Previous | Contents | Index |
DIGITAL C++ implements the Final Draft International ANSI C++ standard (FDIS), with some differences, as described in the DIGITAL C++ Version 6.0 Release Notes for DIGITAL UNIX Systems.
This language differs significantly from The Annotated C++ Reference Manual, implemented by the Version 5.n compilers. When switching from a Version 5.n compiler, you might need to modify your source files, especially if you use the default language mode. In addition, language changes can affect the run-time behavior of your programs. If you want to compile existing source code with minimal source changes, compile using the -std arm option. See Chapter 7 for information on and changes to the Standard Library.
This chapter describes ways to avoid having the DIGITAL C++ compiler reject program code that previously worked with other C++ implementations that adhere less strictly to the C++ language definition. References to applicable portions of The C++ Programming Language, 3rd Edition indicate where you can find additional help.
For compatibility with certain C++ compilers, DIGITAL C++ supports
the -cfront and -ms options to the cxx command. These option direct the compiler
to interpret the source program according to certain rules followed by
other implementations of the C++ language. For more information, see
Section 2.1.
4.1 Using Classes
This section discusses porting issues pertaining to C++ classes.
4.1.1 Friend Declarations
When making friend declarations, use the elaborated form of type specifier. The following code fragment implements the legal and comments out the illegal friend declaration:
class Y; class Z; class X; //friend Y; ** not legal friend class Z; // legal }; |
DIGITAL C++ enforces accessibility rules for public, protected, and private members of a base class. For more
information, see The C++ Programming Language, 3rd Edition.
4.1.3 Base Class Initializers
Unlike some older C++ implementations, DIGITAL C++ requires you to use the base class name in the initializer for a derived class. The following code fragment implements a legal initializer and comments out an illegal initializer:
class Base { // ... public: Base (int); }; class Derived : public Base { // ... public: // Derived(int i) : (i) {/* ...*/} ** not legal Derived(int i) : Base(i) {/* ...*/} // ** legal, supplies class name }; |
For more information, see The C++ Programming Language, 3rd Edition.
4.2 Undefined Global Symbols for Static Data Members
When a static data member is declared, the DIGITAL C++ compiler issues a reference to the external identifier in the object code, which must be resolved by a definition. On DIGITAL UNIX systems, the DIGITAL C++ compiler does not support the declaration anachronism shown in The C++ Programming Language, 3rd Edition.
For example, consider the following code fragment:
int main () { int x; x=C::i; return 0; } |
The DIGITAL C++ compiler does not issue any messages during compilation; however, when you attempt to link a program containing this code, the linker issues a message similar to the following:
ld: Error: Undefined: i__1C |
DIGITAL C++ requires the use of function definitions as described in The C++ Programming Language, 3rd Edition. For examples of outdated syntax not allowed in DIGITAL C++, see The C++ Programming Language, 3rd Edition.
Because all linkage specifications for a name must agree, function prototypes are not permitted if the function is later declared as an inline function. The following code is an example of such a conflicting function declaration:
int f(); inline int f() { return l; } |
In this example, f is declared with both internal and external linkage, which causes a compiler error.
Similarly, the declaration int f(i,j)
causes an error even when the declaration is defined with the "C" linkage specification, because the linkage
specification has no effect on the semantics of the declaration.
4.4 Using Pointers
This section demonstrates how to use pointers effectively in
DIGITAL C++.
4.4.1 Pointer Conversions
In DIGITAL C++, you cannot implicitly convert a const pointer to a nonconstant pointer. For example, char* and const char* are not equivalent; explicitly performing such a cast can lead to unexpected results.
For more information, see The C++ Programming Language, 3rd Edition.
4.4.2 Bound Pointers
Binding a pointer to a member function with a particular object as an
argument to the function is not allowed in DIGITAL C++. For more
information on the illegality of casting bound pointers, see
The C++ Programming Language, 3rd Edition.
4.4.3 Constants in Function Returns
Because the return value cannot be an lvalue, a constant in a function return has no effect on the semantics of the return. However, using a constant in a function return does affect the type signature. For example:
static int f1( int a, int b) {;} const int (* const (f2[])) (int a, int b) = {f1}; |
In this example, the referenced type of the pointer value f1 in the initializer for f2[] is function (signed int, signed int), which returns signed int. This is incompatible with function (signed int, signed int), which returns const signed int.
You can omit the const of int because it affects only the constant return
signature.
4.4.4 Pointers to Constants
The following example shows a type mismatch between a pointer to a char and a pointer to a const char that some compilers, other than the DIGITAL C++ compiler, may not find:
void foo (const char* argv[]) {} int main() { static char* args[2] = {"foo","bar"}; /* 'In this statement, the referenced type of the pointer value "args" is "pointer to char"' which is not compatible with "pointer to const char"'*/ foo (args); return 0; } |
You can correct this example by changing static
char to static const char.
Use an explicit type cast to get an argument match only if no other
option is available; such a cast may break on some C++ implementations.
4.5 Using typedefs
Using a synonym after a class, struct, or union prefix is illegal. Using a synonym in the names for constructors and destructors within the class declaration itself is also illegal.
In the following example, the illegal typedef specifier is commented out:
typedef struct { /* ...*/ } foo; // typedef struct foo foobar; ** not legal |
For more information, see The C++ Programming Language, 3rd Edition.
