[lex.key]
Change: New Keywords
New keywords are added to C++;
see [lex.key].
Rationale:
These keywords were added in order to implement the new
semantics of C++.
Effect on original feature:
Change to semantics of well-defined feature.
Any ISO C programs that used any of these keywords as identifiers
are not valid C++ programs.
Difficulty of converting:
Syntactic transformation.
Converting one specific program is easy.
Converting a large collection
of related programs takes more work.
How widely used:
Common.
[lex.ccon]
Change: Type of character literal is changed from int to char
Rationale:
This is needed for improved overloaded function argument type
matching.
For example:
int function( int i ); int function( char c ); function( 'x' );
It is preferable that this call match the second version of
function rather than the first.
Effect on original feature:
Change to semantics of well-defined feature.
ISO C programs which depend on
sizeof('x') == sizeof(int)
will not work the same as C++ programs.
Difficulty of converting:
Simple.
How widely used:
Programs which depend upon sizeof('x') are probably rare.
Subclause [lex.string]:
Change: String literals made const
The type of a string literal is changed
from “array of char”
to “array of const char.”
The type of a char16_t string literal is changed
from “array of some-integer-type”
to “array of const char16_t.”
The type of a char32_t string literal is changed
from “array of some-integer-type”
to “array of const char32_t.”
The type of a wide string literal is changed
from “array of wchar_t”
to “array of const wchar_t.”
Rationale:
This avoids calling an inappropriate overloaded function,
which might expect to be able to modify its argument.
Effect on original feature:
Change to semantics of well-defined feature.
Difficulty of converting:
Syntactic transformation. The fix is to add a cast:
char* p = "abc"; // valid in C, invalid in C++ void f(char*) { char* p = (char*)"abc"; // OK: cast added f(p); f((char*)"def"); // OK: cast added }
How widely used:
Programs that have a legitimate reason to treat string literals
as pointers to potentially modifiable memory are probably rare.
[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.
[conv.ptr]
Change: Converting void* to a pointer-to-object type requires casting
char a[10];
void *b=a;
void foo() {
char *c=b;
}
ISO C will accept this usage of pointer to void being assigned
to a pointer to object type.
C++ will not.
Rationale:
C++ tries harder than C to enforce compile-time type safety.
Effect on original feature:
Deletion of semantically well-defined feature.
Difficulty of converting:
Could be automated.
Violations will be diagnosed by the C++ translator.
The
fix is to add a cast.
For example:
char *c = (char *) b;
How widely used:
This is fairly widely used but it is good
programming practice to add the cast when assigning pointer-to-void to pointer-to-object.
Some ISO C translators will give a warning
if the cast is not used.
[conv.ptr]
Change: Only pointers to non-const and non-volatile objects may be implicitly converted to void*
Rationale:
This improves type safety.
Effect on original feature:
Deletion of semantically well-defined feature.
Difficulty of converting:
Could be automated.
A C program containing such an implicit conversion from, e.g.,
pointer-to-const-object to void* will receive a diagnostic message.
The correction is to add an explicit cast.
How widely used:
Seldom.
[expr.call]
Change: Implicit declaration of functions is not allowed
Rationale:
The type-safe nature of C++.
Effect on original feature:
Deletion of semantically well-defined feature.
Note: the original feature was labeled as “obsolescent” in ISO C.
Difficulty of converting:
Syntactic transformation.
Facilities for producing explicit function declarations are fairly
widespread commercially.
How widely used:
Common.
[expr.sizeof], [expr.cast]
Change: Types must be declared in declarations, not in expressions
In C, a sizeof expression or cast expression may create a new type.
For example,
p = (void*)(struct x {int i;} *)0;
declares a new type, struct x .
Rationale:
This prohibition helps to clarify the location of
declarations in the source code.
Effect on original feature:
Deletion of a semantically well-defined feature.
Difficulty of converting:
Syntactic transformation.
How widely used:
Seldom.
[expr.cond], [expr.ass], [expr.comma]
Change: The result of a conditional expression, an assignment expression, or a comma expression may be an lvalue
Rationale:
C++ is an object-oriented language, placing relatively
more emphasis on lvalues. For example, functions may
return lvalues.
