[basic.def]
Change: C++ does not have “tentative definitions” as in C
. , at file scope,
int i;
int i;
is valid in C, invalid in C++
.This makes it impossible to define
mutually referential file-local static objects, if initializers are
restricted to the syntactic forms of C
.For example,
struct X { int i; struct X* next; };
static struct X a;
static struct X b = { 0, &a };
static struct X a = { 1, &b };Rationale:
This avoids having different initialization rules for
fundamental types and user-defined types
. Effect on original feature:
Deletion of semantically well-defined feature
. Difficulty of converting:
Semantic transformation
. In C++, the initializer for one of a set of
mutually-referential file-local static objects must invoke a function
call to achieve the initialization
.Rationale:
Class scope is crucial to C++, and a struct is a class
. Effect on original feature:
Change to semantics of well-defined feature
. Difficulty of converting:
Semantic transformation
. 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. The latter is probably rare
.[basic.link] [also
[dcl.type]]
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
. 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
. This feature allows the user to put
const objects in source files that are included
in more than one translation unit
.Effect on original feature:
Change to semantics of well-defined feature
. Difficulty of converting:
Semantic transformation
. [basic.start.main]
Change: The
main function cannot be called recursively and cannot have its address taken
. Rationale:
The
main function may require special actions
. Effect on original feature:
Deletion of semantically well-defined feature
. Difficulty of converting:
Trivial: create an intermediary function such as
mymain(argc, argv). [basic.types]
Change: C allows “compatible types” in several places, C++ does not
.
For example,
otherwise-identical
struct types with different tag names
are “compatible” in C but are distinctly different types
in C++
.Rationale:
Stricter type checking is essential for C++
. Effect on original feature:
Deletion of semantically well-defined feature
. Difficulty of converting:
Semantic transformation
. The “typesafe linkage” mechanism will find many, but not all,
of such problems
.Those problems not found by typesafe linkage will continue to
function properly,
according to the “layout compatibility rules” of this
International Standard
.