There is an existing enum
typedef enum
{
myEnum_front = 11,
myEnum_back = 19
} myEnumSides;
I want to create another enum new_myEnumSides and it's values should be mapped to the values of myEnumSides. Hence forth I would be using new_myEnumSides instead of myEnumSides.
Is the code below ok for this purpose?
typedef enum
{
new_myEnum_front = myEnumSides::myEnum_front,
new_myEnum_back = myEnumSides::myEnum_back
} new_myEnumSides;
Or is there a better way?
I can't possibly imagine why do you need to do it... If you don't need to rename the enum's values, you can just make another variable of the first one, without adding another enum (I believe this is not your case, but still have to point out this opportunity):
typedef enum
{
myEnum_front = 11,
myEnum_back = 19
} myEnumSides, new_myEnumSides;//<-- just add the new variable here
If you do want to rename it (which I believe, is your case), you should not use the :: operator, but simply write:
typedef enum
{
myEnum_front = 11,
myEnum_back = 19
} myEnumSides;
typedef enum
{
new_myEnum_front = myEnum_front,
new_myEnum_back = myEnum_back
} new_myEnumSides;
The :: operator should be used only if the enum is inside a class, structure or namespace, otherwise no :: is needed.
The only reason I can imagine you wanting to do this is to extend an existing enum and create a new one with extra values.
As enum does not offer any form of inheritance, being compile time constants and really just integers, you could do something like this, although I don't particularly recommend it..
// internalEnums.h
myEnum_front = 11,
myEnum_back = 19
// customerEnums.h
someNewValue = 20,
someOtherNewValue = 21
// Wherever you want to define your enums
typedef enum
{
#include "customerEnums.h"
} customerAccessible;
typedef enum
{
#include "internalEnums.h"
#include "customerEnums.h"
} internalUseOnly;
// Now there are two enumerations, sharing items.
customerAccessible::someNewValue // 11
customerAccessible::myEnum_front; // Doesn't exist
EDIT As per your comment, you could expose enumerations to the customer by keeping values in a header specifically for external use.
It's usually the case that you don't want to expose details of third party libraries, but an enumeration that lies there might have the exact members you need.
In that case it's beneficial to create a mapping for the third party enum, so that even if the backend library changes, you can simply provide the enumerators yourself.
The most concise way to do this, is by creating an alias:
using MyEnum = ThirdPartyEnum;
// {
// enumerator1,
// enumerator2
// }
This way, user code won't rely on third party internals,
you avoid doing conversions and even if the third party library is replaced, your users can keep using MyEnum by simply uncommenting the enumerator values (I like to keep them in comments, so users won't have to refer to third party libraries for documentation.
Related
Background
For a UI in an embedded project, I'm looking for a nice generic way to store a "state" and cycle through it with a button press, e.g. a list of menu items.
Normally, I like to use enums for this purpose, for example:
enum class MenuItem {
main,
config,
foo,
bar,
};
Then, in my UI code, I can store a currentMenuItem state like
MenuItem currentMenuItem = MenuItem::MAIN;
and do things depending on the current state by comparing currentMenuItem to any of its possible values, as declared in the enum.
Problem
The thing is, I would now like to advance to the next menu item. For that, I can quite trivially write a function that does that by casting to int, incrementing by one, and casting it back to the enum. I have multiple distinct enums, so I can even use this templated function that does this for arbitrary enums:
template <typename T>
void advanceEnum(T &e) {
e = static_cast<T>(static_cast<int>(e) + 1);
}
The problem with this is that it doesn't wrap around: it will happily keep increasing the underlying integer beyond the number of elements in the actual enum. I could quite easily solve this (by taking the modulo with the number of elements in the above function), if only there was an clean way to get the element count of an enum. Which, as far as I could find, there isn't really.
Custom enum?
I was thinking of writing a custom 'CyclicEnum' class that implements this behaviour, that I can subsequently derive from. That way, I could also write this as an overloaded operator++.
