Related
Below, I provided a minimal example of code I created. I managed to get this code working, but I'm not sure if the practice being employed is sound. In essence, what I am trying to do is have the 'Parameter' class reference select elements in the 'States' class, so variables in States can be changed via Parameters.
Questions I have: is the approach taken OK? If not, is there a better way to achieve what I am aiming for?
Example code:
struct VAR_TYPE{
public:
bool is_fixed; // If is_fixed = true, then variable is a parameter
double value; // Numerical value
std::string name; // Description of variable (to identify it by name)
};
struct NODE{
public:
VAR_TYPE X, Y, Z;
/* VAR_TYPE is a structure of primitive types */
};
class States{
private:
std::vector <NODE_ptr> node; // shared ptr to struct NODE
std::vector <PROP_DICTIONARY_ptr> property; // CAN NOT be part of Parameter
std::vector <ELEMENT_ptr> element; // CAN NOT be part of Parameter
public:
/* ect */
void set_X_reference ( Parameter &T , int i ) { T.push_var( &node[i]->X ); }
void set_Y_reference ( Parameter &T , int i ) { T.push_var( &node[i]->Y ); }
void set_Z_reference ( Parameter &T , int i ) { T.push_var( &node[i]->Z ); }
bool get_node_bool_X( int i ) { return node[i]->X.is_fixed; }
// repeat for Y and Z
};
class Parameter{
private:
std::vector <VAR_TYPE*> var;
public:
/* ect */
};
int main(){
States S;
Parameter P;
/* Here I initialize and set S, and do other stuff */
// Now I assign components in States to Parameters
for(int n=0 ; n<S.size_of_nodes() ; n++ ){
if ( S.get_node_bool_X(n)==true ){
S.set_X_reference ( P , n );
};
// repeat if statement for Y and Z
};
/* Now P points selected to data in S, and I can
* modify the contents of S through P
*/
return 0;
};
Update
The reason this issue cropped up is I am working with Fortran legacy code. To sum up this Fotran code - it's a numerical simulation of a flight vehicle. This code has a fairly rigid procedural framework one must work within, which comes with a pre-defined list of allowable Fortran types. The Fortran glue code can create an instance of a C++ object (in actuality, a reference from the perspective of Fortran), but is not aware what is contained in it (other means are used to extract C++ data into Fortran).
The problem that I encountered is when a C++ module is dynamically linked to the Fortran glue code, C++ objects have to be initialized each instance the C++ code is called. This happens by virtue of how the Fortran template is defined.
To avoid this cycle of re-initializing objects, I plan to use 'State' as a container class. The Fortran code allows a 'State' object, which has an arbitrary definition; but I plan to use it to harness all relevant information about the model. The idea is to use the Parameters class (which is exposed and updated by the Fortran code) to update variables in States.
What you are doing is legal C++ (although some parts of the code are missing, so I have to guess what they do) -- you can take pointers and references to member variables like this. But the question whether this is moral (or "sound") depends heavily on the situation.
It seems to me that you are trying to hide the semantics of which data member (x, etc) a particular object belongs to with your Parameter type. So, when actually accessing the contents of your VAR_TYPE* collection, you will not have that information.
That could be desired or it could be code smell. I'm leaning towards the latter. If x, y and z can serve similar roles, it might be better to replace them by one std::array<VAR_TYPE,3> and access them by index rather than going to the trouble of creating this wrapper type that serves the only purpose to add exactly this indirect access functionality.
The essential problem has basically already been answered elsewhere on this site, but what I really want is opinions on the best way to implement my class in terms of practicality and aesthetics, and if there are any subtleties involved. So bearing that in mind, here's my question:
I have a simple encryption program that I've written and now I want to add xz compression to it, which is written in C. The xz code uses a struct to control data in and out of the compression algos:
/* All of this is in src/liblzma/api/lzma/base.h if you download version 5.0.3
* XZ Utils
*/
typedef struct {
const uint8_t *next_in;
size_t avail_in;
uint64_t total_in;
/* ...
* and so on. Some other members are enums and other structs, but
* this is basically a POD structure
*/
} lzma_stream;
/* This macro is used to initialize lzma_stream objects */
#define LZMA_STREAM_INIT \
{ NULL, 0, 0, NULL, 0, 0, NULL, NULL, \
NULL, NULL, NULL, NULL, 0, 0, 0, 0, \
LZMA_RESERVED_ENUM, LZMA_RESERVED_ENUM }
/* Here's LZMA_RESERVED_ENUM in case anyone's wondering: */
typedef enum {
LZMA_RESERVED_ENUM = 0
} lzma_reserved_enum;
I have a wrapper class to lzma_stream so that if my encryption code throws, the wrapper class destructor can call functions that deallocate any assigned memory in the lzma_stream struct. So, I have:
class Stream {
public:
Stream();
~Stream();
void init();
// ...
private:
lzma_stream stream_;
// ...
};
Stream::~Stream() {
lzma_end( &stream_ );
}
My question is, how would you initialize Stream::stream_ and why? I could initialize the struct's members individually:
Stream::Stream() : stream_(), ... {}
void Stream::init() {
stream_.next_in = NULL;
stream_.avail_in = 0;
// ...
