How to share the scope shared between extended namespaces? - c++

Please take a look at the following code. First, a namespace holding a class is created. Then, the namespace is extended and the class we defined before is used. Here is a live demo.
// File one
namespace system {
class module {
// ...
};
}
// File two including file one
namespace system {
struct entry {
entry(module *module);
module *module;
};
}
Compiling this under gcc 4.8.1 without the permissive flag produces two compiler errors. Explicitly mentioning the namespace like system::module solves the problem, even though it is the same namespace we're already in.
error: declaration of ‘system::module* system::entry::module’ [-fpermissive]
module *module;
^
error: changes meaning of ‘module’ from ‘class system::module’ [-fpermissive]
class module {
^
Why is this code not standard conform? What other options do I have except explicitly mentioning the namespace every time.

As already pointed out in another answer, the compiler is complaining that you have changed the semantics of an identifier from a type to a variable name (module).
You should give your types a name that will make them easily distinguishable from variables. You should give member variables names that can be easily distinguished from regular variables. This increases code readability and maintainability. And, it has the side-effect of making the problem you face a non-issue:
// File one
namespace system {
class module_type {
// ...
};
}
// File two including file one
namespace system {
struct entry {
entry(module_type *module);
module_type *module_;
};
}

In this statement
module *module;
you redefined name module.
So in the member function declaration you have to use elaborated type name
entry( class module *module);
In any case it is a bad idea to redefine names such a way.

// File one
namespace system {
class module {
// ...
};
}
// File two including file one
namespace system {
struct entry {
entry(module *module);
module *module;
};
}
where I cannot really come up with more descriptive names
Naming is one of the most difficult challenges programmer's face, and yet, when shown a way (or 2 or n), it will forever be easier.
I offer the following - from a Coding standard I have long used.
class and namespace names are Capitalized, at least the 1st char
variable names are lower case (or camel case with lower case 1st char)
// File one
namespace System {
class Module {
// ...
};
}
// File two including file one
namespace System {
struct Entry {
Entry(Module* module);
Module* module;
};
}
So?
Search for and read some of the coding standards. They exist for a reason. Once you learn one good idea, it will serve the rest of your career.

Related

Why is it common to not include the word 'class' in C++ .cpp files?

Most classes appear to be separated between declaration and definition in the following form using namespace qualifier to define the class:
// test.h
class test
{
public:
void func1(void);
private:
void func2(void);
};
// test.cpp
void test::func1(void)
{
//whatever
}
void test::func2(void)
{
//whatever
}
Why don't we typically see people use the keyword class in the .cpp file? Like in the following form:
// test.cpp
class test {
void func1(void)
{
//whatever
}
void func2(void)
{
//whatever
}
};
Is it just convention to use the namespace qualifiers? Or because it make more sense when you starting implementing a class via multiple source files?
Let's view this question from another angle...
It is possible to use the same syntax for both, but it's "the other one"; the following is perfectly valid:
namespace ns
{
int foo();
}
int ns::foo() { return 0; }
Looked at like this, it's the opposite question that's interesting, "why is it common to include the word 'namespace' in .cpp files?"
There's one substantial difference between namespaces and classes that makes namespace {} necessary in so many places: namespaces are open to extension, but classes are defined entirely by their (one and only) definition.
Like with classes, you can't add anything to a namespace using the syntax above; you can't add a function bar above with only int ns::bar() { return 9; }, the only way to add names to a namespace is "from within".
And, as many have discovered, it's convenient to wrap an entire file in a namespace and not use the qualified names, even if you're not adding any names to it.
Hence the popularity of "namespace": it's a convenience enabled by the extensibility of namespaces.
Another issue is that the meaning of your "test.cpp" would depend on whether the class definition has already been seen by the compiler – without it, that's a valid and complete definition of a class with two private functions.
This kind of "action from a distance" depending on possibly very distant code is painful to work with.
It's also worth noting that namespaces were added some twenty years after "C with classes" was created, when C++ was a well established language, and changing the meaning of a construct that literally hasn't changed in decades is pretty much unthinkable.
Partularly if all it does is save a few keystrokes.

