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I am trying to write a calculation intensive program. And I need char* to be the fields of comparsion for the composite_key_compare of the multi_index_container. However, it doesn't seem to work. Code as below:

struct MyStruct
{
    char* firstName;
    char* secondName;
    int age;
};

struct equal_char
{   // functor for operator<=
    inline bool operator()(const char* left, const char* right) const
    {   // apply operator<= to operands
        bool result=(strcmp(left,right)==0);
        return result;
    }
};

typedef composite_key
    <MyStruct*,
    BOOST_MULTI_INDEX_MEMBER(MyStruct, char*, firstName),
    BOOST_MULTI_INDEX_MEMBER(MyStruct, char*, secondName)
    > comp_key;
typedef multi_index_container
    <
    MyStruct*, 
    indexed_by
        <
        ordered_unique
            <
                comp_key,
                composite_key_compare
                <equal_char, equal_char> 
            >
        >
    > MyContainer;

boost::ptr_vector<MyStruct> vec;
MyStruct* struct1=new MyStruct();
struct1->firstName="Michael";
struct1->secondName="Mike";
struct1->age=20;
vec.push_back(struct1);



MyContainer myContainer;
myContainer.insert(struct1);
char* first="Michael";
char* second="Mike";
auto it=myContainer.find(boost::make_tuple(first, second));
if(it!=myContainer.end())
    cout << (*it)->age << endl;

I did trace into the equal_char, and found out it did return true on the first comparison of "Michael" to "Michael", but I also found that the equal_char is not called for the second comparsion of "Mike" to "Mike". Anyone who can help me with this? How sould I write the composite_key_compare?

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1 Answer

up vote 0 down vote

I traced the program, and found out that boost expects something like std::less() for the compare operation for "ordered unique".

So, I added below code, and it worked.

struct CompareLess
{   // functor for operator<=
#pragma region For 1 variable
    static inline int compare(const char* left, const char* right)
    {
        return strcmp(left, right);
    }
    inline bool operator()(const char* left, const char* right) const
    {   // apply operator<= to operands
        return compare(left, right)<0;
    }

    static inline int compare(const boost::tuple<char*>& x, const char*y)
    {
        return compare(x.get<0>(),y);
    }
    inline bool operator()(const boost::tuple<char*>& x, const char*y) const
    {
        return compare(x,y)<0;
    }

    static inline int compare(const      boost::multi_index::composite_key_result<comp_key>& k, const boost::tuple<char*>& y)
    {
        return -compare(y,(const char*)(k.value->firstName));
    } 
    inline bool operator()(const boost::multi_index::composite_key_result<comp_key>& k, const boost::tuple<char*>& y) const
    {
        return compare(k,y)<0;
    }

    static inline int compare(const boost::tuple<char*>& y, const boost::multi_index::composite_key_result<comp_key>& k)
    {
        return compare(y,(const char*)(k.value->firstName));
    }
    inline bool operator()(const boost::tuple<char*>& y, const boost::multi_index::composite_key_result<comp_key>& k) const
    {
        return compare(y,k) <0;
    }

#pragma endregion For 1 variable
#pragma region For 2 variables
    static inline int compare(const boost::multi_index::composite_key_result<comp_key>& k, const boost::tuple<char*, char*>& y)
    {
        int val=strcmp(k.value->firstName, y.get<0>());
        if(val!=0)
            return val;
        else
            return compare(y.get<1>(),(const char*)(k.value->secondName));
    }
    inline bool operator()(const boost::multi_index::composite_key_result<comp_key>& k, const boost::tuple<char*, char*>& y) const
    {
        return compare(k,y) <0;
    }

    static inline int compare(const boost::tuple<char*, char*>& y, const boost::multi_index::composite_key_result<comp_key>& k)
    {
        return -compare(k,y);
    }
    inline bool operator()(const boost::tuple<char*, char*>& y, const boost::multi_index::composite_key_result<comp_key>& k) const
    {
        return compare(y,k)<0;
    }
#pragma endregion For 2 variables
#pragma region For all variables
    inline bool operator()(const boost::multi_index::composite_key_result<comp_key>& k1, 
        const boost::multi_index::composite_key_result<comp_key>& k2) const
    {
        return CompareLess()((const char*)(k1.value->firstName), (const char*)(k2.value->firstName));
    }
#pragma endregion For all variables



};

typedef multi_index_container
    <
    MyStruct*, 
    indexed_by
        <
        ordered_unique
            <
                comp_key,
                CompareLess
            >
        >
    > MyContainer;

But I got more question, this is very lengthy code, and suppose if I have more fields, it is going to be much much more longer. Is there any smart way to do this?

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