[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)