Why namespace is needed in a header file?

I am familiar with the following use of namespaces.
In the header file (for example people.h) I describe interface of a name space. For example:
namespace people{
int getAge(str Name);
void setName(str Name);
}
Then in people.cpp I define the methods from the space name:
#include "people.h"
int people::getAge(str Name) {
something_1;
something_2;
}
void people::setName(str Name) {
something_1;
}
However, in a header file that I have I see that in addition to the namespace people there are also interfaces of other namespaces (for example namespace dogs). And these name spaces are not defined in the people.cpp file.
So, I assumed that (because of some strange reason) the interface for the namespace dogs is put into the people.h and then then the name space dog is defined in the "dogs.cpp" file. So, in other words I assumed that two different name spaces are defined in two different cpp files but their interface is described in one header file. However, this assumption seems to be wrong because I found that there are many header files that declare "namespace dogs".
So, I assume that namespace dogs in the 2people.h" file has another function but I cannot figure out what function it is. Could anybody please help me with that?
ADDED
The code that I try to understand is written not by me and it works fine. So, it should make sense. May be I was not clear enough. So, I try to give an example:
In the header file (people.h) I have:
namespace etet
{
class date;
}
namespace xsystem{
class estimation_module;
}
namespace people {
a_lot_of_different_stuff;
}
Then the people.cpp defines all the methods that belong to the people name space.
The "namespace interface" is a misleading concept. A namespace is just a bounch of names grouped together under a surname (like you and your brothers and sisters). It has no "interface" because there is no namespace "obejct".
The
#include "people.h"
int people::getAge(str Name) {
something_1;
something_2;
}
void people::setName(str Name) {
something_1;
}
is perfectly equivalent to
#include "people.h"
namespace people
{
int getAge(str Name) {
something_1;
something_2;
}
void setName(str Name) {
something_1;
}
}
may be this is more familiar, or may be not.
The fact an header declares functions not present in a cpp, just means they are probably present in another one.
About the fact that the namespace name { ..... } declaration can be repeated in many files, each containing various function is perfectly normal, since the namespace keyword does not declare an object. It just group names. And -in fact- sayning a namespace is "declared" is a common language abuse. What is declared is the name of the namespace.
And different names declare in different places can belong to a same group. there is nothing mysterious in that.
You lexicon makes me thinking you are confusing namespaces with classes and structs
ADDED:
After your clarification, it looks like the a_lot_of_different_stuff contains declarations that use etet::date and xsystem::estimation_module;
This names (and only the names) must be known to the compiler, but the header cannot recursively include ther respective headers because they most likely already included people.h.
A typical "curculare reference" problem, like in here, but involving different namespaces.
You're confusing namespaces and classes. Typically, a class definition occurs in a header file (.h) and the implementation of its member functions appear in the corresponding implementation file (.cpp).
A namespace works differently to a class. If a class is defined in multiple translation units, it must have precisely the same tokens in all of them. You can't even reorder members, even if it would result in the exact same class. It's easy to meet this requirement by using the above described header files. Each translation unit that needs a class foo contains the contents of foo.h because they do #include "foo.h" when they need it. Of course they all contain precisely the same definition of foo bceause they all included foo.h.
However, this is very different to namespaces. A namespace can be introduced multiple times across the same and different translation units without it being the same tokens every time. Something like this is totally fine:
namespace bar {
void baz();
struct x;
}
// some stuff
namespace bar {
void do_something(x);
}
Each occurence of namespace bar introduces some declarations to that namespace.
You will often have many classes defined in the same namespace. Each header for those classes will do namespace whatever { ... } and introduce the class definition into that namespace.
Sometimes you will even want to introduce things to multiple namespaces or nested namespaces in a single header file. There's nothing to stop you doing that. A possible situation for doing that is if you want to forward declare something from another namespace. Let's say you have a class defined in people.h like so:
namespace people {
class person {
dogs::dog* pet_dog;
};
}
Now, this class needs to know about the type dog in the dogs namespace. One way to do his would be to #include "dogs.h". However, since pet_dog is only a pointer, we can do with an incomplete type, so we can forward declare dog like so:
namespace dogs {
class dog;
}
namespace people {
class person {
dogs::dog* pet_dog;
};
}

How to overcome the namespace evil of C++ header files?