Effect on original feature:
Change to semantics of well-defined feature. Some C
expressions that implicitly rely on lvalue-to-rvalue
conversions will yield different results. For example,
char arr[100]; sizeof(0, arr)
yields
100
in C++ and
sizeof(char*)
in C.
Difficulty of converting:
Programs must add explicit casts to the appropriate rvalue.
How widely used:
Rare.
[stmt.switch], [stmt.goto]
Change: It is now invalid to jump past a declaration with explicit or implicit initializer (except across entire block not entered)
Rationale:
Constructors used in initializers may allocate
resources which need to be de-allocated upon leaving the
block.
Allowing jump past initializers would require
complicated run-time determination of allocation.
Furthermore, any use of the uninitialized object could be a
disaster.
With this simple compile-time rule, C++ assures that
if an initialized variable is in scope, then it has assuredly been
initialized.
Effect on original feature:
Deletion of semantically well-defined feature.
Difficulty of converting:
Semantic transformation.
How widely used:
Seldom.
[stmt.return]
Change: It is now invalid to return (explicitly or implicitly) from a function which is
declared to return a value without actually returning a value
Rationale:
The caller and callee may assume fairly elaborate
return-value mechanisms for the return of class objects.
If
some flow paths execute a return without specifying any value,
the implementation must embody many more complications.
Besides,
promising to return a value of a given type, and then not returning
such a value, has always been recognized to be a questionable
practice, tolerated only because very-old C had no distinction between
void functions and int functions.
Effect on original feature:
Deletion of semantically well-defined feature.
Difficulty of converting:
Semantic transformation.
Add an appropriate return value to the source code, such as zero.
How widely used:
Seldom.
For several years, many existing C implementations have produced warnings in
this case.
[dcl.stc]
Change: In C++, the static or extern specifiers can only be applied to names of objects or functions
Using these specifiers with type declarations is illegal in C++.
In C, these specifiers are ignored when used on type declarations.
Example:
static struct S { // valid C, invalid in C++
int i;
};
Rationale:
Storage class specifiers don't have any meaning when associated
with a type.
In C++, class members can be declared with the static storage
class specifier.
Allowing storage class specifiers on type
declarations could render the code confusing for users.
Effect on original feature:
Deletion of semantically well-defined feature.
Difficulty of converting:
Syntactic transformation.
How widely used:
Seldom.
[dcl.typedef]
Change: A C++ typedef name must be different from any class type name declared
in the same scope (except if the typedef is a synonym of the class name with the
same name). In C, a typedef name and a struct tag name declared in the same scope
can have the same name (because they have different name spaces)
Example:
typedef struct name1 { /*...*/ } name1; // valid C and C++
struct name { /*...*/ };
typedef int name; // valid C, invalid C++
Rationale:
For ease of use, C++ doesn't require that a type name be prefixed
with the keywords class, struct or union when used in object
declarations or type casts.
Example:
class name { /*...*/ };
name i; // i has type class name
Effect on original feature:
Deletion of semantically well-defined feature.
Difficulty of converting:
Semantic transformation.
One of the 2 types has to be renamed.
How widely used:
Seldom.
[dcl.type] [see also [basic.link]]
Change: const objects must be initialized in C++ but can be left uninitialized in C
Rationale:
A const object cannot be assigned to so it must be initialized
to hold a useful value.
Effect on original feature:
Deletion of semantically well-defined feature.
Difficulty of converting:
Semantic transformation.
How widely used:
Seldom.
[dcl.type]
Change: Banning implicit int
In C++ a decl-specifier-seq must contain a type-specifier, unless it is followed by a declarator for a constructor, a destructor, or a conversion function. In the following example, the left-hand column presents valid C; the right-hand column presents equivalent C++ :
void f(const parm); void f(const int parm);
const n = 3; const int n = 3;
main() int main()
/* ... */ /* ... */
Rationale:
In C++, implicit int creates several opportunities for
ambiguity between expressions involving function-like
casts and declarations.
Explicit declaration is increasingly considered
to be proper style.
Liaison with WG14 (C) indicated support for (at least)
deprecating implicit int in the next revision of C.
Effect on original feature:
Deletion of semantically well-defined feature.
Difficulty of converting:
Syntactic transformation.
Could be automated.
How widely used:
Common.