4.6 Initializing References
DIGITAL C++ warns against initializing nonconstant references to refer to temporary objects. The following example demonstrates the problems that can result:
static void f() { int i = 5; i++; // OK int &ri = 23; ri++; // In the initializer for ri, the initialization of a // non-const reference requires a temporary for "23". } |
The issue of reference initialization arises most often in assignment operators and in copy constructors. Wherever possible, declare all reference arguments as const.
For more information, see The C++ Programming Language, 3rd Edition.
4.7 Using the switch and goto Statements
Branching around a declaration with an explicit or implicit initializer is not legal, unless the declaration is in an inner block that is completely bypassed. To satisfy this constraint, enclose the declaration in a block. For example:
int i; switch (i) { case 1: int l = 0; //not initialized at this case label myint m = 0; //not initialized at this case label { int j = 0; // legal within the braces myint m = 0; // legal within the braces } case 2: break; // ... } |
For more information on using the switch
statement, see of The C++ Programming Language, 3rd Edition.
4.8 Using Volatile Objects
You must supply the meaning of copy constructing and assigning from volatile objects, because the compiler generates no copy constructors or assignment operators that copy or assign from volatile objects. The following example contains examples of such errors, as noted in the comments:
class A { public: A() { } // A(volatile A&) { } // operator=(volatile A&) { return 0; } }; void foo() { volatile A va; A a; A cca(va); // error - cannot copy construct from volatile object a = va; // error - cannot assign from volatile object return; } |
For more information, see The C++ Programming Language, 3rd Edition.
4.9 Preprocessing
DIGITAL C++ allows identifiers, but not expressions, on the #ifdef preprocessor directive. For example:
// this is not legal // #ifdef KERNEL && !defined(__POSIX_SOURCE) |
The following is the legal alternative:
// use this instead #if defined(KERNEL) && !defined(__POSIX_SOURCE) |
For more information, see The C++ Programming Language, 3rd Edition.
4.10 Managing Memory
The proper way to manage memory for a class is to overload the new and delete operators. This is in contrast to some older C++ implementations, which let you manage memory through assignment to the this pointer.
For more information, see The C++ Programming Language, 3rd Edition.
Program developers must take care that any user-defined new operators always return pointers to
quadword aligned memory.
4.11 Size-of-Array Argument to delete Operator
If a size-of-array argument accompanies a delete operator, DIGITAL C++ ignores the argument and issues a warning. The following example includes an anachronistic use of the delete operator:
int main() { int *a = new int [20]; int *b = new int [20]; delete[20] a; //old-style; argument ignored, warning issued delete[] b; return 0; } |
Do not depend on the newline character (\n) to flush your terminal output buffer. A
previous stream implementation might have done so, but this behavior is
not in conformance with Version 2.0 of the AT&T iostream library. If you want to flush the
output buffer, use the endl manipulator
or the flush member function.
4.13 Missing Parenthesis Error Message
Situations occur in which a simple typographical error generates a missing parenthesis error message. In the following example, the class name CaseSensitive is incorrectly specified as Casesensitive in the constructor declaration:
class CaseSensitive { void Test( const Casesensitive &foo ); }; |
As the compiler parses the argument declaration list, it first sees
const, which it interprets as a type
specifier. The compiler then sees Casesensitive, which it interprets as a dname. Among the next legal tokens are the
equal sign, comma, and closing parenthesis. Upon finding an ampersand,
the compiler expects a closing parenthesis. With all other
possibilities exhausted, the compiler has what appears to be a legal
argument declaration list, after which the closing parenthesis is the
only allowable token. The compiler expected one thing but encountered
something else. Often, inserting newline characters can isolate the
offending token.
4.14 Segmentation Faults
If you encounter a segmentation fault (signal SEGV) while running your program, it might be
because you built your binary with the wrong linker. Use the cxx command, not the ld command, to build a binary; otherwise, the
run-time startup and other crucial code will not build properly.
4.15 Source File Extensions
DIGITAL C++ automatically treats files with a .c extension as C language source files and passes the files to the cc command. For DIGITAL C++ to compile them, your files must have one of the following extensions:
.cxx .CXX .cpp .CPP .cc .CC .C |
You also can use the -x option to direct
the compiler to ignore file-name extensions and treat all named files,
other than those with an .a or .o extension, as C++ source files.
4.16 Incrementing Enumerations
Some other C++ implementations let you perform integer arithmetic,
including ++, on enumerated types; DIGITAL C++ does not allow this.
4.17 Variables Declared More than Once
Some other C++ implementation might handle scoping differently from DIGITAL C++, as shown by the following code fragment:
foo(int dup) // first dup declared { // first dup used for(int dup; dup < 10; dup++) // second dup declared { // second dup used int dup; // third dup declared // third dup used } // first dup used } main() { foo(5); } |
In this example, dup is used three times:
once in the formal parameter declaration, once in the for statement declaration, and once in the
for block. The third declaration is
correct, because it is within a block. The second declaration, should
be flagged as an error because it is not within a separate block.
DIGITAL C++ will flag this as an error.
4.18 Guidelines for Writing Clean 64-Bit Code
Paying careful attention to data types can ensure that your code works on both 32-bit and 64-bit systems. Use the following guidelines to write clean 64-bit code:
long foo, bar; foo = 1L << bar; |
For more information, see the ULTRIX to DIGITAL UNIX Migration Guide.
Previous | Next | Contents | Index |