 Basics"): basics [diff.basic]
+
+[1](#1)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/compatibility.tex#L2851)
+
+**Affected subclause:** [[basic.def]](basic.def)  
+
+
+**Change:** C++ does not have “tentative definitions” as in C.
+
+[*Example [1](#example-1)*:
+
+At file scope,int i;int i; is valid in C, invalid in C++[.](#1.sentence-1)
+
+— *end example*]
+
+This makes it impossible to define
+mutually referential file-local objects with static storage duration,
+if initializers are restricted to the syntactic forms of C[.](#1.sentence-2)
+
+[*Example [2](#example-2)*: struct X { int i; struct X* next; };
+
+static struct X a;static struct X b = { 0, &a };static struct X a = { 1, &b }; — *end example*]
+
+  
+
+
+**Rationale:** This avoids having different initialization rules for
+fundamental types and user-defined types[.](#1.sentence-3)
+
+  
+
+
+**Effect on original feature:** Deletion of semantically well-defined feature[.](#1.sentence-4)
+
+  
+
+
+**Difficulty of converting:** Semantic transformation[.](#1.sentence-5)
+
+In C++, the initializer for one of a set of
+mutually-referential file-local objects with static storage
+duration must invoke a function
+call to achieve the initialization[.](#1.sentence-6)
+
+  
+
+
+**How widely used:** Seldom[.](#1.sentence-7)
+
+[2](#2)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/compatibility.tex#L2888)
+
+**Affected subclause:** [[basic.scope]](basic.scope)  
+
+
+**Change:** A struct is a scope in C++, not in C.
+
+[*Example [3](#example-3)*:
+
+struct X {struct Y { int a; } b;};struct Y c; is valid in C but not in C++, which would require X​::​Y c;[.](#2.sentence-1)
+
+— *end example*]
+
+  
+
+**Rationale:** Class scope is crucial to C++, and a struct is a class[.](#2.sentence-3)
+
+  
+
+
+**Effect on original feature:** Change to semantics of well-defined feature[.](#2.sentence-4)
+
+  
+
+
+**Difficulty of converting:** Semantic transformation[.](#2.sentence-5)
+
+  
+
+
+**How widely used:** C programs use struct extremely frequently, but the
+change is only noticeable when struct, enumeration, or enumerator
+names are referred to outside the struct[.](#2.sentence-6)
+
+The latter is probably rare[.](#2.sentence-7)
+
+[3](#3)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/compatibility.tex#L2912)
+
+**Affected subclause:** [[basic.link]](basic.link) [also [[dcl.type]](dcl.type "9.2.9 Type specifiers")]  
+
+**Change:** A name of file scope that is explicitly declared const, and not explicitly
+declared extern, has internal linkage, while in C it would have external linkage[.](#3.sentence-2)
+
+  
+
+
+**Rationale:** Because const objects may be used as values during translation in
+C++, this feature urges programmers to provide an explicit initializer
+for each const object[.](#3.sentence-3)
+
+This feature allows the user to put const objects in source files that are included
+in more than one translation unit[.](#3.sentence-4)
+
+  
+
+
+**Effect on original feature:** Change to semantics of well-defined feature[.](#3.sentence-5)
+
+  
+
+
+**Difficulty of converting:** Semantic transformation[.](#3.sentence-6)
+
+  
+
+
+**How widely used:** Seldom[.](#3.sentence-7)
+
+[4](#4)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/compatibility.tex#L2929)
+
+**Affected subclause:** [[basic.start.main]](basic.start.main)  
+
+
+**Change:** The main function cannot be called recursively and cannot have its address taken[.](#4.sentence-1)
+
+  
+
+
+**Rationale:** The main function may require special actions[.](#4.sentence-2)
+
+  
+
+
+**Effect on original feature:** Deletion of semantically well-defined feature[.](#4.sentence-3)
+
+  
+
+
+**Difficulty of converting:** Trivial: create an intermediary function such asmymain(argc, argv)[.](#4.sentence-4)
+
+  
+
+
+**How widely used:** Seldom[.](#4.sentence-5)
+
+[5](#5)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/compatibility.tex#L2942)
+
+**Affected subclause:** [[basic.types]](basic.types)  
+
+
+**Change:** C allows “compatible types” in several places, C++ does not[.](#5.sentence-1)
+
+  
+
+
+For example,
+otherwise-identical struct types with different tag names
+are “compatible” in C but are distinctly different types
+in C++[.](#5.sentence-2)
+
+  
+
+
+**Rationale:** Stricter type checking is essential for C++[.](#5.sentence-3)
+
+  
+
+
+**Effect on original feature:** Deletion of semantically well-defined feature[.](#5.sentence-4)
+
+  
+
+
+**Difficulty of converting:** Semantic transformation[.](#5.sentence-5)
+
+The “typesafe linkage” mechanism will find many, but not all,
+of such problems[.](#5.sentence-6)
+
+Those problems not found by typesafe linkage will continue to
+function properly,
+according to the “layout compatibility rules” of this
+document[.](#5.sentence-7)
+
+  
+
+
+**How widely used:** Common[.](#5.sentence-8)