}
But I would like to use LZMA_STREAM_INIT because that would mean that I would not need to worry about changes in the xz library. With that in mind, alternatively, I could create a temp:
Stream::Stream() : stream_(), ... {}
void Stream::init() {
lzma_stream const temp = LZMA_STREAM_INIT;
stream_ = temp;
// ...
}
Preliminary question: Is there a way I could do the initialization in the Stream ctor (edit: I mean, in the initialization list)? (I take it not, right?) I'm trying to avoid c++0x initialization lists, by the way, for compiler portability reasons.
As I said above, these are they ways to solve the problem and that's already been said elsewhere; but what I'd like to know is which way would you guys do it (if there's not some other way I don't know about)? I can already guess that you'd say the latter method, but I have the sneaky feeling that there's a catch involved: is there?
OK, lots of useful info and solutions provided below. Thanks for all help, guys.
Yes, you can use the ctor-initializer, you just need to add a helper function:
Stream::Stream() : stream_(def_stream) {}
static lzma_stream def_stream()
{
lzma_stream tmpStream = LZMA_STREAM_INIT;
return tmpStream;
}
Among other things, this allows you to initialize a const aggregate member. And almost all compilers will elide creation of the temporary.
If you run into a compiler that doesn't, you can use this variation:
static const lzma_stream& def_stream()
{
static lzma_stream tmpStream = LZMA_STREAM_INIT;
return tmpStream;
}
In C++0x, you'll be able to write:
Stream::Stream() : stream_ LZMA_STREAM_INIT {}
which takes advantage of the "uniform initializer syntax".
Why not just put the initialization in the ctor?
Stream::Stream() : ...
{
lzma_stream const temp = LZMA_STREAM_INIT;
stream_ = temp;
// ...
}
Note that you don't need a mem-initializer for stream_. When a POD member has no mem-initializer, C++ will leave it "uninitialized", just like a local variable lzma_stream stream;. But as long as you then assign to it right away, that's not really a bad thing.
I also note that lzma itself recommends this "temp" object assignment pattern (in base.h) for cases when a direct initialization can't be done. In C, it's talking more about when the lzma_stream memory was malloc-ed, but it also applies to a C++03 class member. (C++0x does have a way of doing this in a mem-initializer, yes.)
Sure you could:
Stream::Stream() : your_init_list {
lzma_stream tmpStream = LZMA_STREAM_INIT;
stream_ = tmpStream;
}
A constructor has a body like any other function or method, and structs have implicit copy constructors and assignment operators. If you're using GCC, you can even skip the temporary and assign directly to the field.
Edit: It's not possible to construct a struct in the init list unless it has an appropriate constructor defined (i.e. there is no implicit constructor that takes arguments for every field.) That's why the constructor has a body--so you can do setup for the object that goes beyond basic field assignment.
Edit #2: As Ben points out, you can use a helper function for this, but directly initializing the field in the init list (something like stream_(LZMA_STREAM_INIT)) isn't possible.
So this is what I am trying to accomplish. I am trying to use a sax parser to parse some XML. it looks like I need to call all their methods as statics. So if I want to pass a value back from say startElement it is static void startElement. Which brings me to my example code. I have been pulling my hair on how to update a value in a Nesting class from a static member function.
I have looked at several things such as defining OuterClass * oc; then trying to reference oc->allRecords, but since it is a static method inside, it fails. I am sure I am doing something wrong architecturally, so any feedback on what would be the right way to do this would be a great help. Thanks.
class Attribute {
string AttributeName;
string AttributeValue;
};
typedef shared_ptr<Attribute> AttributePtr;
class AttributeSet {
vector<AttributePtr> Attributes;
};
typedef shared_ptr<AttributeSet> AttributeSetPtr;
class OuterClass {
public :
vector<AttributeSetPtr> allRecords;
class InnerClass {
public:
static mymethod1() {
// I need to be able to set attributes here :
// This would be the characters method for sax parsing
// What is the right way to Attributes.push_back(new Attribute(Name,Value));
}
static mymethod2() {
// I also need to be able to add Records here :
// This would be the endElement for sax parsing
// What is the right way to allRecords.push_back(AttributeSet);
}
};
// EDIT: CALLING CODE GOES HERE (WAS EDITED - SEE BELOW)
};
// ADDING INFORMATION REGARDING HOW METHOD 1 & 2 are called
xmlSAXHandler saxHandler;
memset(&saxHandler, 0, sizeof(saxHandler));
saxHandler.initialized = XML_SAX2_MAGIC;
...
saxHandler.endElementsNs = &InnerClass::method2;
saxHandler.characters = &InnerClass::method1;
...
InnerClass innerXmlParsingClass
xmlSaxUserParseMemory( &saxHandler, &innerXmlParsingClass, xmlString, xmlString.length());
Your mistake is using an inner class (are you coming from Java?).
I don't know what you believe you are are achieving with an inner class, but it won't work. Don't use inner classes in C++ unless you really know what it does (for inner classes, protected and private members of the outer classes are seen as if they were public).
Now, as the solution to your problem, I guess it depends on the implementation you're using (I used once Apache's Xerces SAX, but I know Microsoft offers its own SAX implementation, and that there should be a lot other alternatives, so...)
