c++ compile-time string concatenation using boost-mpl

Question

I'm trying to concatenate strings at compile-time using boost-mpl but am getting errors from gcc. Here's the sample -

using namespace boost;
using namespace std;

template<class A>
struct type {};

template<>
struct type<int> {
    typedef mpl::string < 'i' > value;
};

template<>
struct type<char> {
    typedef mpl::string < 'c' > value;
};

struct empty {
};

template<class A, class B, class C, class D>
struct converter;

template<class A, class B = empty, class C = empty, class D = empty>

struct converter {
    typedef mpl::push_back< type<A>::value, converter<B,C,D>::value >::type value ;
};

template<>
struct converter<empty, empty, empty, empty> {
    typedef mpl::string < '\0' > value;
};

So, what i'm trying to achieve is :

converter<int,char,int> == "ici\0" // true. 

Problem is that the above code in gcc throws the following errors:

main.cpp:37: error: type/value mismatch at argument 1 in template parameter list for ‘template<class Sequence, class T> struct boost::mpl::push_back’
main.cpp:37: error:   expected a type, got ‘type::value’
main.cpp:37: error: type/value mismatch at argument 2 in template parameter list for ‘template<class Sequence, class T> struct boost::mpl::push_back’
main.cpp:37: error:   expected a type, got ‘converter::value’

Could anyone point out the problem with the above code and explain the right way to do it ? Thanks

EDIT 1: corrected formatting and few typos

EDIT 2: After Lambdageek, Andy's suggestion the code does compile but when i try to print the result

int main(int argc, char** argv) {
    cout << mpl::c_str< converter<int,char>::value >::value << endl;
    return 0;
}

, compiler complains -

/usr/local/include/boost/mpl/string.hpp:534:   instantiated from ‘boost::mpl::c_str<boost::mpl::push_back<boost::mpl::string<105, 0, 0, 0, 0, 0, 0, 0>, boost::mpl::push_back<boost::mpl::string<105, 0, 0, 0, 0, 0, 0, 0>, boost::mpl::string<0, 0, 0, 0, 0, 0, 0, 0> > > >’
main.cpp:49:   instantiated from here

/usr/local/include/boost/mpl/string.hpp:228: error: ‘value’ is not a member of ‘boost::mpl::push_back<boost::mpl::string<105, 0, 0, 0, 0, 0, 0, 0>, boost::mpl::string<0, 0, 0, 0, 0, 0, 0, 0> >’

/usr/local/include/boost/mpl/string.hpp: In instantiation of ‘boost::mpl::c_str<boost::mpl::push_back<boost::mpl::string<105, 0, 0, 0, 0, 0, 0, 0>, boost::mpl::push_back<boost::mpl::string<105, 0, 0, 0, 0, 0, 0, 0>, boost::mpl::string<0, 0, 0, 0, 0, 0, 0, 0> > > >’:
main.cpp:49:   instantiated from here
/usr/local/include/boost/mpl/string.hpp:548: error: no type named ‘value_type’ in struct boost::mpl::push_back<boost::mpl::string<105, 0, 0, 0, 0, 0, 0, 0>, boost::mpl::push_back<boost::mpl::string<105, 0, 0, 0, 0, 0, 0, 0>, boost::mpl::string<0, 0, 0, 0, 0, 0, 0, 0> > >’
main.cpp: In function ‘int main(int, char**)’:
main.cpp:49: error: ‘value’ is not a member of ‘boost::mpl::c_str<boost::mpl::push_back<boost::mpl::string<105, 0, 0, 0, 0, 0, 0, 0>, boost::mpl::push_back<boost::mpl::string<105, 0, 0, 0, 0, 0, 0, 0>, boost::mpl::string<0, 0, 0, 0, 0, 0, 0, 0> > > >’
make[2]: *** [build/Debug/GNU-Linux-x86/main.o] Error 1
make[1]: *** [.build-conf] Error 2

I admit that i'm very new to template programming, so am sure the problem must be something elementary. Thanks for all the help

EDIT 3: Changed the push_back line in converter struct.

Errors:

main.cpp:41: error: type ‘boost::mpl::push_back<typename type<A>::value, typename converter<B, C, D, empty>::value>’ is not derived from type ‘converter<A, B, C, D>’
main.cpp:41: error: expected ‘;’ before ‘value’

Eastern

Answer 1

OK, as per your final edits, I see several problems here.

First, you can use mpl::push_back to add an element to a sequence. Now you're concatenating two sequences. I changed the type of type<>::value to mpl::char_, then changed the order of the mpl::push_back arguments (first the sequence, then the element). Also, you have to use push_front, not push_back in this code. Finally, I added a ::type after the push_front, because you have to extract the actual type here. Here is the code for reference:

using namespace boost;
using namespace std;

template<class A>
struct type {};

template<>
struct type<int> {
    typedef mpl::char_ < 'i' > value;
};

template<>
struct type<char> {
    typedef mpl::char_ < 'c' > value;
};

struct empty {
};

template<class A, class B, class C, class D>
struct converter;


template<class A, class B = empty, class C = empty, class D = empty>
struct converter {
        typedef typename mpl::push_front< typename converter<B,C,D>::value, typename type<A>::value >::type value ;
};


template<>
struct converter<empty, empty, empty, empty> {
    typedef mpl::string < '\0' > value;
};

Now this code works as expected:

int
main (void)
{
        cout << mpl::c_str< converter<int,char>::value >::value << endl;
        return 0;
}

(prints ic).

Saltatory

That works. Thanks a ton. But just curious, is there a way to do the same using mpl::string and not char_ ? Also, i upvoted but can't mark it as the answer as the original question was already answered by lambdageek - sorry abt that – Eastern

@Code, the answer to your question doesn't generate a valid code :) