Was std::string.npos ever valid? (As opposed to the correct std::string::npos.)
I am seeing it a lot in an old project I'm working on, and it does not compile with VS2010.
Is it something from the pre-standard days?
The C with classes syntax for naming a class member was, in fact, a dot:
class X {
public:
void f();
};
void X.f() // a dot! see D&E 2.3
{
}
However, the :: syntax had not yet been invented. The std namespace didn't exist yet either. Thus the std::string.npos wasn't ever valid as either C with classes or standard C++.
I suspect std::string.npos is purely Microsoft's extension (or a bug?). It might be inspired by the old syntax, and might be not.
No, std::string.npos was never valid, and no, it's not something from the pre-standard days.
I see other answers mentioning that MSVC has allowed that notation.
However, MSVC is not a very compliant compiler. For example, it lets you freely bind a temporary to a reference to non-const. For another example, for Windows GUI subsystem applications you have to use not well-documented switches to make it accept a standard main. Much has improved since Microsoft hired Herb Sutter (and other guy that I don't remember the name of right now) to fix up their monstrous compiler. And in relative terms it has been really great, but in absolute terms, well that compiler is still a bit lacking.
Access to any static member via class name and dot was unfortunately allowed by prior versions of MSVC.
#include <iostream>
struct A
{
static int a;
};
int A::a;
int main()
{
std::cout << A.a;
}
This code is happily accepted by MSVC9.0 with a warning
Warning 1 warning C4832: token '.' is
illegal after UDT 'A'
The C++ standard obviously disallows access to a static member via className.memberName (although it is perfectly legal to access a static member via an object object.staticMemberName).
My common sense tells me that if MSVC is aware that this is not standard and gives a warning, then we can turn that extension off. We go to Project Propertied -> C/C++ -> Language and set Disable Language Extensions to Yes. Do you think anything changes? Of course not, the compiler still accepts the illegal code with the same warning. I sometimes wonder what Disable Language Extensions actually does...
Related
I am learning C, and after starting out learning C++ as my first compiled language, I decided to "go back to basics" and learn C.
There are two questions that I have concerning the ways each language deals with functions.
Firstly, why does C "not care" about the scope that functions are defined in, whereas C++ does?
For example,
int main()
{
donothing();
return 0;
}
void donothing() { }
the above will not compile in a C++ compiler, whereas it will compile in a C compiler. Why is this? Isn't C++ mostly just an extension on C, and should be mostly "backward compatible"?
Secondly, the book that I found (Link to pdf) does not seem to state a return type for the main function. I check around and found other books and websites and these also commonly do not specify return types for the main function. If I try to compile a program that does not specify a return type for main, it compiles fine (although with some warnings) in a C compiler, but it doesn't compile in a C++ compiler. Again, why is that? Is it better style to always specify the return type as an integer rather than leaving it out?
Thanks for any help, and just as a side note, if anyone can suggest a better book that I should buy that would be great!
Firstly, why does C "not care" about the scope that functions are defined in, whereas C++ does?
Actually, C does care. It’s just that C89 allows implicitly declared functions and infers its return type as int and its parameters from usage. C99 no longer allows this.
So in your example it’s as if you had declared a prototype as
int dosomething();
The same goes for implicit return types: missing return types are inferred as int in C89 but not C99. Compiling your code with gcc -std=c99 -pedantic-errors yields something similar to the following:
main.c: In function 'main':
main.c:2:5: error: implicit declaration of function 'donothing' [-Wimplicit-function-declaration]
main.c: At top level:
main.c:5:6: error: conflicting types for 'donothing'
main.c:2:5: note: previous implicit declaration of 'donothing' was her
For the record, here’s the code I’ve used:
int main() {
donothing();
return 0;
}
void donothing() { }
It's because C++ supports optional parameters. When C++ sees donothing(); it can't tell if donothing is:
void donothing(void);
or
void donothing(int j = 0);
It has to pass different parameters in these two cases. It's also because C++ is more strongly typed than C.
int main() {
donothing();
return 0;
}
void donothing() { }
Nice minimum working example.