[dcl.spec.auto]
Change:
The keyword auto cannot be used as a storage class specifier.
void f() {
auto int x; // valid C, invalid C++
}
Rationale: Allowing the use of auto to deduce the type
of a variable from its initializer results in undesired interpretations of
auto as a storage class specifier in certain contexts.
Effect on original feature: Deletion of semantically well-defined feature.
Difficulty of converting: Syntactic transformation.
How widely used: Rare.
[dcl.enum]
Change: C++ objects of enumeration type can only be assigned values of the same enumeration type.
In C, objects of enumeration type can be assigned values of any integral type
Example:
enum color { red, blue, green };
enum color c = 1; // valid C, invalid C++
Rationale:
The type-safe nature of C++.
Effect on original feature:
Deletion of semantically well-defined feature.
Difficulty of converting:
Syntactic transformation.
(The type error produced by the assignment can be automatically
corrected by applying an explicit cast.)
How widely used:
Common.
[dcl.enum]
Change: In C++, the type of an enumerator is its enumeration. In C, the type of an enumerator is int.
Example:
enum e { A };
sizeof(A) == sizeof(int) // in C
sizeof(A) == sizeof(e) // in C++
/* and sizeof(int) is not necessarily equal to sizeof(e) */
Rationale:
In C++, an enumeration is a distinct type.
Effect on original feature:
Change to semantics of well-defined feature.
Difficulty of converting:
Semantic transformation.
How widely used:
Seldom.
The only time this affects existing C code is when the size of an
enumerator is taken.
Taking the size of an enumerator is not a
common C coding practice.
[dcl.fct]
Change: In C++, a function declared with an empty parameter list takes no arguments.
In C, an empty parameter list means that the number and type of the function arguments are unknown.
Example:
int f(); // means int f(void) in C++ // int f( unknown ) in C
Rationale:
This is to avoid erroneous function calls (i.e., function calls
with the wrong number or type of arguments).
Effect on original feature:
Change to semantics of well-defined feature.
This feature was marked as “obsolescent” in C.
Difficulty of converting:
Syntactic transformation.
The function declarations using C incomplete declaration style must
be completed to become full prototype declarations.
A program may need to be updated further if different calls to the
same (non-prototype) function have different numbers of arguments or
if the type of corresponding arguments differed.
How widely used:
Common.
[dcl.fct] [see [expr.sizeof]]
Change: In C++, types may not be defined in return or parameter types. In C, these type definitions are allowed
Example:
void f( struct S { int a; } arg ) {} // valid C, invalid C++
enum E { A, B, C } f() {} // valid C, invalid C++
Rationale:
When comparing types in different compilation units, C++ relies
on name equivalence when C relies on structural equivalence.
Regarding parameter types: since the type defined in an parameter list
would be in the scope of the function, the only legal calls in C++
would be from within the function itself.
Effect on original feature:
Deletion of semantically well-defined feature.
Difficulty of converting:
Semantic transformation.
The type definitions must be moved to file scope, or in header files.
How widely used:
Seldom.
This style of type definitions is seen as poor coding style.
[dcl.fct.def]
Change: In C++, the syntax for function definition excludes the “old-style” C function.
In C, “old-style” syntax is allowed, but deprecated as “obsolescent.”
Rationale:
Prototypes are essential to type safety.
Effect on original feature:
Deletion of semantically well-defined feature.
Difficulty of converting:
Syntactic transformation.
How widely used:
Common in old programs, but already known to be obsolescent.
[dcl.init.string]
Change: In C++, when initializing an array of character with a string, the number of
characters in the string (including the terminating '\0') must not exceed the
number of elements in the array. In C, an array can be initialized with a string even if
the array is not large enough to contain the string-terminating '\0'
Example:
char array[4] = "abcd"; // valid C, invalid C++
Rationale:
When these non-terminated arrays are manipulated by standard
string routines, there is potential for major catastrophe.
Effect on original feature:
Deletion of semantically well-defined feature.
Difficulty of converting:
Semantic transformation.
The arrays must be declared one element bigger to contain the
string terminating '\0'.
How widely used:
Seldom.
This style of array initialization is seen as poor coding style.