Edit
After the comment, I found the following tutorial:
http://www.jamesh.id.au/articles/libxml-sax/libxml-sax.html
I must say that, coming from Java to C++, and using a C API, you have a kind of courage...
:-D
If you are not familiar enough with function pointers, and C in general, using libxml2 will be a challenge. Be sure that in the end, you will understand those notions... Note that C have a way to handle the data that C++, Java or C# developers associate to this. The C way is to pass a pointer to your data (the user data) to a function, and when the callback is called, it passes back this pointer, typed as a void *. You must then cast it back to its right type, and voilà, you have your this back.
:-)
Anyway, reading the doc, I see that when you parse the file, you'll call the following C function:
int xmlSAXUserParseFile( xmlSAXHandlerPtr sax,
void * user_data,
const char * filename);
the user_data part is the one that interest you because it enables you to have a context. So, wrapping this function in a C++ class, you could have something like:
// MySaxBase.hpp
class MySaxBase
{
public :
MySaxBase() ;
int parseFile(const std::string & p_filename) ;
virtual void startDocument() ;
virtual void endDocument() ;
private :
static void do_startDocument(void *p_user_data) ;
static void do_endDocument(void *p_user_data) ;
xmlSAXHandler m_sax ;
}
.
// MySaxBase.cpp
extern "C"
{
void do_startDocument(void *p_user_data)
{
// this static method will convert the p_user_data into
// the this pointer...
MySaxBase * saxBase = static_cast<MySaxBase *>(p_user_data) ;
// ...and call the right virtual method
saxBase->startDocument() ;
}
void do_endDocument(void *p_user_data)
{
// this static method will convert the p_user_data into
// the this pointer...
MySaxBase * saxBase = static_cast<MySaxBase *>(p_user_data) ;
// ...and call the right virtual method
saxBase->endDocument() ;
}
} // extern "C"
MySaxBase::MySaxBase()
{
// the m_sax structure must be set to zero to NULL all its
// pointers to functions
memset(&m_sax, 0, sizeof(xmlSAXHandler)) ;
// Now, we initialize some pointers to the static method we
// want to be called
this->m_sax.startDocument = do_startDocument ;
this->m_sax.endDocument = do_endDocument ;
}
int MySaxBase::parseFile(const std::string & p_filename)
{
// the important thing, here, is the this pointer, passed as
// a user_data parameter
return xmlSAXUserParseFile(&m_sax, this, p_filename.c_str()) ;
}
void MySaxBase::startDocument()
{
// The document started. Override this method to
// actually do something
}
void MySaxBase::endDocument()
{
// The document ended. Override this method to
// actually do something
}
I did not test this, and I never used libxml2, but I guess the code must be Ok, and this should be enough for you to continue on your own: Just add the methods you want to support, initialize the sax handler with the relevant function pointers, and you'll have your class complete.
The MySaxBase::startDocument and MySaxBase::endDocument methods are virtual just for you to derive from MySaxBase and then override those methods.
Edit 2
I'll reproduce here Steve Jessop's excellent comment:
+1. One tiny quibble - I don't think that static member functions are guaranteed by the C++ standard to have C linkage / calling convention, but to use them as a callback from a C API, that's what they need. I don't specifically know what implementations it makes a difference, but for safety do_startDocument should be a free function declared with extern "C". On the same subject: a Java programmer may not realise you have make sure that the function can't throw an exception (because C doesn't have them). So you'd normally want to see a try/catch(...) in the wrapper function. – Steve Jessop
Following this, and after reading Johannes Schaub - litb (who else?) no less excellent answer at static vs extern "C"/"C++" , I modified the code to make do_startDocument and do_endDocument real C functions (i.e. wrapped in an extern "C" block). This usually is not important (I never encountered this kind of problem), but, better safe than sorry.
Your basic problem is that static methods are not per-instance, so there is no this pointer. You somehow need to get a OuterClass* passed to mymethod1 and mymethod2.
If you show us how mymethod1 and mymethod2 are called, we can help you further.
If it's simply called by you someplace where you have a OuterClass object, then your solution is simple:
class OuterClass
{
// ...
static void mymethod1(OuterClass* oc)
{
oc->all_records.push_back( something );
}
};
void some_func()
{
OuterClass oc;
OuterClass::method1(&oc);
}
Since you updated your question here is how you should do this:
class OuterClass {
public:
vector<AttributeSetPtr> allRecords;
void characters(const xmlChar* ch, int len)
{
// do here whatever you want
allRecords.push_back(bla bla);
}
static void static_characters(void* ctx, const xmlChar* ch, int len) {
// retrieve this pointer from ctx
static_cast<OuterClass*>(ctx)->characters(ch, len);
}
};
saxHandler.characters = &OuterClass::static_characters;
...
OuterClass outerClass;
xmlSaxUserParseMemory(&saxHandler, static_cast<void*>(&outerClass), xmlString, xmlString.length());
I've got way too much information to work with, so for now I'll consider this question answered until I can sort it all out and decide on the final implementation! Thanks a ton gf and Simon Buchan. I wish I could accept both of your answers, since they're both definite possibilities!
Additional / Revised Conceptual Information as suggested:
What I am aiming to do;
I am making a game. In this game every object used is an instance of the DOBJ class. The TUR class extends the DOBJ class. The SHO class extends the TUR class.