However, I still haven't devised how to can get an enum-like thing without actually using an enum. For example, I got to something like this:
class CyclicEnum {
public:
uint8_t v;
CyclicEnum& operator++() {
v = (v+1) % count;
return *this;
}
CyclicEnum operator++(int) {
CyclicEnum old = *this;
operator++();
return old;
}
private:
uint8_t count;
protected:
CyclicEnum(uint8_t v, uint8_t count) : v(v), count(count) {}
};
struct Tab : public CyclicEnum {
enum Value {
main,
config,
foo,
bar,
};
Tab(Value v) : CyclicEnum(v, 4) {}
};
However, as you can see, this still uses an enum inside the custom CyclicEnum class, and I'm back to the same issue: I can't count the number of Enum elements, so I have to specify that manually (which I think is not nice because it's redundant). Secondly, this way I also have to override the constructor in the derived class which I would like to avoid to keep it as clean as possible.
Upon searching this issue, many people apparently recommend to add a "dummy" value at the end of the enum as a trick to get the size:
enum Value {
main,
config,
foo,
bar,
_count,
};
but frankly, I find that just ugly, since _count is now actually a valid option.
Is there any way around this? Am I abusing enums? Looking at the fact that enums are apparently (by design) so hard to count, probably. But what would be a nice way to have a structure like this with named values like an enum provides?
EDIT:
OK, I'm convinced that using a _count element at the end isn't so bad of an idea. Still, I'd like to encapsulate this in some kind of structure, like a class.
I also thought of instead of using inheritance, using a class template to accomplish this, something like this:
(caution, this doesn't compile):
template<typename T>
struct CyclicEnum {
T v;
enum Values = T;
CyclicEnum& operator++() {
v = (v+1) % T::_count;
return *this;
}
CyclicEnum operator++(int) {
CyclicEnum old = *this;
operator++();
return old;
}
CyclicEnum(T v) : v(v) {}
};
struct MenuItem : public CyclicEnum<enum class {
main,
config,
foo,
bar,
_count,
}> {};
However, this does not work because "ISO C++ forbids forward references to 'enum' types" and "anonymous enum cannot be defined in a type specifier"...
Is there another way to make this idea work (template a class with an enum), or is this not going to work?
The thing is, I would now like to advance to the next menu item.
++ comes to mind. Keep it simple.
That way, I could also write this as an overloaded operator++
Yeah... or again, keep it simple, you could just drop the whole class. There's no need to write an abstraction layer around a simple integer. Really.
Upon searching this issue, many people apparently recommend to add a "dummy" value at the end of the enum as a trick to get the size
Sure why not. This is incredibly common practice.
but frankly, I find that just ugly, since LAST is now actually a valid option.
It's canonical code, it isn't ugly. Just give it a sensible name, maybe something like MENU_ITEMS_N to suggest that this a counter variable then use it as such. for(int i=0; i<MENU_ITEMS_N; i++) ...
Am I abusing enums?
Enums are just named integer values. Don't over-engineer your code. It's bad for performance, it's bad for maintenance, it adds needless complexity.
You could use the magic_enum library to reflect on enums.
Example which gets the names of all enum elements as std::array < std::string_view > and prints them.
#include <algorithm>
#include <iostream>
#include <magic_enum.hpp>
enum struct Apple
{
Fuji = 2,
Honeycrisp = -3,
Envy = 4
};
int
main ()
{
constexpr auto &appleNames = magic_enum::enum_names<Apple> (); // get an std::array<std::string_view> with the names for the enum sorted by value
std::copy (appleNames.begin (), appleNames.end (), std::ostream_iterator<std::string_view> (std::cout, "\n")); // print all the names
}
Prints:
Honeycrisp
Fuji
Envy
There are some limitations please read magic enum limitations
Rather then using _count, set the last "sentinel" value to the last actual value.
enum Value {
main,
config,
foo,
bar,
last = bar
};
Then you avoid the problem of having an enum value that is not a valid menu option. in your increment for example instead of :
v = static_cast<Value>( (static_cast<int>(v) + 1) %
static_cast<int>(Value::_count) );
you'd have:
v = static_cast<Value>( (static_cast<int>(v) + 1) %
(static_cast<int>(Value::last) + 1) ) ;
If in fact these enums simply cause different menu item handler functions to be called, then you could instead used an array of pointer-to-functions rather then an enum/switch or whatever.