With one of my projects I will head into the C++ field. Basically I am coming
from a Java background and was wondering how the concept of Java packages
is realized in the C++ world. This led me to the C++ concept of namespaces.
I am absolutely fine with namespaces so far but when it comes to header files
things are becoming kind of inefficient with respect to fully qualified class
names, using-directives and using-declarations.
A very good description of the issue is this article by Herb Sutter.
As I understand it this all boils down to: If you write a header file always
use fully qualified type names to refer to types from other namespaces.
This is almost unacceptable. As a C++ header commonly provides the declaration
of a class, a maximum of readability has top priority. Fully qualifying each
type from a different namespace creates a lot of visual noise, finally
diminishing readability of the header to a degree which raises the question
whether to use namespaces at all.
Nevertheless I want to take advantage of C++ namespaces and so put some thought into
the question: How to overcome the namespace evil of C++ header files? After
some research I think typedefs could be a valid cure to this problem.
Following you will find a C++ sample program which demonstrates how I would
like to use public class scoped typedefs to import types from other namespaces.
The program is syntactically correct and compiles fine on MinGW W64. So far so
good, but I am not sure whether this approach happily removes the using keyword
from the header but brings in another problem which I am simply not aware of.
Just something tricky like the things described by Herb Sutter.
That is I kindly ask everybody who has a thorough understanding of C++ to
review the code below and let me know whether this should work or not. Thanks
for your thoughts.
MyFirstClass.hpp
#ifndef MYFIRSTCLASS_HPP_
#define MYFIRSTCLASS_HPP_
namespace com {
namespace company {
namespace package1 {
class MyFirstClass
{
public:
MyFirstClass();
~MyFirstClass();
private:
};
} // namespace package1
} // namespace company
} // namespace com
#endif /* MYFIRSTCLASS_HPP_ */
MyFirstClass.cpp
#include "MyFirstClass.hpp"
using com::company::package1::MyFirstClass;
MyFirstClass::MyFirstClass()
{
}
MyFirstClass::~MyFirstClass()
{
}
MySecondClass.hpp
#ifndef MYSECONDCLASS_HPP_
#define MYSECONDCLASS_HPP_
#include <string>
#include "MyFirstClass.hpp"
namespace com {
namespace company {
namespace package2 {
/*
* Do not write using-declarations in header files according to
* Herb Sutter's Namespace Rule #2.
*
* using std::string; // bad
* using com::company::package1::MyFirstClass; // bad
*/
class MySecondClass{
public:
/*
* Public class-scoped typedefs instead of using-declarations in
* namespace package2. Consequently we can avoid fully qualified
* type names in the remainder of the class declaration. This
* yields maximum readability and shows cleanly the types imported
* from other namespaces.
*/
typedef std::string String;
typedef com::company::package1::MyFirstClass MyFirstClass;
MySecondClass();
~MySecondClass();
String getText() const; // no std::string required
void setText(String as_text); // no std::string required
void setMyFirstInstance(MyFirstClass anv_instance); // no com::company:: ...
MyFirstClass getMyFirstInstance() const; // no com::company:: ...
private:
String is_text; // no std::string required
MyFirstClass inv_myFirstInstance; // no com::company:: ...
};