Each TUR class has an array of SHO's stored in it's SHOARR array. Each SHO instance needs to be given a set of instructions.
I know for a fact I could make 1000's of different SHO classes that have their instructions set during construction.
However, considering I will have so many different acting SHO instances, I was interested in another way to pass a set of instructions. Through the contruction of the SHO would be the ideal.
The instructions I am attempting to pass to each SHO are simple if statements;
if(frame > 64) { rotation += 4; };
if(state == 0 && frame < 32) { xs = 12; ys = 12; state = 1; };
Original question
Migration from ActionScript3.0 to C++ is proving to be a trial indeed. Thanks to those who have answered my questions thus far and also to those who opened stackoverflow in the first place. Onto the question... (TL;DR near the bottom to get straight to the question)
I'm attempting to apply the same logic that I could apply in AS3.0 to my project in C++ and it's just not going very well.
In AS3.0 I was used to slapping any and every datatype into an Array. It made things pretty simple. Now that I've run into C++ dev, I realized that I can't exactly do that anymore.
So now I'm stuck with this problem of rewriting a little AI system in a new language, where the driving point of the system isn't even compatible!
Here's an example of a piece of the code I was writing in AS3.0;
AI[NUM][1]( OBJ, AI[NUM][2], AI[NUM][3] );
AI being an array, NUM being an integer, OBJ being an instance of a class.
This line obviously called the function in the second element of the first array in the main array with the arguments being a class in which to perform the function on, whatever was in the third element of the first array of the main array, and likewise the fourth element.
In this case;
AI[NUM][1] would be a function
AI[NUM][2] would be a variable
AI[NUM][3] would be a number
Generally, my AI was run on calling a function to change or compare the variable with a number.
An example would be;
CompareST( someObject, "x", 500 );
and return true if someObject's x variable was smaller than (ST) 500.
The AI array itself was just filled with arrays of calls similar to this.
Quite new to C++ I had no idea how to go about this, so I did a bit of searching and reading of many different websites and came to the conclusion that I should look into function pointers.
However, after reading a bit into them, I've come to the conclusion that it won't help me realize my goal. While it did help me call functions like I wanted to call them, it doesn't help me stack different datatypes into one large array of arrays.
TL;DR
EDIT++:
What I need for each object is a set of instructions to be checked every frame. However, for each instance of the class, the instructions have to be different.
I plan on having a LOT of different instances, so making a class for each one is unreasonable.
Thus, I needed a way to pass a set of instructions to each one through it's constructor and read + execute them at any time their think() function is called.
My ultimate goal (aside from finding out about a better way to go about this) would be to be able to have an array of function calls, like;
A[n][0]( O, A[n][1], A[n][2] );
Where;
O is the instance the function is altering
A[n][0] is a function (Equality or Comparison)
A[n][1] is the variable, eg; "x", O["x"] (or a pointer to that variable in the case of C++)
A[n][2] is the value to alter the variable by, or compare it to.
And I'm not sure how I would rewrite this into C++, or alter it to work in another way.
Aftermath / Additional Information
What I'm actually aiming to do is be able to give an object a set of instructions at the time of it's creation, through the constructor. For example upon creation give an object instructions to wait 64 frames, and then rotate in the opposite direction, would have been something like this;
t.AI = [ [ 1, AIF.CompareET, "STATE", 0, AIF.CompareGT, "FRAME", 64, 0, AIF.EqualityAT, "baseRotation", 180, AIF.EqualityET, "STATE", 1 ] ];
In pseudocode;
(The 1 in the array denotes how to read the rest of the array, in this case everything before the odd 0 [ The one that comes after 64 ] is a comparison. If any of those fail, anything after the 0 will not be looked at )
Compare STATE is equal to (ET) 0, if true
Compare FRAME is greather than (GT) 64, if true
Add 180 to (AT) baseRotation, Set STATE equal to 1
Sorry that this turned out really long. I hope it's understandable, and I'm not asking something stupidly difficult to explain.
You can store functions using function pointers or functors. Variant types though are not natively supported by C++, you have to use custom solutions there.
One possibility would be to use Boost.Any (or better, Boost.Variant if you only use a fixed set of types):
typedef void (*Function)(Object*, const std::string&, boost::any&);
std::vector<Function> functions;
Given some function:
void f(Object* obj, const std::string& name, boost::any& value) {
// ...