I have a situation where I need two enums to hold one member of the same name. My code is in C++, Using IAR Embeddedj Workbench IDE. The code snippet is as follows:
enum Port_e : uint32_t
{
PortA = 0,
PortB,
PortC,
PortD,
PortE,
PortF,
PortG,
PortH,
PortI,
PortJ,
PortK,
NONE
};
enum Pin_e : uint32_t
{
Pin0 = 0, Pin1, Pin2, Pin3, Pin4, Pin5, Pin6, Pin7,
Pin8, Pin9, Pin10, Pin11, Pin12, Pin13, Pin14, Pin15,NONE
};
If you notice here both enums have the last member as NONE.
This code does not compile. Gives Error as NONE is already defined.
Is there any way to make it build while keeping the name as it is?
I also do not want to change the type to "enum class" as it will break the rest of my application code.
Is there any way to make it build while keeping name as it is?
Not without changes. If you wrap Port_e and Pin_e in a namespace or class you can resolve the name collision, but that would still alter the way the enums are used.
I also do not want to change the type to "enum class" as it will break the rest of my application code.
I strongly suggest you to do that anyway, and take the time to fix your application code. enum class was designed exactly to solve this problem.
You can use class enums or nest your enums in appropriate classes or namespaces.
#include <iostream>
enum class A :uint32_t { One, Two, Three };
enum class B :uint32_t { One, Two, Three };
int main()
{
std::cout << int(A::One) << std::endl;
}
Identifiers of enumeration are names of integral constants belonging to namespace enum is declared in. Class enums changed this situation, but you cant cast from and to them implicitly. Any other approach eventually would cause ODR breakage if you would use simple enogh names. In your code it leads to ambiguity: is NONE equal to 13? Or to 16?
Suppose I have an enum:
enum Types
{
TYPE_ASCENDING,
TYPE_DESCENDING
};
and I use it to it... anywhere in the code. Say if(bla < TYPE_ASCENDING), or with a switch/case. The real enum is much larger.
Whatever the results of the checks (or anything else), it needs to be std::cout in a prettier way to let the user know what happened. So, continuing the if() example, it might be something like this:
if(bla < TYPE_ASCENDING)
std::cout << "Ascending.\n";
All these happen inside a class. My question: is there a way to define some type of variable/STL/anything that would allow storing both enum-like and std::string-like variables, but would also let me use, separately, both types?
One thought was a namespace, but it seems it can't be used inside a class. To exemplify, here's what it would have looked like:
namespace Type
{
enum Types
{
ASCENDING,
DESCENDING
};
std::string s[2] {"Ascending", "Descending"};
};
and it would have been called as Type::ASCENDING for the if(), and Type::s[0] for the string. But, no namespace inside a class, so it's not a solution.
Using std::map only lets me use int as the index, so I can only use this:
std::map<Types, std::string> m {{TYPE_ASCENDING, "Ascending}, {TYPE_DESCENDING, "Descending"}};
as m[0], or m[TYPE_ASCENDING], but I can't call it for it's index to be used inside the if(). For that I have to call the enum, separately, which means I have both an enum and a map, two variables. I need one, unified, to avoid chasing variable names all over the code.
If I use a struct, I can't access directly Struct::TYPE_DESENDING, I need to create an object.
I can use an enum and a std::string array/vector, but that means that, again, I have to call two variables, separately, and I'd like them to be unified.
Is what I want possible?
You don't really have that mechanism in native C++. You can write a map / mapper function.
enum class E
{
ONE,
TWO
};
std::unordered_map<E,std::string> eStrings { {E::ONE,"ONE"},{E::TWO,"two"}};
While this is C++11 you can do the same for older C++ versions
Then you can use this like
std::cout << eStrings[E::ONE];
The issue here is you have to maintain this manually. So when you add a new value to the enum you have to manually add a new entry to the map.
The same would be true for writing a class or functions to have this behavior. You always have to duplicate the code of enum declaration and the mapping to the string.
A solution here would be to use some tool to generate these.
You can define in some file your enum (this is just some random format and only intended for explaining this. Chose whatever you want in your own defenition file)
E
- ONE
- TWO
And then generate the C++ enum and Map in a header and/or cpp file.
enum class <name>
{
<foreach:> <value>,
};
std::unordered_map< <name> ,std::string> eStrings
{
<foreach:> {<name>::<value>,"<value>"},
};
If you don't like having a map this approach is pretty flexible. You can also generate a switch case statement if you like
std::string getString(<name> e)
{
switch(e)
{
<foreach:> case <name>::<value>: return "<value>";
}
}
The syntax here is no standard for anything just some "pseudocode" to visualize the concept. There are several ways to generate c++ code out there. You can choose whatever you want or write your own program for this.