} // namespace package2
} // namespace company
} // namespace com
#endif /* MYSECONDCLASS_HPP_ */
MySecondClass.cpp
#include "MySecondClass.hpp"
/*
* According to Herb Sutter's "A Good Long-Term Solution" it is fine
* to write using declarations in a translation unit, as long as they
* appear after all #includes.
*/
using com::company::package2::MySecondClass; // OK because in cpp file and
// no more #includes following
MySecondClass::MySecondClass()
{
}
MySecondClass::~MySecondClass()
{
}
/*
* As we have already imported all types through the class scoped typedefs
* in our header file, we are now able to simply reuse the typedef types
* in the translation unit as well. This pattern shortens all type names
* down to a maximum of "ClassName::TypedefTypeName" in the translation unit -
* e.g. below we can simply write "MySecondClass::String". At the same time the
* class declaration in the header file now governs all type imports from other
* namespaces which again enforces the DRY - Don't Repeat Yourself - principle.
*/
// Simply reuse typedefs from MySecondClass
MySecondClass::String MySecondClass::getText() const
{
return this->is_text;
}
// Simply reuse typedefs from MySecondClass
void MySecondClass::setText(String as_text)
{
this->is_text = as_text;
}
// Simply reuse typedefs from MySecondClass
void MySecondClass::setMyFirstInstance(MyFirstClass anv_instance)
{
this->inv_myFirstInstance = anv_instance;
}
// Simply reuse typedefs from MySecondClass
MySecondClass::MyFirstClass MySecondClass::getMyFirstInstance() const
{
return this->inv_myFirstInstance;
}
Main.cpp
#include <cstdio>
#include "MySecondClass.hpp"
using com::company::package2::MySecondClass; // OK because in cpp file and
// no more #includes following
int main()
{
// Again MySecondClass provides all types which are imported from
// other namespaces and are part of its interface through public
// class scoped typedefs
MySecondClass *lpnv_mySecCls = new MySecondClass();
// Again simply reuse typedefs from MySecondClass
MySecondClass::String ls_text = "Hello World!";
MySecondClass::MyFirstClass *lpnv_myFirClsf =
new MySecondClass::MyFirstClass();
lpnv_mySecCls->setMyFirstInstance(*lpnv_myFirClsf);
lpnv_mySecCls->setText(ls_text);
printf("Greetings: %s\n", lpnv_mySecCls->getText().c_str());
lpnv_mySecCls->setText("Goodbye World!");
printf("Greetings: %s\n", lpnv_mySecCls->getText().c_str());
getchar();
delete lpnv_myFirClsf;
delete lpnv_mySecCls;
return 0;
}
Pain is mitigated by reducing complexity. You're bending C++ into Java. (That works just as bad as trying the other way.)
Some hints:
Remove the "com" namespace level. (This is just a java-ism that you don't need)
Drop the "company" namespace, maybe replace by "product" or "library" namespace (i.e. boost, Qt, OSG, etc). Just pick something that's unique w.r.t. the other libs you're using.
You don't need to fully declare names that are in the same namespace you're in (caveat emptor: template classe, see comment). Just avoid any using namespace directives in the headers. (And use with care in C++ files, if at all. Inside functions is preferred.)
Consider namespace aliases (in functions/cpp files), i.e namespace bll = boost::lambda;. This creates shortcuts that are quite neat.
Also, by hiding private members/types using the pimpl pattern, your header have less types to expose.
P.S: Thanks to #KillianDS a few good tips in comments (that were deleted when I edited them into the question.)