}
you could store and call it similar to your example:
functions.push_back(&f);
functions[0](obj, "x", boost::any(500));
To utilize a declarative syntax, there are three options that come to my mind:
you use a similar approach and have central "interpreter" function, e.g. based on a switch (don't forget to switch to integers or pointers-to-members instead of strings if you need performance)
you invent your own language and generate C++ code from description files
you compose function objects in a declarative way
To do composition, you could use Boost.Bind or something like custom objects that represent operations:
struct Operation {
virtual ~Operation() {}
virtual bool operator()(Object&) = 0;
};
template<class T>
struct GreaterThen : Operation {
typedef T Object::*Member;
Member member;
const T value;
CompareGT(Member member, const T& value) : member(member), value(value) {}
bool operator()(Object& obj) { return (obj.*member > value); }
};
template<class T>
struct SetTo : Operation {
typedef T Object::*member;
Member member;
const T value;
SetTo(Member member, const T& value) : member(member), value(value) {}
bool operator()(Object& obj) { obj.*member = value; return true; }
};
Now we can build operation lists:
typedef std::vector<Operation*> OpList;
OpList operation;
operations.push_back(new GreaterThen<int>(&Object::Frame, 64));
operations.push_back(new SetTo<int>(&Object::State, 1));
We can use helper functions to avoid having to specify the template types:
template<class T>
Operation* opGreaterThen(T Object::*mem, const T& val) {
return new GreaterThen<T>(mem, val);
}
Assuming a similar helper for SetTo and using Boost.Assign the above becomes:
OpList operations = boost::assign::list_of
(opGreaterThen(&Object::Frame, 64))
(opSetTo (&Object::State, 1));
Executing the operations becomes the following then:
OpList::iterator it = operation.begin();
for( ; it != operations.end(); ++it) {
Operation& op = *it; // just for readability
if(!op(someObject)) break; // stop if operation returns false
}
Wow.
Reading through that slowly suggests what you're trying to end up with is an array of function calls and you can choose a different function with the same parameters (but different implementation) for different actions and choose the correct one for the correct case.
If that is the case, you're looking for function pointers. Try this tutorial.
You should be able to use a function pointer with an argument set and point it to the correct function based on your needs. You won't need an array of function pointers for this either - any function that matches the definition should do. From the tutorial, declare a function pointer like this:
int (TMyClass::*functptr)(classname, int, int) = NULL; // C++
Then assign it later:
this.functptr = &TMyClass::doitthisway;
While it is possible (although a pain) to have an array of arbitrary types, you pretty much never need it, since you have to know something about what is where to do anything interesting with it: for example, your 'TL;DR' example seems to look something like:
struct AIRule {
// Can only handle comparing ints, see later for more general solution.
typedef bool compare_type(AIObject*, AIObject::*int, int);
compare_type* compare;
AIObject* object;
AIObject::int* member;
int comparand;
};
So now you can do something like:
bool ai_equal(AIObject* object, AIObject::int* member, int comparand) {
return object->*member == comparand;
}
...
ai[n].compare = &ai_equal;
ai[n].object = some_object;
ai[n].member = &AIObject::some_member;
ai[n].comparand = 50;
...
if (ai[n].compare(ai[n].object, ai[n].member, ai[n].comparand)) {
...
}
This just moves the any type problem from the rules array to member though. C++ needs to know at least how many bytes a member is, and a string (for example) can be much bigger than an int. You can get around this by using pointers: which essentially is C++'s version of any, but you then need to delete it yourself (or you will leak memory!), at which point the interface method below becomes simpler.
If I was doing what you seem to want, I would use inheritance:
struct Sprite {
int frame;
double rotation;
Sprite() {
frame = 0;
rotation = 0.0;
}
virtual ~Sprite() {}
virtual void think() {
++frame;
}
virtual void draw() {
...
}
};
struct RotatingSprite : public Sprite {
int state;
MyShape() {
state = 0;
}
void think() {
Sprite::think();
if (state == 0 && frame > 64) {
state = 1;
rotation += 180.0;
}
}
};
Or a function pointer:
struct Sprite {
int frame;
double rotation;
void (*think)(Sprite*);
Sprite() {
frame = 0;
rotation = 0.0;
}
};
void rotate_think(Sprite* sprite) {
if (sprite->state == 0 && sprite->frame > 64) {
sprite->state = 1;
sprite->rotation += 180.0;
}
}
...
sprite->think = &rotate_think;
If you really need to do it dynamically I would recommend using the ++ part of C++. For the predicates (a predicate is just something that returns a boolean, like isLowerCase()) create an AIPredicate interface, and the actions an AIAction interface:
struct AIPredicate {
// "When you delete an AIPredicate, delete the full type, not just this interface."
virtual ~AIPredicate() {}
// "You can treat this as a function (operator()) but I'm not providing an implementation here ( = 0)"
virtual bool operator()(AIObject* object) = 0;
};
struct AIAction {
virtual ~AIAction() {}
virtual void operator()(AIObject* object) = 0;
};
struct AIRule {
// std::auto_ptr (or std::unique_ptr if you can use C++0x) will delete predicate for you.
// Add "#include <memory>" to your includes if it complains (most std headers will include it already)
std::auto_ptr<AIPredicate> predicate;
std::auto_ptr<AIAction> action;
};
Now you can make types like:
struct AIFrame : public AIPredicate {
// Implement the operator() member AICondition promises.
bool operator()(AIObject* object) {
return object->foo < 100;
}
};
...
// Use .reset() instead of = if you use std::unique_ptr.
ai[n].predicate = new AIFooIsLow();
If you want to have a very general predicate type, you can use the very powerful (and complicated) templates feature:
// The naming convention I'm using here is 'T'TitleCase for template parameters, TitleCase for types,
// lower_case for arguments and variables and '_'lower_case for members.
template<typename TMemberType, AIObject::TMemberType* TMember>
struct AIMemberEquals : public AIPredicate {
// Constructor: Initializes a new instance after it is created.
AIMemberEquals(TMemberType comparand) {
// Save comparand argument so we can use it in operator().
_comparand = comparand;
}
bool operator()(AIObject* object) {
return object->*TMember == comparand;
}
// Stores the value to compare.
