unable to apply std::set_intersection on different types of structs with a common field

Question

I am trying to use use std::set_intersection to find common elements between 2 completely different types of data structures that have a common binding 'name' field.

I looked at the following enter link description here but it seems to force me down the route to do a custom conversion between the 2 different struct types which I was trying to avoid (as these types are from a 3rd party)

The code snippet below shows what I am trying to achieve.

// common field used for set intersection
typedef struct StructA {
    std::string mCommonField;
    float mFloatValue;
} StructA;

typedef struct StructB {
    std::string mCommonField;
    int mValue1;
    short mValue2;
} StructB;

// initially unsorted list
std::vector<StructA> aStructs = {{"hello", 1.0f}, {"goodbye", 2.0f}, {"foo", 3.0f}};
// initially unsorted list
std::vector<StructB> bStructs = {{"hello", 1, 2}, {"goodbye", 3, 4}, {"bar", 5, 6}};
// sorting both sets before calling std::intersection
std::sort(aStructs.begin(), aStructs.end(),
    [](const StructA& lhs, const StructA& rhs) {
        return lhs.mCommonField < rhs.mCommonField;
    });
std::sort(bStructs.begin(), bStructs.end(),
    [](const StructB& lhs, const StructB& rhs) {
    return lhs.mCommonField < rhs.mCommonField;
});

std::vector<StructA> intersection;
std::set_intersection(
    aStructs.begin(), aStructs.end(),
    bStructs.begin(), bStructs.end(),
    std::back_inserter(intersection),
    [](const StructA& lhs, const StructB& rhs){
        return lhs.mCommonField < rhs.mCommonField;
    });

I am using Visual Studio 2013 to compile the above, however the above code spits out a plethora of errors as shown below. Reading through the std::set_intersection I am having a problem putting together a compatible StrictWeakOrdering comp last argument. I would ideally like to implement this as a one off lambda.

template <class InputIterator1, class InputIterator2, class OutputIterator,
          class StrictWeakOrdering>
OutputIterator set_intersection(InputIterator1 first1, InputIterator1 last1,
                                InputIterator2 first2, InputIterator2 last2,
                                OutputIterator result, 
                                StrictWeakOrdering comp);

1>C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\include\algorithm(3591): error C2664: 'bool (__vectorcall *)(const main::StructA &,const main::StructB &)' : cannot convert argument 1 from 'main::StructB' to 'const main::StructA &' 1>
Reason: cannot convert from 'main::StructB' to 'const main::StructA' 1> No user-defined-conversion operator available that can perform this conversion, or the operator cannot be called 1>
C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\include\algorithm(3625) : see reference to function template instantiation '_OutIt std::_Set_intersection<_InIt1,_InIt2,_OutIt,_Pr>(_InIt1,_InIt1,_InIt2,_InIt2,_OutIt,_Pr)' being compiled 1> with 1> [ 1>
_OutIt=std::back_insert_iterator>> 1> , _InIt1=main::StructA * 1> ,
_InIt2=main::StructB * 1> , _Pr=main:: 1> ] 1> C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\include\algorithm(3654) : see reference to function template instantiation '_OutIt std::_Set_intersection2(_InIt1,_InIt1,_InIt2,_InIt2,_OutIt,_Pr,std::true_type)' being compiled 1> with 1> [ 1>
_OutIt=std::back_insert_iterator>> 1> , _Pr=main:: 1> , _InIt1=main::StructA * 1> ,
_InIt2=main::StructB * 1> ] 1> ....\src\dlf\main.cpp(111) : see reference to function template instantiation '_OutIt std::set_intersection>>,std::_Vector_iterator>>,std::back_insert_iterator>>,main::>(_InIt1,_InIt1,_InIt2,_InIt2,_OutIt,_Pr)' being compiled 1> with 1> [ 1>
_OutIt=std::back_insert_iterator>> 1> , _Ty=main::StructA 1> ,
_InIt1=std::_Vector_iterator>> 1> ,
_InIt2=std::_Vector_iterator>> 1> , _Pr=main:: 1> ]