[class.name] [see also [dcl.typedef]]
Change: In C++, a class declaration introduces the class name into the scope where it is
declared and hides any object, function or other declaration of that name in an enclosing
scope. In C, an inner scope declaration of a struct tag name never hides the name of an
object or function in an outer scope
Example:
int x[99];
void f() {
struct x { int a; };
sizeof(x); /* size of the array in C */
/* size of the struct in C++ */
}
Rationale:
This is one of the few incompatibilities between C and C++ that
can be attributed to the new C++ name space definition where a
name can be declared as a type and as a non-type in a single scope
causing the non-type name to hide the type name and requiring that
the keywords class, struct, union or enum be used to refer to the type name.
This new name space definition provides important notational
conveniences to C++ programmers and helps making the use of the
user-defined types as similar as possible to the use of fundamental
types.
The advantages of the new name space definition were judged to
outweigh by far the incompatibility with C described above.
Effect on original feature:
Change to semantics of well-defined feature.
Difficulty of converting:
Semantic transformation.
If the hidden name that needs to be accessed is at global scope,
the :: C++ operator can be used.
If the hidden name is at block scope, either the type or the struct
tag has to be renamed.
How widely used:
Seldom.
[class.nest]
Change: In C++, the name of a nested class is local to its enclosing class. In C
the name of the nested class belongs to the same scope as the name of the outermost enclosing class.
Example:
struct X {
struct Y { /* ... */ } y;
};
struct Y yy; // valid C, invalid C++
Rationale:
C++ classes have member functions which require that classes
establish scopes.
The C rule would leave classes as an incomplete scope mechanism
which would prevent C++ programmers from maintaining locality
within a class.
A coherent set of scope rules for C++ based on the C rule would
be very complicated and C++ programmers would be unable to predict
reliably the meanings of nontrivial examples involving nested or
local functions.
Effect on original feature:
Change of semantics of well-defined feature.
Difficulty of converting:
Semantic transformation.
To make the struct type name visible in the scope of the enclosing
struct, the struct tag could be declared in the scope of the
enclosing struct, before the enclosing struct is defined.
Example:
struct Y; // struct Y and struct X are at the same scope struct X { struct Y { /* ... */ } y; };
All the definitions of C struct types enclosed in other struct
definitions and accessed outside the scope of the enclosing
struct could be exported to the scope of the enclosing struct.
Note: this is a consequence of the difference in scope rules,
which is documented in [basic.scope].
How widely used:
Seldom.
[class.nested.type]
Change: In C++, a typedef name may not be redeclared in a class definition after being used in that definition
Example:
typedef int I;
struct S {
I i;
int I; // valid C, invalid C++
};
Rationale:
When classes become complicated, allowing such a redefinition
after the type has been used can create confusion for C++
programmers as to what the meaning of 'I' really is.
Effect on original feature:
Deletion of semantically well-defined feature.
Difficulty of converting:
Semantic transformation.
Either the type or the struct member has to be renamed.
How widely used:
Seldom.
[class.copy]
Change: Copying volatile objects
The implicitly-declared copy constructor and implicitly-declared copy assignment operator cannot make a copy of a volatile lvalue. For example, the following is valid in ISO C:
struct X { int i; };
volatile struct X x1 = {0};
struct X x2(x1); // invalid C++
struct X x3;
x3 = x1; // also invalid C++
Rationale:
Several alternatives were debated at length.
Changing the parameter to
volatile
const
X&
would greatly complicate the generation of
efficient code for class objects.
Discussion of
providing two alternative signatures for these
implicitly-defined operations raised
unanswered concerns about creating
ambiguities and complicating
the rules that specify the formation of
these operators according to the bases and
members.
Effect on original feature:
Deletion of semantically well-defined feature.
Difficulty of converting:
Semantic transformation.
If volatile semantics are required for the copy,
a user-declared constructor or assignment must
be provided. [ Note: This user-declared
constructor may be explicitly defaulted. — end note ]
If non-volatile semantics are required,
an explicit
const_cast
can be used.
How widely used:
Seldom.
[cpp.predefined]
Change: Whether __STDC__ is defined and if so, what its value is, are
implementation-defined
Rationale:
C++ is not identical to ISO C.
Mandating that __STDC__
be defined would require that translators make an incorrect claim.
Each implementation must choose the behavior that will be most
useful to its marketplace.
Effect on original feature:
Change to semantics of well-defined feature.
Difficulty of converting:
Semantic transformation.
How widely used:
Programs and headers that reference __STDC__ are
quite common.