How to wrap a C struct in a C++ class and keep the same name?

Let's say the Acme company releases a useful library with an extremely ugly C API. I'd like to wrap the structs and related functions in C++ classes. It seems like I can't use the same names for the wrapper classes, because the original library is not inside a namespace.
Something like this is not possible, right?
namespace AcmesUglyStuff {
#include <acme_stuff.h> // declares a struct Thing
}
class Thing {
public:
...
private:
AcmesUglyStuff::Thing thing;
};
Linking will be a problem.
The only way I can think to wrap the library, and not pollute my namespace with the C library names, is a hack like this, reserving space in the class:
// In mything.h
namespace wrapper {
class Thing {
public:
...
private:
char impl[SIZE_OF_THING_IN_C_LIB];
};
}
// In thing.cc
#include <acme_stuff.h>
wrapper::Thing::Thing() {
c_lib_function((::Thing*)impl); // Thing here referring to the one in the C lib
}
Is that the only way? I'd like to avoid putting prefixes on all my class names, like XYThing, etc.
Seems like you're making this harder than it needs to be.
#include "acme_stuff.h" // puts all of its names in global namespace
namespace acme {
class Thing {
public:
// whatever
private:
::Thing thing;
};
}
Now just use acme::Thing rather than Thing.
If it's really important to you to not have the C names in the global namespace, then you need a level of indirection:
namespace acme {
class Thing {
public:
Thing();
~Thing();
// whatever
private:
void *acme_thing;
};
}
In your implementation file, #include "acme_stuff.h", in your constructor create a new ::Thing object and store its address in acme_thing, in your destructor delete it, and in your member functions cast acme_thing to type ::Thing*.
It's not a good idea to try to name something the exact same thing as something else. (I mean equal fully-qualified names, including all namespaces.) If some library has already grabbed the obvious best name in the global namespace, you'll need to pick a different name.
You could put your class Thing in a namespace as Pete Becker suggests, and then use ::Thing to access Acme's Thing. That would be fine if you're prepared to always access your class through it's fully namespace-qualified name (e.g. My::Thing). It's tempting to try using My::Thing; or using namespace My;, but that won't work, because any translation unit that includes the definition of your class (e.g. via a header file you create) must necessarily pull Acme's Thing into the global namespace first (otherwise an "Undefined symbol" compilation error would occur when parsing the definition of My::Thing).
Is it really a C API? Try to extern "C" {} to whole included header to solve the linking problem.
namespace AcmesUglyStuff {
extern "C" {
#include <acme_stuff.h>
}
}

Creating a C++ namespace in header and source (cpp)

Is there any difference between wrapping both header and cpp file contents in a namespace or wrapping just the header contents and then doing using namespace in the cpp file?
By difference I mean any sort performance penalty or slightly different semantics that can cause problems or anything I need to be aware of.
Example:
// header
namespace X
{
class Foo
{
public:
void TheFunc();
};
}
// cpp
namespace X
{
void Foo::TheFunc()
{
return;
}
}
VS
// header
namespace X
{
class Foo
{
public:
void TheFunc();
};
}
// cpp
using namespace X;
{
void Foo::TheFunc()
{
return;
}
}
If there is no difference what is the preferred form and why?
The difference in "namespace X" to "using namespace X" is in the first one any new declarations will be under the name space while in the second one it won't.
In your example there are no new declaration - so no difference hence no preferred way.
Namespace is just a way to mangle function signature so that they will not conflict. Some prefer the first way and other prefer the second version. Both versions do not have any effect on compile time performance. Note that namespaces are just a compile time entity.
The only problem that arises with using namespace is when we have same nested namespace names (i.e) X::X::Foo. Doing that creates more confusion with or without using keyword.
There's no performance penalties, since the resulting could would be the same, but putting your Foo into namespace implicitly introduces ambiguity in case you have Foos in different namespaces. You can get your code fubar, indeed. I'd recommend avoiding using using for this purpose.
And you have a stray { after using namespace ;-)
If you're attempting to use variables from one to the other, then I'd recommend externalizing them, then initializing them in the source file like so:
// [.hh]
namespace example
{
extern int a, b, c;
}
// [.cc]
// Include your header, then init the vars:
namespace example
{
int a, b, c;
}
// Then in the function below, you can init them as what you want:
void reference
{
example::a = 0;
}
If the second one compiles as well, there should be no differences. Namespaces are processed in compile-time and should not affect the runtime actions.
But for design issues, second is horrible. Even if it compiles (not sure), it makes no sense at all.
The Foo::TheFunc() is not in the correct namespacein the VS-case. Use 'void X::Foo::TheFunc() {}' to implement the function in the correct namespace (X).
In case if you do wrap only the .h content you have to write using namespace ... in cpp file otherwise you every time working on the valid namespace. Normally you wrap both .cpp and .h files otherwise you are in risk to use objects from another namespace which may generate a lot of problems.
I think right thing to do here is to use namespace for scoping.
namespace catagory
{
enum status
{
none,
active,
paused
}
};
void func()
{
catagory::status status;
status = category::active;
}
Or you can do the following:
// asdf.h
namespace X
{
class Foo
{
public:
void TheFunc();
};
}
Then
// asdf.cpp
#include "asdf.h"
void X::Foo::TheFunc()
{
return;
}