TMemberType _comparand;
};
Unfortunately, creating templates looks a bit crazy:
ai[n].predicate = new AIMemberEquals<int, &AIObject::some_member>(100);
Read it as "create a new instance of (the type that AIMemberEquals applied to int and (the some_member member of AIObject) creates), with the argument 100".
When you have multiple predicates memory management becomes a bit more difficult without C++0x's unique_ptr or shared_ptr, types that will delete the object for you, since std::auto_ptr doesn't work in containers:
#include <vector>
struct AIData {
// vector is fairly close to AS3's Array type, it is a good default for
// arrays of changing or unknown size.
std::vector<AIPredicate*> predicates;
// Destructor: will be run before the memory for this object is freed.
~AIData() {
for (int i = 0; i != predicates.size(); ++i) {
delete predicates[i];
}
}
};
...
ai[n].predicates.push_back(new AIFooIsLow());
...
for (int i = 0; i != ai[n].predicates.size(); ++i) {
(*ai[n].predicates[i])(ai[n].object);
}
In C++0x:
struct AIData {
// unique_ptr will delete it for you, so no ~AIData() needed.
std::vector<unique_ptr<AIPredicate>> predicates;
};
Your final example could in C++ look something like:
std::auto_ptr<Shape> shape(new Shape());
...
std::auto_ptr<AIRule> rule(new AIRule());
rule->predicates.push(new AIMemberEquals<int, &Shape::state>(0));
rule->predicates.push(new AIMemberGreater<int, &Shape::frame>(64));
rule->actions.push(new AIAddMember<double, &Shape::rotation>(180.0));
rule->actions.push(new AISetMember<int, &Shape::state>(1));
shape->ai.push(rule); // .push(std::move(rule)); if you are using unique_ptr
Certainly not as pretty, but it works and is fairly flexible.
The problem:
I have a C++ class with gajillion (>100) members that behave nearly identically:
same type
in a function, each member has the same exact code done to it as other members, e.g. assignment from a map in a constructor where map key is same as member key
This identicality of behavior is repeated across many-many functions (>20), of course the behavior in each function is different so there's no way to factor things out.
The list of members is very fluid, with constant additions and sometimes deletions, some (but not all) driven by changing columns in a DB table.
As you can imagine, this presents a big pain-in-the-behind as far as code creation and maintenance, since to add a new member you have to add code to every function
where analogous members are used.
Example of a solution I'd like
Actual C++ code I need (say, in constructor):
MyClass::MyClass(SomeMap & map) { // construct an object from a map
intMember1 = map["intMember1"];
intMember2 = map["intMember2"];
... // Up to
intMemberN = map["intMemberN"];
}
C++ code I want to be able to write:
MyClass::MyClass(SomeMap & map) { // construct an object from a map
#FOR_EACH_WORD Label ("intMember1", "intMember2", ... "intMemberN")
$Label = map["$Label"];
#END_FOR_EACH_WORD
}
Requirements
The solution must be compatible with GCC (with Nmake as make system, if that matters).
Don't care about other compilers.
The solution can be on a pre-processor level, or something compilable. I'm fine with either one; but so far, all of my research pointed me to the conclusion that the latter is just plain out impossible in C++ (I so miss Perl now that I'm forced to do C++ !)
The solution must be to at least some extent "industry standard" (e.g. Boost is great, but a custom Perl script that Joe-Quick-Fingers created once and posted on his blog is not. Heck, I can easily write that Perl script, being much more of a Perl expert than a C++ one - I just can't get bigwigs in Software Engineering at my BigCompany to buy into using it :) )
The solution should allow me to declare a list of IDs (ideally, in only one header file instead of in every "#FOR_EACH_WORD" directive as I did in the example above)
The solution must not be limited to "create an object from a DB table" constructor. There are many functions, most of them not constructors, that need this.
A solution of "Make them all values in a single vector, and then run a 'for' loop across the vector" is an obvious one, and can not be used - the code's in a library used by many apps, the members are public, and re-writing those apps to use vector members instead of named members is out of the question, sadly.
Boost includes a great preprocessor library that you can use to generate such code:
#include <boost/preprocessor/repetition.hpp>
#include <boost/preprocessor/stringize.hpp>
#include <boost/preprocessor/cat.hpp>
typedef std::map<std::string, int> SomeMap;
class MyClass
{
public:
int intMember1, intMember2, intMember3;
MyClass(SomeMap & map)
{
#define ASSIGN(z,n,_) BOOST_PP_CAT(intMember, n) = map[ BOOST_PP_STRINGIZE(BOOST_PP_CAT(intMember, n))];
BOOST_PP_REPEAT_FROM_TO(1, 4, ASSIGN, nil)
}
};
Boost.Preprocessor proposes many convenient macros to perform such operations. Bojan Resnik already provided a solution using this library, but it assumes that every member name is constructed the same way.
Since you explicitely required the possibily to declare a list of IDs, here is a solution that should better fulfill your needs.
