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HP OpenVMS SystemsC++ Programming Language |
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HP C++
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This appendix describes HP C++ compatibility with other C++ compilers, and documents compatibility concerns between the Version 5.n and Version 6.n compilers.
For porting and compatibility between Alpha and I64 systems, see Chapter 4.
HP C++ implements the C++ International Standard, with some differences, as described in the C++ release notes.
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 /STANDARD=ARM qualifier option. See Chapter 7 for information on and changes to the Standard Library.
This chapter describes ways to avoid having the 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.
In default mode (/STANDARD=RELAXED), the compiler implements most features of the C++ International Standard, including:
For compatibility with previous versions, the compiler provides the following language mode options:
/STANDARD=RELAXED
Specify this option if you want an ANSI C++ compiler that supports some commonly used extensions and is somewhat less strict than the standard. This is the default compiler mode. Please note that /STANDARD=ANSI is accepted as a synonym for /STANDARD=RELAXED to be compatible with previous compiler versions.If you want to use RELAXED mode but find that the compiler generates too many diagnostics in that mode, you can use the /QUIET option with the /STANDARD=RELAXED option. The /QUIET option relaxes error checking and suppresses or reduces the severity of many diagnostics. It also suppresses many warnings that are generated in RELAXED mode but were not issued by Version 5.n compilers. For information on message control options, see Section 2.5.
/STANDARD=ARM
Specify this option if you want to compile programs developed using Version 5.n and want to minimize source changes.
HP C++ Version 6.n and higher also provides support for other C++ dialects and language modes. You can specify the following options:
/STANDARD=MS
Specify this option if you want the compiler to accept additional Microsoft Visual C++ extensions./STANDARD=STRICT_ANSI
Enforce the ANSI standard strictly but permit some ANSI violations that should be errors to be warnings. To force ANSI violations to be issued with Error instead of Warning severity, use /WARNINGS=ANSI_ERRORS in addition to /STANDARD=STRICT_ANSI./STANDARD=LATEST
Use the latest C standard dialect. /STANDARD=LATEST is currently equivalent to /STANDARD=C99, but is subject to change when newer versions of the C standard are released.
With /STANDARD=MS you may also want to specify /QUIET to reduce the number of diagnostic messages generated.
This section provides details about differences between the Version 6.n and later compilers, and the Version 5.6 and earlier compilers:
Users should be aware of the following language differences between Version 6.n and higher (denoted simply as Version 6.n in the following list), and previous versions of the compiler.
bool, const_cast, explicit, export, false, mutable, dynamic_cast, reinterpret_cast, static_cast, true, typeid, typename, wchar_t |
and, and_eq, bitand, bitor, compl, not, not_eq, or, or_eq, xor, xor_eq |
typedef void (*PF)(); // Pointer to an extern "C++" function extern "C" typedef void (*PCF)(); // Pointer to an extern "C" function void f(PF); void f(PCF); |
#include <iostream.h>
struct A {
int a;
};
struct D : public virtual A
{
A* toA(){ return (A*) this; }
};
main ()
{
D* d = NULL;
A* ad = d->toA(); // will ACCVIO
if (ad==NULL) cout << "ok";
}
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typedef void Z::* any_ptom; |
Users should be aware of the following implementation differences between Version 6.n compilers, and previous versions of the compiler:
f(const int p1);
f(int p1);
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f(i++, i++);
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The template instantiation model was completely redesigned for C++ Version 6.0. The changes include:
The automatic template instantiation model new with Version 6.0 is not directly compatible with previous template instantiation mechanisms. When linking applications using Version 6.0 and later, instantiations might not be resolved from existing Version 5.n repositories. Where possible, it is safest to start fresh with an empty repository and create the required instantiations by compiling all source files. If this is not possible, there are some strategies that can be used to link mixed generation instantiations.
