DEC C++ implements the C++ language as defined in The Annotated C++ Reference Manual, and closely follows that text's language definition.

This chapter describes ways to avoid having the DEC 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, 2nd Edition indicate where you can find additional help.

For compatibility with certain C++ compilers, DEC 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 };

4.1.2 Member Access

DEC C++ enforces accessibility rules for public, protected, and private members of a base class. For more information, see §r.11.2 of The C++ Programming Language, 2nd Edition.

4.1.3 Base Class Initializers

Unlike some older C++ implementations, DEC 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 §r.12.6.2 and §r.18.3.2 of The C++ Programming Language, 2nd Edition.

4.2 Undefined Global Symbols for Static Data Members

When a static data member is declared, the DEC 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 DEC C++ compiler does not support the declaration anachronism shown in §r.18.3 of The C++ Programming Language, 2nd Edition.

For example, consider the following code fragment:

int main () { int x; x=C::i; return 0; }

The DEC 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

DEC C++ requires the use of function definitions as described in §r.8.3 of The C++ Programming Language, 2nd Edition. For examples of outdated syntax not allowed in DEC C++, see §r.18.3.1 of The C++ Programming Language, 2nd 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 DEC C++.

4.4.1 Pointer Conversions

In DEC 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 §r.4.6 of The C++ Programming Language, 2nd 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 DEC C++. For more information on the illegality of casting bound pointers, see §18.3.4 of The C++ Programming Language, 2nd 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 DEC 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 §r.7.1.3 of The C++ Programming Language, 2nd Edition.

4.6 Initializing References

DEC 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 §r.8.4.3 of The C++ Programming Language, 2nd 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 §r.6.4.2 of The C++ Programming Language, 2nd 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 §r.7.1.6 of The C++ Programming Language, 2nd Edition.

4.9 Preprocessing

DEC 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 §r.16.5 of The C++ Programming Language, 2nd 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 §r.18.3.3 of The C++ Programming Language, 2nd 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, DEC 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 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

DEC C++ automatically treats files with a .c extension as C language source files and passes the files to the cc command. For DEC 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; DEC C++ does not allow this.

4.17 Overloading Disambiguation

DEC C++ applies the conversions specified in The Annotated C++ Reference Manual more strictly than do some C++ implementations. This can cause DEC C++ to interpret some calls to overloaded functions as ambiguous, even though some other C++ implementations may be able to process them. The following example shows one of these ambiguous calls to overloaded functions:

#include <iostream.hxx> enum logical { false, true }; logical false1 = false; /* const unsigned int */ unsigned int false2 = 0; const int false3 = 0; void foo(void*) { cout << "foo(void*)\\n"; } void foo(int) { cout << "foo(int)\\n"; } main() { foo(false1); // Constant expression that is not an exact match for // int. Can be converted using standard conversions // to both int and void* foo(false2); // not a constant expression foo(false3); // exact match of int }

In this example, false1 is ambiguous. As an unsigned const int with a value of 0, it can be converted either to void* or to int.

The following example shows how to properly declare the logical:

enum logical { false = 1, true };

4.18 Variables Declared More than Once

Some other C++ implementation might handle scoping differently from DEC 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. DEC C++ will flag this as an error.

4.19 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 MIPS 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 MIPS 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;

• Using a 0 where you should use NULL generates a 32-bit constant. On Alpha systems, this could yield 0 in the low 32 bits and useless data in the high 32 bits. Using NULL from the <stdio.h> header file provides the correct value on both MIPS and Alpha systems.
• 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.