DIGITAL C++ implements the C++ International Standard, with some differences, as described in the DIGITAL C++ Version 6.1 Release Notes for OpenVMS Alpha.

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, 3nd Edition indicate where you can find additional help.

For compatibility with certain C++ compilers, DIGITAL C++ supports the /standard=cfront and /standard=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 };

4.1.2 Member Access

Unlike some older C++ implementations, DIGITAL C++ strictly enforces accessibility rules for public, protected, and private members of a base class. For more information, see The C++ Programming Language, 3nd 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, 3nd 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, 3nd Edition.

For example, consider the following code fragment:

class C { static int i; }; //missing definition //int C::i = 5; 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

4.3 Functions and Function Declaration Considerations

DIGITAL C++ requires the use of function definitions as described in The C++ Programming Language, 3nd Edition. For examples of outdated syntax not allowed in DIGITAL C++, see The C++ Programming Language, 3nd 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, 3nd 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, 3nd 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, 3nd 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, 3nd 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, 3nd 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, 3nd 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, 3nd 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, 3nd 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; }

4.12 Flushing the Output Buffer

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 Access Violations

If you encounter an access violation (ACCVIO) while running your program, it might be because you built your binary with the wrong linker. Use the cxxlink command to build a binary; otherwise, the run-time startup and other crucial code will not build properly.

4.14 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.

4.15 Incrementing Enumerations

Some other C++ implementations let you perform integer arithmetic, including ++, on enumerated types; DIGITAL C++ does not allow this.

4.16 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:

• Variables that should be 32 bits in size on both 32-bit systems and 64-bit Alpha systems should be declared as int (not long). Even better would be to declare them as int-32, which is a macro you define to be int.
• Variables that should be 32 bits in size on a 32-bit system and 64 bits in size on a 64-bit Alpha system should be declared as long. Even better would be to declare them as long-int, which is a macro you define to be long.
• Check any variables in your code that are declared long. If the variable must be 32 bits in size, you must change its type to int.
• Variables declared as int should not be used to hold an address; sizeof (int) does not equal sizeof (char *) on Alpha systems.
• Remember that register variables and unsigned variables default to int (32 bits).
• Constants are 32-bit quantities by default. Performing shift operations or bit operations on constants will give 32-bit results. You must add L to the constant to get a 64-bit result. For example:

  long foo, bar; foo = 1L << bar;

• Assigning to a char is not atomic on Alpha systems. You will get a load of 32 or 64 bits, followed by byte operations to extract, mask, and shift the byte, followed by a store of 32 or 64 bits.
• Bit-fields declared as int on Alpha systems generate load/store 32 bits. Bit-fields declared as long on Alpha systems generate load/store 64 bits.
• If you do not explicitly declare the formal parameters to functions, their sizes may not match the caller sizes. The default is int, which truncates 64-bit addresses.
• The %d and %x format specifiers print 32 bits of data. Use %ld or %lx with printf to print 64 bits of data. You can use %p on both 32- and 64-bit systems to print the value of pointers.

For more information, see the ULTRIX to DIGITAL UNIX Migration Guide.