With gcc 4.2.1, the above code gets a warning regarding the conflicting types for void donothing() with default compiler settings. That's what the C89 standard says to do with this kind of problem. With clang, the above code fails on void donothing(). The C99 standard is a bit stricter.
It's a good idea to compile your C++ code with warnings enabled and set to a high threshold. This becomes even more important in C. Compile with warnings enabled and treat implicit function declarations as an error.
Another difference between C and C++: In C++ there is no difference between the declarations void donothing(void); and void donothing(); There is a huge difference between these two in C. The first is a function that takes no parameters. The latter is a function with an unspecified calling sequence.
Never use donothing() to specify a function that takes no arguments. The compiler has no choice but to accept donothing(1,2,3) with this form. It knows to reject donothing(1,2,3) when the function is declared as void donothing(void).
he above will not compile in a C++ compiler, whereas it will compile in a C compiler. Why is this?
Because C++ requires a declaration (or definition) of the function to be in scope at the point of the call.
Isn't C++ mostly just an extension on C
Not exactly. It was originally based on a set of C extensions, and it refers to the C standard (with a few modifications) for the definitions of the contents of standard headers from C. The C++ "language itself" is similar to C but is not an extension of it.
and should be mostly "backward compatible"?
Emphasis on "mostly". Most C features are available in C++, and a lot of the ones removed were to make C++ a more strictly typed language than C. But there's no particular expectation that C code will compile as C++. Even when it does, it doesn't always have the same meaning.
I check around and found other books and websites and these also commonly do not specify return types for the main function
The C and C++ standards have always said that main returns int.
In C89, if you omit the return type of a function it is assumed to be int. C++ and C99 both lack this implicit int return type, but a lot of C tutorial books and tutorials (and compilers and code) still use the C89 standard.
C has some allowances for implementations to accept other return types, but not for portable programs to demand them. Both languages have a concept of a "freestanding implementation", which can define program entry and exit any way it likes -- again, because this is specific to an implementation it's not suitable for general teaching of C.
IMO, even if you're going to use a C89 compiler it's worth writing your code to also be valid C99 (especially if you have a C99 compiler available to check it). The features removed in C99 were considered harmful in some way. It's not worth even trying to write code that's both C and C++, except in header files intended for inter-operation between the languages.
I decided to "go back to basics" and learn C.
You shouldn't think of C as a prerequisite or "basic form" of C++, because it isn't. It is a simpler language, though, with fewer features for higher-level programming. This is often cited as an advantage of C by users of C. And an advantage of C++ by users of C++. Sometimes those users are the same people using the languages for different purposes.
Typical coding style in C is different from typical coding style in C++, and so you might well learn certain basics more readily in C than in C++. It is possible to learn low-level programming using C++, and the code you write when you do so may or may not end up looking a lot like C code.
So, what you learn while learning C may or may not inform the way you write C++. If it does, that may or may not be for the better.
C++ has changed these rules on purpose, to make C++ a more typesafe language.
C.1.4 Clause 5: expressions [diff.expr]
5.2.2
Change: Implicit declaration of functions is not allowed
Rationale: The type-safe nature of C++.
Effect on original feature: Deletion of semantically well-defined feature. Note: the original feature was
labeled as “obsolescent” in ISO C.
Difficulty of converting: Syntactic transformation. Facilities for producing explicit function declarations
are fairly widespread commercially.
How widely used: Common.
You can find other similar changes in appendix C of this Draft C++ standard
Isn't C++ mostly just an extension on C
No. If you think of C++ as "C with Classes", you're doing it very, very wrong. Whilst strictly, most valid C is valid C++, there's virtually no good C that's good C++. The reality is that good C++ code is vastly different to what you'd see as good C code.
Firstly, why does C "not care" about the scope that functions are
defined in, whereas C++ does?
Essentially, because not enforcing the same rules as C++ makes doing this in C hideously unsafe and in fact, nobody sane should ever do that. C99 tightened this up, along with implicit-int and other defects in the C language.
In C++ using void in a function with no parameter, for example:
class WinMessage
{
public:
BOOL Translate(void);
};
is redundant, you might as well just write Translate();.
I, myself generally include it since it's a bit helpful when code-completion supporting IDEs display a void, since it ensures me that the function takes definitely no parameter.