Note:
This is also just a general concept. You can wrap this functioniality / map etc into another class, make it static etc. for optimizations and not put it in global scope.
If you need something more fancy than just a map to lookup the string you can create a class with this concept or another map which does just the reverse lookup. It's more about the fact that you most likely have to generate the code by an external tool.
Reading Hayts answer I see that what I originally wrote may be relevant for the auto generation of code aspect. So I leave it here.
Seeing as regular old enums are implicitly convertible to int (as opposed to enum classes), you can simply use a map<int, string>.
Now, to the interesting part, generating it semi-automatically.
#include <iostream>
#include <map>
#include <string>
struct Thing {
enum Type {
# define ENUM_DEF(v, s) v,
ENUM_DEF(TYPE_ASCENDING, "Ascending")
ENUM_DEF(TYPE_DESCENDING, "Descending")
# undef ENUM_DEF
};
std::map<int, std::string> string;
Thing() {
# define ENUM_DEF(v, s) string[v] = s;
ENUM_DEF(TYPE_ASCENDING, "Ascending")
ENUM_DEF(TYPE_DESCENDING, "Descending")
# undef ENUM_DEF
}
};
int main() {
Thing t;
std::cout << t.string[0];
return 0;
}
I used a technique known as X-Macros. The premise is that you pass all the argument needed for your enum into the macro. Then you define the macro, depending on how you need the arguments to be used. So firstly:
# define ENUM_DEF(v, s) v,
This just expands the enum token as you'd provide it to a regular enum defintion.
Then, in Things c'tor:
# define ENUM_DEF(v, s) string[v] = s;
It expands to the statement you need to populate the map.
And to address one last point you may have issue with: Do you really have to do all this repetition, retyping ENUM_DEF all the time?
Well, luckily you don't. You can move those statements into their own file, let's call it type_enum.def:
#ifdef ENUM_DEF
ENUM_DEF(TYPE_ASCENDING, "Ascending")
ENUM_DEF(TYPE_DESCENDING, "Descending")
#endif //ENUM_DEF
And the original code becomes:
#include <iostream>
#include <map>
#include <string>
struct Thing {
enum Type {
# define ENUM_DEF(v, s) v,
# include "type_enum.def"
# undef ENUM_DEF
};
std::map<int, std::string> string;
Thing() {
# define ENUM_DEF(v, s) string[v] = s;
# include "type_enum.def"
# undef ENUM_DEF
}
};
int main() {
Thing t;
std::cout << t.string[0];
return 0;
}
There are several questions on these forums about the inheritance of C++ enums for extending (which is actually the thing without the logic). But what about inheritance just for setting specific values?
Currently, there is something like the following in my code:
//lib_impl.h
enum class X {
a = 13, // these values are
b = 42 // implementation dependent
}
//lib.h
#include "lib_impl.h"
void some_func(X param) {
X x = X::a;
}
I just want to avoid the dependecy of the 'lib' from its implementation. Probably, something other than enums must be used for that. As even in C++11 we have the ability only to declare forward enum name, but not its enumerators:
//lib.h
enum class X { a, b } // this is both declaration and definition, unfortunately
void some_func(X param) {
X x = X::a;
}
//lib_impl.h
#include "lib.h"
enum class X { // redefenition, compilation error
a = 13,
b = 42
}
What is the best compile-time solution for such problems?
--
As it seems to be unimplementable in c++, what is the most common way to resolve such issues? Leave the dependency of the 'lib' from the 'impl' as it is? Probably, 'impl' could be split into two parts, small which will be included before the 'lib.h' and other, bigger, to be included after it. Is it ok or I need to abandon the use of enums in favor of abstract classes?
Expose an enum with nominal values (start with 0, sequential say). Inside your library, remap these values to an internal enum with implementation dependent values (say an array for speed, using the external value as index). Reverse the mapping if you export said enum values to the outside (the reverse mapping will be slower).
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.