I also tried to use a custom comparator struct to do the comparison, but the errors were even more confusing as per:

struct comparator {
    bool operator()(const StructA& lhs, const StructB& rhs) const {
        return lhs.mCommonField < rhs.mCommonField;
    }
    bool operator()(const StructB& lhs, const StructA& rhs) const {
        return lhs.mCommonField < rhs.mCommonField;
    }
};

std::vector<StructA> intersection;
std::set_intersection(
    aStructs.begin(), aStructs.end(),
    bStructs.begin(), bStructs.end(),
    std::back_inserter(intersection),
    comparator());

which resulted in the following verbose error output. I was hoping to avoid customizing the structures (as the actual ones I am trying to use are from a 3rd party) to have converters from StructA to StructB and vise-versa, is there any way I can avoid that and just have some simple lambda to achieve a simple binding between 2 relatively simple structs with a common field?
Thanks in advance.

1>C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\include\xutility(521): error C2664: 'bool main::comparator::operator ()(const main::StructA &,const main::StructB &) const' : cannot convert argument 1 from 'main::StructA' to 'const main::StructB &' 1> Reason: cannot convert from 'main::StructA' to 'const main::StructB' 1> No user-defined-conversion operator available that can perform this conversion, or the operator cannot be called 1> C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\include\xutility(625) : see reference to function template instantiation 'bool std::_Debug_lt_pred<_Pr,main::StructA&,main::StructA&>(_Pr,_Ty1,_Ty2,std::_Dbfile_t,std::_Dbline_t)' being compiled 1> with 1> [ 1>
_Pr=main::comparator 1> , _Ty1=main::StructA & 1> , _Ty2=main::StructA & 1> ] 1> C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\include\xutility(636) : see reference to function template instantiation 'void std::_Debug_order2<_InIt,_Pr>(_FwdIt,_FwdIt,_Pr,std::_Dbfile_t,std::_Dbline_t,std::forward_iterator_tag)' being compiled 1> with 1> [ 1>
_InIt=std::_Vector_iterator>> 1> , _Pr=main::comparator 1> ,
_FwdIt=std::_Vector_iterator>> 1> ] 1> C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\include\algorithm(3649) : see reference to function template instantiation 'void std::_Debug_order<_InIt1,_Pr>(_InIt,_InIt,_Pr,std::_Dbfile_t,std::_Dbline_t)' being compiled 1> with 1> [ 1>
_InIt1=std::_Vector_iterator>> 1> , _Pr=main::comparator 1> ,
_InIt=std::_Vector_iterator>> 1> ] 1> ....\src\dlf\main.cpp(118) : see reference to function template instantiation '_OutIt std::set_intersection>>,std::_Vector_iterator>>,std::back_insert_iterator>>,main::comparator>(_InIt1,_InIt1,_InIt2,_InIt2,_OutIt,_Pr)' being compiled 1> with 1> [ 1>
_OutIt=std::back_insert_iterator>> 1> , _Ty=main::StructA 1> ,
_InIt1=std::_Vector_iterator>> 1> ,
_InIt2=std::_Vector_iterator>> 1> , _Pr=main::comparator 1> ] 1>C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\include\xutility(523): error C2664: 'bool main::comparator::operator ()(const main::StructA &,const main::StructB &) const' : cannot convert argument 1 from 'main::StructA' to 'const main::StructB &' 1> Reason: cannot convert from 'main::StructA' to 'const main::StructB' 1> No user-defined-conversion operator available that can perform this conversion, or the operator cannot be called 1>C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\include\xutility(521): error C2664: 'bool main::comparator::operator ()(const main::StructA &,const main::StructB &) const' : cannot convert argument 2 from 'main::StructB' to 'const main::StructA &' 1> Reason: cannot convert from 'main::StructB' to 'const main::StructA' 1> No user-defined-conversion operator available that can perform this conversion, or the operator cannot be called 1> C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\include\xutility(625) : see reference to function template instantiation 'bool std::_Debug_lt_pred<_Pr,main::StructB&,main::StructB&>(_Pr,_Ty1,_Ty2,std::_Dbfile_t,std::_Dbline_t)' being compiled 1> with 1> [ 1>
_Pr=main::comparator 1> , _Ty1=main::StructB & 1> , _Ty2=main::StructB & 1> ] 1> C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\include\xutility(636) : see reference to function template instantiation 'void std::_Debug_order2<_InIt,_Pr>(_FwdIt,_FwdIt,_Pr,std::_Dbfile_t,std::_Dbline_t,std::forward_iterator_tag)' being compiled 1> with 1> [ 1>
_InIt=std::_Vector_iterator>> 1> , _Pr=main::comparator 1> ,
_FwdIt=std::_Vector_iterator>> 1> ] 1> C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\include\algorithm(3650) : see reference to function template instantiation 'void std::_Debug_order<_InIt2,_Pr>(_InIt,_InIt,_Pr,std::_Dbfile_t,std::_Dbline_t)' being compiled 1> with 1> [ 1>
_InIt2=std::_Vector_iterator>> 1> , _Pr=main::comparator 1> ,
_InIt=std::_Vector_iterator>> 1> ] 1>C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\include\xutility(523): error C2664: 'bool main::comparator::operator ()(const main::StructA &,const main::StructB &) const' : cannot convert argument 2 from 'main::StructB' to 'const main::StructA &' 1> Reason: cannot convert from 'main::StructB' to 'const main::StructA' 1> No user-defined-conversion operator available that can perform this conversion, or the operator cannot be called

