[basic.def]
Change: C++ does not have “tentative definitions” as in C
E.g., 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.
Rationale:
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.
How widely used:
Seldom.
[basic.scope]
Change: A struct is a scope in C++, not in C
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 can be used as compile-time values in
C++, this feature urges programmers to provide explicit initializer
values for each const.
This feature allows the user to put constobjects in header files that are included
in many compilation units.
Effect on original feature:
Change to semantics of well-defined feature.
Difficulty of converting:
Semantic transformation
How widely used:
Seldom
[basic.start]
Change: Main 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).
How widely used:
Seldom
[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.
How widely used:
Common.