If you used both Version 6.n and Version 5.n to build applications, HP strongly recommends that you use different repositories to contain automatic template instantiations for Version 6.n and Version 5.n compilations. The default repository name is the same for Version 6.n as for prior versions. Thus, if you use Version 6.n with older pre-6.n versions, you should do compilations in a different directory for each compiler or explicitly specify a different repository for each using the /REPOSITORY qualifier.
When linking applications using Version 6.n against instantiations created with Version 5.n, it is necessary to complete the Version 5.n instantiation process, to create instantiation object files. If old_repository is a Version 5.n repository then you would create the Version 5.n instantiation object files by using the Version 5.n cxxlink :
CXXLINK/NOEXE /REPOSITORY=[.old_repository] <Version 5.n object files> |
<Version 5.n object files> are the object files that were created using the Version 5.n compiler; old_repository now contains the instantiation object files. Create a library of these object files as follows:
LIBRARY/CREATE/OBJECT lib_old_repository/LOG
LIBRARY/INSERT/OBJECT lib_old_repository/LOG
[old_repository]*.obj
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When linking using Version 6.n, specify lib_old_repository.olb after all of the Version 5.n object files that are being linked.
In a similar way, you can create a library of Version 6.n instantiation object files to link into a Version 5.n application being linked using C++ Version 5.n. If new_repository is the Version 6.n repository, then a library of the instantiations would be created by:
LIBRARY/CREATE/OBJECT lib_new_repository/LOG LIBRARY/INSERT/OBJECT lib_new_repository/LOG [new_repository]*.obj |
When linking using Version 5.n, specify lib_new_repository.olb after all of the Version 6.n object files that are being linked.
Aspects of memory allocation and deallocation have changed from the V5.n and earlier compilers to the Version 6.n compilers. See the description of /[NO]STDNEW and /[NO]GLOBAL_ARRAY_NEW in The C++ Standard Library .
This section discusses porting issues pertaining to C++ classes.
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 }; |
Unlike some older C++ implementations, HP C++ strictly enforces accessibility rules for public , protected , and private members of a base class. For more information, see The C++ Programming Language, 3rd Edition.
Unlike some older C++ implementations, HP 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
};
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For more information, see The C++ Programming Language, 3rd Edition.
When a static data member is declared, the compiler issues a reference to the external identifier in the object code, which must be resolved by a definition. The compiler does not support the declaration anachronism shown in The C++ Programming Language, 3rd 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;
}
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The compiler does not issue any messages during compilation; however, when you attempt to link a program containing this code, the linker issues an unresolved symbol error message for the variable C::i .
HP C++ requires the use of function definitions as described in The C++ Programming Language, 3rd Edition. For examples of outdated syntax not allowed in HP 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; }
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In this example, f is declared with both internal and external linkage, which causes a compiler error.
This section demonstrates how to use pointers effectively in HP C++.
In HP 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.
Binding a pointer to a member function with a particular object as an argument to the function is not allowed in HP C++. For more information on the illegality of casting bound pointers, see The C++ Programming Language, 3rd Edition.
Because the return value cannot be an lvalue, the const keyword in a function return has no effect on the semantics of the return. However, using the const keyword 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};
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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.
The following example shows a type mismatch between a pointer to a char and a pointer to a const char that some other compilers might 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;
}
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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.
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
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For more information, see The C++ Programming Language, 3rd Edition.
HP 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".
}
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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.
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;
// ...
}
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For more information on using the switch statement, see The C++ Programming Language, 3rd Edition.
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;
}
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For more information, see The C++ Programming Language, 3rd Edition.
HP 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.
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.
If a size-of-array argument accompanies a delete operator, HP 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;
}
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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.
When linking applications, use CXXLINK instead of LINK. See Section 1.3 (ALPHA ONLY) and Section 1.4 (I64 ONLY).
Some other C++ implementations let you perform integer arithmetic, including ++, on enumerated types; HP C++ does not allow this.
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; |
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