Answer 1

Thanks to the expressiveness of C++, there are a few ways you can solve this problem. The following is by no means an exhaustive list.

1. Implicitly convert both types to a wrapper struct for comparison

If you're attached to using lambdas, define a type that can be implicitly constructed from both StructA and StructB and wraps the fields used for comparison. This can allow for additional logic to be performed to the fields in the constructor before comparison. For example:

struct Common {
    std::string const& mCommonField;
    Common(StructA const& sa) : mCommonField{sa.mCommonField} {};
    Common(StructB const& sb) : mCommonField{sb.mCommonField} {};
};

Then your comparison lambda can be written

auto cmp = [](Common const& lhs, Common const& rhs) {
    return lhs.mCommonField < rhs.mCommonField;
};

and used like

std::sort(aStructs.begin(), aStructs.end(), cmp);
std::sort(bStructs.begin(), bStructs.end(), cmp);
// ...
std::set_intersection(aStructs.begin(), aStructs.end(),
                      bStructs.begin(), bStructs.end(),
                      std::back_inserter(intersection),
                      cmp
                      );

Live example on Coliru Viewer.

2. Use a comparator with templated operator().

Instead of using a lambda, define a functor with a templated operator().

struct comparator
{
    template<typename T, typename U>
    bool operator()(T const& lhs, U const& rhs) const {
        return lhs.mCommonField < rhs.mCommonField;
    }
};

Then, it's as easy as:

std::sort(aStructs.begin(), aStructs.end(), comparator{});
std::sort(bStructs.begin(), bStructs.end(), comparator{});
// ...
std::set_intersection(aStructs.begin(), aStructs.end(),
                      bStructs.begin(), bStructs.end(),
                      std::back_inserter(intersection),
                      comparator{}
                      );

Just note that as there is a template in the comparator, it must be declared outside of function scope. Live example on Coliru Viewer.

3. Wait for C++14

And with generic lambdas added to C++14, you can use the following with a conformant compiler:

auto cmp = [](auto lhs, auto rhs) { return lhs.mCommonField < rhs.mCommonField; };
// ...
std::sort(aStructs.begin(), aStructs.end(), cmp);
std::sort(bStructs.begin(), bStructs.end(), cmp);
// ...
std::set_intersection(aStructs.begin(), aStructs.end(),
                      bStructs.begin(), bStructs.end(),
                      std::back_inserter(intersection),
                      cmp);

Again, live example on Coliru Viewer.

---

Also, C-style struct typedefs are unnecessary in C++ (and arguably unclear most places in C), so anywhere you have

typedef struct Foo {
    // ...
} Foo;

you can replace it with

struct Foo {
    // ...
};

without any other changes required of your code.