My question is, Is adding void to parameter-less functions a good practice? Should it be encouraged in modern code?
In C++
void f(void);
is identical to:
void f();
The fact that the first style can still be legally written can be attributed to C.
n3290 § C.1.7 (C++ and ISO C compatibility) states:
Change: In C++, a function declared with an empty parameter list takes
no arguments.
In C, an empty parameter list means that the number and
type of the function arguments are unknown.
Example:
int f(); // means int f(void) in C++
// int f( unknown ) in C
In C, it makes sense to avoid that undesirable "unknown" meaning. In C++, it's superfluous.
Short answer: in C++ it's a hangover from too much C programming. That puts it in the "don't do it unless you really have to" bracket for C++ in my view.
I see absolutely no reason for this. IDEs will just complete the function call with an empty argument list, and 4 characters less.
Personally I believe this is making the already verbose C++ even more verbose. There's no version of the language I'm aware of that requires the use of void here.
I think it will only help in backward compatibility with older C code, otherwise it is redundant.
I feel like no. Reasons:
A lot more code out there has the BOOL Translate() form, so others reading your code will be more comfortable and productive with it.
Having less on the screen (especially something redundant like this) means less thinking for somebody reading your code.
Sometimes people, who didn't program in C in 1988, ask "What does foo(void) mean?"
Just as a side note. Another reason for not including the void is that software, like starUML, that can read code and generate class diagrams, read the void as a parameter. Even though this may be a flaw in the UML generating software, it is still annoying to have to go back and remove the "void"s if you want to have clean diagrams
I am wondering if this is a compiler specific problem or not. I've seen examples of the use of constructors for a struct in C++.
I have something like:
struct Example
{
Example()
{
}
};
I still get this compiler error "Functions may not be part of a struct or union". I am using the very old Borland 4.5 compiler (best not to ask why...).
(And yes, this is done in C++).
I can't help myself: Why?
Using a compiler that announces it's support for Windows 95 is sort of interesting. The C++ standard is from 1998, so anything published before that is perhaps not up to date? :-)
Other than that, the code is perfectly ok.
Hi i have to port some stuff written on c++ from unix bases os to windows visual studio 2008.
The following code implements array data type with void ** - pointer to the data.
struct array
{
int id;
void **array; // store the actual data of the array
// more members
}
When i compile with g++ on Unix it's ok but when i try with MSVS 2008 I get the error - error C2461: 'array' : constructor syntax missing formal parameters. When i change the member from 'array' to something else it works, so it seems that the compiler thinks that the member name 'array' is actually the constructor of the struct array. It's obviously not a good practice to name the member like the struct but it's already written that way. Can i tell the MSVS compiler to ignore this problem or i should rename all members that are the same as the struct name.
You are dealing with a bug in GCC compiler. C++ language explicitly prohibits having data members whose name is the same as the name of the class (see 9.2/13). MS compiler is right to complain about it. Moreover, any C++ compiler is required to issue a diagnostic message in this case. Since GCC is silent even in '-ansi -pedantic -Wall' mode, it is a clear bug in GCC.
Revison: What I said above is only correct within the "classic" C++98 specification of C++ language. In the most recent specification this requirement only applies to static data members of the class. Non-static data members can now share the name with the class. I don't know whether this change is already in the official version of the revised standard though.
That means that both compilers are correct in their own way. MS compiler sticks to the "classic" C++98 specification of the language, while GCC seems to implement a more recent one.
I'd say that if you're doing something that you yourself describe as "not a good practice", then you should change it.
I would rename your attribute to not have the same name as the class. This will make your code more portable. If you have to move to yet another compiler in the future, you won't run in to this problem again then.
struct elem
{
int i;
char k;
};
elem user; // compile error!
struct elem user; // this is correct
In the above piece of code we are getting an error for the first declaration. But this error doesn't occur with a C++ compiler. In C++ we don't need to use the keyword struct again and again.
So why doesn't anyone update their C compiler, so that we can use structure without the keyword as in C++ ?
Why doesn't the C compiler developer remove some of the glitches of C, like the one above, and update with some advanced features without damaging the original concept of C?
Why it is the same old compiler not updated from 1970's ?
