Object with attributes of any type with easy storage and iteration

Question

Object with attributes of any type with easy storage and iteration

I have an "object" with different attributes stored as key / value. The key is a string and the value can be any basic type. My first idea was to use a template class:

template <class T>
class Attribute {
public:
    Attribute<T>(const std::string& key, T value) :
        m_key(key),
        m_value(value)
    {
    }

    ~Attribute(){}

    T getValue() const
    {
        return m_value;
    }

    std::string getKey() const
    {
        return m_key;
    }
private:
    std::string m_key;
    T m_value;
};

But now the problem is, in my object class, I have to declare fields and overloading functions for every possible attribute type:

class MyObject {
public:
    MyObject(int value) :
        m_value(value)
    {
    }

    ~MyObject()
    {
    }

    int getValue() const
    {
        return m_value;
    }

    void addAttribute(Attribute<int> attribute)
    {
        m_intAttributes.push_back(attribute);
    }

    void addAttribute(Attribute<double> attribute)
    {
        m_doubleAttributes.push_back(attribute);
    }

    const std::list<Attribute<int> >& getIntAttributes() const
    {
        return m_intAttributes;
    }

    const std::list<Attribute<double> >& getDoubleAttributes() const
    {
        return m_doubleAttributes;
    }

private:
    int m_value;
    std::list<Attribute<int> > m_intAttributes;
    std::list<Attribute<double> > m_doubleAttributes;
};

Also, iterating through attributes is not very convenient and finding an attribute of a given name is very difficult:

void showMyObject(const MyObject& myObject)
{
    std::list<Attribute<int> > intAttributes;
    std::list<Attribute<int> >::const_iterator itInt;
    std::list<Attribute<double> > doubleAttributes;
    std::list<Attribute<double> >::const_iterator itDouble;

    std::cout << "Value in myObject " << myObject.getValue() << std::endl;

    intAttributes = myObject.getIntAttributes();
    for(itInt = intAttributes.begin() ; itInt != intAttributes.end() ; itInt++)
    {
        std::cout << itInt->getKey() << " = " << itInt->getValue() << std::endl;
    }

    doubleAttributes = myObject.getDoubleAttributes();
    for(itDouble = doubleAttributes.begin() ; itDouble != doubleAttributes.end() ; itDouble++)
    {
        std::cout << itDouble->getKey() << " = " << itDouble->getValue() << std::endl;
    }
}

FYI, my main function looks like this:

int main(int argc, char* argv[])
{
    MyObject object(123);

    object.addAttribute(Attribute<double>("testDouble", 3.23));
    object.addAttribute(Attribute<double>("testDouble2", 99.654));
    object.addAttribute(Attribute<int>("testInt", 3));
    object.addAttribute(Attribute<int>("testInt2", 99));

    showMyObject(object);

    return 0;
}

My guess is that if we want to guarantee type safety, there must be a list of functions with the correct return type in the signature (getTYPEAttributes in my example).

However, I was wondering if there is a more elegant solution, and if a design pattern that I am not aware of can help me deal with this problem correctly.

+3

c ++ design-patterns templates

morandg Apr 30 At 15:05

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1 answer

Barry · Accepted Answer · 2015-04-30T15:34:25+0000

Sounds like a job for Boost.TypeErasure . Do you want to store different things that have common features (are streaming, have a key), but can be explicitly accessible and do not need a common base? Store your attributes this way:

namespace mpl = boost::mpl
using namespace boost::type_erasure;

BOOST_TYPE_ERASURE_MEMBER((has_getKey), getKey, 0)

using AnyAttribute = any<mpl::vector<
                         copy_constructible<>,
                         typeid_<>,
                         ostreamable<>, // add a stream operator for Attribute
                         has_getKey<std::string(), const _self>
                         > >;

std::vector<AnyAttribute> attributes;

Adding an attribute will look like this:

template <typename T>
void addAttribute(const std::string& key, const T& value) {
    // since Attribute<T> is copy-constructible, streamable,
    // and has a member function with the signature std::string getKey() const
    // we can construct an AnyAttribute with it.
    attributes.push_back(Attribute<T>(key, value));
}

Printing all attributes:

void showMe() {
    for (const auto& attr : attributes) {
        std::cout << attr << ' '; // since we specified ostreamable<>,
                                  // everything we put into this any<> is streamable
                                  // so the any<> is too
    }
    std::cout << '\n';
}

Searching for an attribute by name and specified type returns nullptr

if not found or of wrong type:

template <typename T>
const Attribute<T>* lookupAttribute(const std::string& key) {
    // can use getKey() even if they're all different types
    // because we added has_getKey<> as a concept
    auto it = std::find_if(attributes.begin(), attributes.end(), 
        [=](const AnyAttribute& a) {
            return a.getKey() == key;
        }); 

    if (it != attributes.end()) {
        // this will return a valid Attribute<T>* you specified the
        // correct type, nullptr if you specified the incorrect type
        // it is not possible to query the type.
        return any_cast<Attribute<T>*>(&*it);
    }   
    else {
        return nullptr;
    }   
}

There is a simpler object with a type, which is simple Boost.Any

, but there you cannot have any general functionality - to make it difficult to implement either search or print operations that I illustrated above.

Object with attributes of any type with easy storage and iteration

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