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Compaq C++ implements the C++ International Standard, with some differences, as described in the C++ 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 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.
4.1 Compatibility with Other C++ Compilers
In default mode ( /standard=ansi ), 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=ansi
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.If you want to use ANSI mode but find that the compiler generates too many diagnostics in that mode, you can use the /quiet option with the /standard=ansi 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 ANSI mode but were not issued by Version 5.6. 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.
Compaq C++ Version 6.n 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=cfront
Specify this option if you want to compile programs developed using cfront or a compiler based on cfront./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 .
With either
/standard=ms
or
/standard=cfront
you may also want to specify
/quiet
to reduce the number of diagnostic messages generated.
4.2 Compatibility With Previous Versions
This section provides details about differences between the current and previous compiler versions:
Users should be aware of the following language differences between the current 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";
}
|
typedef void Z::* any_ptom; |
Users should be aware of the following implementation differences between the current and previous versions of the compiler:
f(const int p1);
f(int p1);
|
f(i++, i++);
|
Aspects of memory allocation and deallocation have changed. See the
description of
/[no]stdnew
and
/[no]global_array_new
in
The C++ Standard Library
.
4.3 Using Classes
This section discusses porting issues pertaining to C++ classes.
4.3.1 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 }; |
Unlike some older C++ implementations, Compaq 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.
4.3.3 Base Class Initializers
Unlike some older C++ implementations, Compaq 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, 3nd Edition.
4.4 Undefined Global Symbols for Static Data Members
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. On Compaq Tru64 UNIX systems, the 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 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
.
4.5 Functions and Function Declaration Considerations
Compaq C++ requires the use of function definitions as described in The C++ Programming Language, 3nd Edition. For examples of outdated syntax not allowed in Compaq 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; }
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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.
4.6 Using Pointers
This section demonstrates how to use pointers effectively in
Compaq C++.
4.6.1 Pointer Conversions
In Compaq 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.
4.6.2 Bound Pointers
Binding a pointer to a member function with a particular object as an
argument to the function is not allowed in Compaq C++. For more
information on the illegality of casting bound pointers, see
The C++ Programming Language, 3nd Edition.
4.6.3 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.
4.6.4 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 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;
}
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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.
4.7 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.
4.8 Initializing References
Compaq 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, 3nd Edition.
4.9 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;
// ...
}
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For more information on using the
switch
statement, see of The C++ Programming Language, 3nd Edition.
4.10 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;
}
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For more information, see The C++ Programming Language, 3nd Edition.
4.11 Preprocessing
Compaq 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.
4.12 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.
4.13 Size-of-Array Argument to delete Operator
If a size-of-array argument accompanies a delete operator, Compaq 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.
4.15 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.
4.16 Source File Extensions
Compaq C++ automatically treats files with a
.c
extension as C language source files and passes the files to the
cc
command. For Compaq C++ to compile them, your files must have one of
the following extensions:
.cxx
,
.cpp
, or
.cc
.
4.17 Incrementing Enumerations
Some other C++ implementations let you perform integer arithmetic,
including ++, on enumerated types; Compaq C++ does not allow this.
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; |
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