Look at visual studio etc.. It is frequently updated with new releases and for every new release we have to learn some new function usage (even though it is a problem we can cope up with it). We will also get updated with the new compiler if there is any.
Don't take this as a silly question. Why it is not possible? It could be developed without any incompatibility issues (without affecting the code that was developed on the present / old compiler)
Ok, lets develop the new C language, C+, which is in between C and C++ which removes all glitches of C and adds some advanced features from C++ while keeping it useful for specific applications like system level applications, embedded systems etc.
Because it takes years for a new Standard to evolve.
They are working on a new C++ Standard (C++0x), and also on a new C standard (C1x), but if you remember that it usually takes between 5 and 10 years for each iteration, i don't expect to see it before 2010 or so.
Also, just like in any democracy, there are compromises in a Standard. You got the hardliners who say "If you want all that fancy syntactic sugar, go for a toy language like Java or C# that takes you by the hand and even buys you a lollipop", whereas others say "The language needs to be easier and less error-prone to survive in these days or rapidly reducing development cycles".
Both sides are partially right, so standardization is a very long battle that takes years and will lead to many compromises. That applies to everything where multiple big parties are involved, it's not just limited to C/C++.
typedef struct
{
int i;
char k;
} elem;
elem user;
will work nicely. as other said, it's about standard -- when you implement this in VS2008, you can't use it in GCC and when you implement this even in GCC, you certainly not compile in something else. Method above will work everywhere.
On the other side -- when we have C99 standard with bool type, declarations in a for() cycle and in the middle of blocks -- why not this feature as well?
First and foremost, compilers need to support the standard. That's true even if the standard seems awkward in hindsight. Second, compiler vendors do add extensions. For example, many compilers support this:
(char *) p += 100;
to move a pointer by 100 bytes instead of 100 of whatever type p is a pointer to. Strictly speaking that's non-standard because the cast removes the lvalue-ness of p.
The problem with non-standard extensions is that you can't count on them. That's a big problem if you ever want to switch compilers, make your code portable, or use third-party tools.
C is largely a victim of its own success. One of the main reasons to use C is portability. There are C compilers for virtually every hardware platform and OS in existence. If you want to be able to run your code anywhere you write it in C. This creates enormous inertia. It's almost impossible to change anything without sacrificing one of the best things about using the language in the first place.
The result for software developers is that you may need to write to the lowest common denominator, typically ANSI C (C89). For example: Parrot, the virtual machine that will run the next version of Perl, is being written in ANSI C. Perl6 will have an enormously powerful and expressive syntax with some mind-bending concepts baked right into the language. The implementation, though, is being built using a language that is almost the complete opposite. The reason is that this will make it possible for perl to run anywhere: PCs, Macs, Windows, Linux, Unix, VAX, BSD...
This "feature" will never be adopted by future C standards for one reason only: it would badly break backward compatibility. In C, struct tags have separate namespaces to normal identifiers, and this may or may not be considered a feature. Thus, this fragment:
struct elem
{
int foo;
};
int elem;
Is perfectly fine in C, because these two elems are in separate namespaces. If a future standard allowed you to declare a struct elem without a struct qualifier or appropriate typedef, the above program would fail because elem is being used as an identifier for an int.
An example where a future C standard does in fact break backward compatibiity is when C99 disallowed a function without an explicit return type, ie:
foo(void); /* declare a function foo that takes no parameters and returns an int */
This is illegal in C99. However, it is trivial to make this C99 compliant just by adding an int return type. It is not so trivial to "fix" C programs if suddenly struct tags didn't have a separate namespace.
I've found that when I've implemented non-standard extensions to C and C++, even when people request them, they do not get used. The C and C++ world definitely revolves around strict standard compliance. Many of these extensions and improvements have found fertile ground in the D programming language.
Walter Bright, Digital Mars
Most people still using C use it because they're either:
Targeting a very specific platform (ie, embedded) and therefore must use the compiler provided by that platform vendor
Concerned about portability, in which case a non-standard compiler would defeat the purpose
Very comfortable with plain C and see no reason to change, in which case they just don't want to.
As already mentioned, C has a standard that needs to be adhered to. But can't you just write your code using slightly modified C syntax, but use a C++ compiler so that things like
struct elem
{
int i;
char k;
};
elem user;
will compile?