#include <boost/preprocessor/seq/for_each.hpp>
#include <boost/preprocessor/stringize.hpp>
// sequence of member names (can be declared in a separate header file)
#define MEMBERS (foo)(bar)
// macro for the map example
#define GET_FROM_MAP(r, map, member) member = map[BOOST_PP_STRINGIZE(member)];
BOOST_PP_SEQ_FOR_EACH(GET_FROM_MAP, mymap, MEMBERS)
// generates
// foo = mymap["foo"]; bar = mymap["bar];
-------
//Somewhere else, we need to print all the values on the standard output:
#define PRINT(r, ostream, member) ostream << member << std::endl;
BOOST_PP_SEQ_FOR_EACH(PRINT, std::cout, MEMBERS)
As you can see, you just need to write a macro representing the pattern you want to repeat, and pass it to the BOOST_PP_SEQ_FOR_EACH macro.
You could do something like this: create an adapter class or modify the existing class to have a vector of pointers to those fields, add the addresses of all member variables in question to that vector in the class constructor, then when needed run the for-loop on that vector. This way you don't (or almost don't) change the class for external users and have a nice for-loop capability.
Of course, the obvious question is: Why do you have a class with 100 members? It doesn't really seem sane.
Assuming it is sane nevertheless -- have you looked at boost preprocessor library? I have never used it myself (as one friend used to say: doing so leads to the dark side), but from what I heard it should be the tool for the job.
Surreptitiously use perl on your own machine to create the constructor. Then ask to increase your salary since you're succesfully maintaining such a huge chunk of code.
You could use the preprocessor to define the members, and later use the same definition to access them:
#define MEMBERS\
MEMBER( int, value )\
SEP MEMBER( double, value2 )\
SEP MEMBER( std::string, value3 )\
struct FluctuatingMembers {
#define SEP ;
#define MEMBER( type, name ) type name
MEMBERS
#undef MEMBER
#undef SEP
};
.. client code:
FluctuatingMembers f = { 1,2., "valuesofstringtype" };
std::cout <<
#define SEP <<
#define MEMBER( type, name ) #name << ":" << f.##name
MEMBERS;
#undef MEMBER
#undef SEP
It worked for me, but is hard to debug.
You can also implement a visitor pattern based on pointer-to-members. After the preprocessor solution, this one turns out way more debuggeable.
struct FluctuatingMembers {
int v1;
double v2;
std::string v3;
template<typename Visitor> static void each_member( Visitor& v );
};
template<typename Visitor> void FluctuatingMembers::each_member( Visitor& v ) {
v.accept( &FluctuatingMembers::v1 );
v.accept( &FluctuatingMembers::v2 );
v.accept( &FluctuatingMembers::v3 );
}
struct Printer {
FluctuatingMembers& f;
template< typename pt_member > void accept( pt_member m ) const {
std::cout << (f::*m) << "\n";
}
};
// you can even use this approach for visiting
// multiple objects simultaneously
struct MemberComparer {
FluctuatingMembers& f1, &f2;
bool different;
MemberComparer( FluctuatingMembers& f1, FluctuatingMembers& f2 )
: f1(f1),f2(f2)
,different(false)
{}
template< typename pt_member > void accept( pt_member m ) {
if( (f1::*m) != (f2::*m) ) different = true;
}
};
... client code:
FluctuatingMembers object1 = { 1, 2.2, "value2" }
, object2 = { 1, 2.2, "valuetoo" };
Comparer compare( object1, object2 );
FluctuatingMembers::each_member( compare );
Printer pr = { object1 };
FluctuatingMembers::each_member( pr );
Why not do it at run time? (I really hate macro hackery)
What you really are asking for, in some sense, is class metadata.
So I would try something like:
class AMember{
......
};
class YourClass{
AMember member1;
AMember member2;
....
AMember memberN;
typedef AMember YourClass::* pMember_t;
struct MetaData : public std::vector<std::pair<std::string,pMember_t>>{
MetaData(){
push_back(std::make_pair(std::string("member1"),&YourClass::member1));
...
push_back(std::make_pair(std::string("memberN"),&YourClass::memberN));
}
};
static const MetaData& myMetaData() {
static const MetaData m;//initialized once
return m;
}
YourClass(const std::map<std::string,AMember>& m){
const MetaData& md = myMetaData();
for(MetaData::const_iterator i = md.begin();i!= md.end();++i){
this->*(i->second) = m[i->first];
}
}
YourClass(const std::vector<std::pair<std::string,pMember_t>>& m){
const MetaData& md = myMetaData();
for(MetaData::const_iterator i = md.begin();i!= md.end();++i){
this->*(i->second) = m[i->first];
}
}
};
(pretty sure I've got the syntax right but this is a machinery post not a code post)
RE:
in a function, each member has the same exact code done to it as other members, e.g. assignment from a map in a constructor where map key is same as member key
this is handled above.
RE:
The list of members is very fluid, with constant additions and sometimes deletions, some (but not all) driven by changing columns in a DB table.
When you add a new AMember, say newMember, all you have to do is update the MetaData constructor with an:
push_back(make_pair(std::string("newMember"),&YourClass::newMember));
RE:
This identicality of behavior is repeated across many-many functions (>20), of course the behavior in each function is different so there's no way to factor things out.
You have the machinery to apply this same idiom to build the functions
eg: setAllValuesTo(const AMember& value)
YourClass::setAllValuesTo(const AMember& value){
const MetaData& md = myMetaData();
for(MetaData::const_iterator i = md.begin();i!= md.end();++i){
this->*(i->second) = value;
}
}
If you are a tiny bit creative with function pointers or template functionals you can factor out the mutating operation and do just about anything you want to YourClass' AMember's on a collection basis. Wrap these general functions (that may take a functional or function pointer) to implement your current set of 20 public methods in the interface.
If you need more metadata just augment the codomain of the MetaData map beyond a pointer to member. (Of course the i->second above would change then)
Hope this helps.
You can do something like his:
#define DOTHAT(m) m = map[#m]
DOTHAT(member1); DOTHAT(member2);
#undef DOTHAT
That doesn't fully fit your description, but closest to it that saves you typing.
Probably what I'd look to do would be to make use of runtime polymorphism (dynamic dispatch). Make a parent class for those members with a method that does the common stuff. The members derive their class from that parent class. The ones that need a different implementation of the method implement their own. If they need the common stuff done too, then inside the method they can downcast to the base class and call its version of the method.
Then all you have to do inside your original class is call the member for each method.
I would recommend a small command-line app, written in whatever language you or your team are most proficient in.
Add some kind of template language to your source files. For something like this, you don't need to implement a full-fledged parser or anything fancy like that. Just look for an easily-identified character at the beginning of a line, and some keywords to replace.
Use the command-line app to convert the templated source files into real source files. In most build systems, this should be pretty easy to do automatically by adding a build phase, or simply telling the build system: "use MyParser.exe to handle files of type *.tmp"
Here's an example of what I'm talking about:
MyClass.tmp
MyClass::MyClass(SomeMap & map) { // construct an object from a map
▐REPLACE_EACH, LABEL, "intMember1", "intMember2, ... , "intMemberN"
▐ LABEL = map["$Label"];
}
I've used "▐" as an example, but any character that would otherwise never appear as the first character on a line is perfectly acceptable.
Now, you would treat these .tmp files as your source files, and have the actual C++ code generated automatically.
If you've ever heard the phrase "write code that writes code", this is what it means :)
There are already a lot of good answers and ideas here, but for the sake of diversity I'll present another.
In the code file for MyClass would be:
struct MemberData
{
size_t Offset;
const char* ID;
};
static const MemberData MyClassMembers[] =
{
{ offsetof(MyClass, Member1), "Member1" },
{ offsetof(MyClass, Member2), "Member2" },
{ offsetof(MyClass, Member3), "Member3" },
};
size_t GetMemberCount(void)
{
return sizeof(MyClassMembers)/sizeof(MyClassMembers[0]);
}
const char* GetMemberID(size_t i)
{
return MyClassMembers[i].ID;
}
int* GetMemberPtr(MyClass* p, size_t i) const
{
return (int*)(((char*)p) + MyClassMembers[i].Offset);
}
Which then makes it possible to write the desired constructor as:
MyClass::MyClass(SomeMap& Map)
{
for(size_t i=0; i<GetMemberCount(); ++i)
{
*GetMemberPtr(i) = Map[GetMemberID(i)];
}
}
And of course, for any other functions operating on all the members you would write similar loops.
Now there are a few issues with this technique:
Operations on members use a runtime loop as opposed to other solutions which would yield an unrolled sequence of operations.
This absolutely depends on each member having the same type. While that was allowed by OP, one should still evaluate whether or not that might change in the future. Some of the other solutions don't have this restriction.
If I remember correctly, offsetof is only defined to work on POD types by the C++ standard. In practice, I've never seen it fail. However I haven't used all the C++ compilers out there. In particular, I've never used GCC. So you would need to test this in your environment to ensure it actually works as intended.
Whether or not any of these are problems is something you'll have to evaluate against your own situation.
Now, assuming this technique is usable, there is one nice advantage. Those GetMemberX functions can be turned into public static/member functions of your class, thus providing this generic member access to more places in your code.
class MyClass
{
public:
MyClass(SomeMap& Map);
int Member1;
int Member2;
int Member3;
static size_t GetMemberCount(void);
static const char* GetMemberID(size_t i);
int* GetMemberPtr(size_t i) const;
};
And if useful, you could also add a GetMemberPtrByID function to search for a given string ID and return a pointer to the corresponding member.
One disadvantage with this idea so far is that there is a risk that a member could be added to the class but not to the MyClassMembers array. However, this technique could be combined with xtofl's macro solution so that a single list could populate both the class and the array.
changes in the header:
#define MEMBERS\
MEMBER( Member1 )\
SEP MEMBER( Member2 )\
SEP MEMBER( Member3 )\
class MyClass
{
public:
#define SEP ;
#define MEMBER( name ) int name
MEMBERS;
#undef MEMBER
#undef SEP
// other stuff, member functions, etc
};
and changes in the code file:
const MemberData MyClassMembers[] =
{
#define SEP ,
#define MEMBER( name ) { offsetof(MyClass, name), #name }
MEMBERS
#undef MEMBER
#undef SEP
};
Note: I have left error checking out of my examples here. Depending on how this would be used, you might want to ensure the array bounds are not overrun with debug mode asserts and/or release mode checks that would return NULL pointers for bad indexes. Or some use of exceptions if appropriate.
Of course, if you aren't worried about error checking the array bounds, then GetMemberPtr could actually be changed into something else that would return a reference to the member.