Actually, many C compilers do add features - doesn't pretty much every C compiler support C++ style // comments?
Most of the features added to updates of the C standard (C99 being the most recent) come from extensions that 'caught on'.
For example, even though the compiler I'm using right now on an embedded platform does not claim to conform to the C99 standard (and it is missing quite a bit from it), it does add the following extensions (all of which are borrowed from C++ or C99) to it's 'C90' support:
declarations mixed with statements
anonymous structs and unions
inline
declaration in the for loop initialization expression
and, of course, C++ style // comments
The problem I run into with this is that when I try to compile those files using MSVC (either for testing or because the code is useful on more than just the embedded platform), it'll choke on most of them (I'm honestly not sure about anonymous structs/unions).
So, extensions do get added to C compilers, it's just that they're done at different rates and in different ways (so code using them becomes more difficult to port) and the process of moving them into a standard occurs at a near glacial pace.
We have a typedef for exactly this purpose.
And please do not change the standard we have enough compatibility problems already....
# Manoj Doubts comment
I have no problem with you or somebody else to define C+ or C- or Cwhatever unless you don't touch C :)
I still need a language that capable to complete my task - have a same piece of code (not a small one) to be able to run on tens of Operating system compiled by significant number of different compilers and be able to run on tens of different hardware platform at the moment there is only one language that allow me complete my task and i prefer not to experiment with this ability :) Especially for reason you provided. Do you really think that ability to write
foo test;
instead
struct foo test;
will make you code better from any point of view ?
The following program outputs "1" when compiled as standard C or something else, probably 2, when compiled as C++ or your suggested syntax. That's why the C language can't make this change, it would give new meaning to existing code. And that's bad!
#include <stdio.h>
typedef struct
{
int a;
int b;
} X;
int main(void)
{
union X
{
int a;
int b;
};
X x;
x.a = 1;
x.b = 2;
printf("%d\n", x.a);
return 0;
}
Because C is Standardized. Compiler could offer that feature and some do, but using it means that the source code doesn't follow the standard and could only be compiled on that vendor's compiler.
Well,
1 - None of the compilers that are in use today are from the 70s...
2 - There are standarts for both C and C++ languages and compilers are developed according to those standarts. They can't just change some behaviour !
3 - What happens if you develop on VS2008 and then try to compile that code by another compiler whose last version was released 10 years ago ?
4 - What happens when you play with the options on the C/C++ / Language tab ?
5 - Why don't Microsoft compilers target all the possible processors ? They only target x86, x86_64 and Itanium, that's all...
6 - Believe me , this is not even considered as a problem !!!
You don't need to develop a new language if you want to use C with C++ typedefs and the like (but without classes, templates etc).
Just write your C-like code and use the C++ compiler.
As far as new functionality in new releases go, Visual C++ is not completely standard-conforming (see http://msdn.microsoft.com/en-us/library/x84h5b78.aspx), By the time Visual Studio 2010 is out, the next C++ standard will likely have been approved, giving the VC++ team more functionality to change.
There are also changes to the Microsoft libraries (which have little or nothing to do with the standard), and to what the compiler puts out (C++/CLI). There's plenty of room for changes without trying to deviate from the standard.
Nor do you need anything like C+. Just write in C, use whatever C++ features you like, and compile as C++. One of the Bjarne Stroustrup's original design goals for C++ was to make it unnecessary to write anything in C. It should compile perfectly efficiently provided you limit the C++ features you use (and even then will compile very efficiently; modern C++ compilers do a very good job).
And the unanswered question: Why would you want to use non-standard C, when you could write standard C or standard C++ with almost equal facility?
This sounds like the embrace and extend concept.
Life under your scenario.
I develop code using a C compiler that has the C "glitches" removed.
I move to a different platform with another C compiler that has the C "glitches" removed, but in a slightly different way.
My code doesn't compile or runs differently on the new platform, I waste time "porting" my code to the new platform.
Some vendors actually like to fix "glitches" because this tends to lock people into a single platform.
If you want to write in standard C, follow the standards. That's it.
If you want more freedom use C# or C++.NET or anything else your hardware supports.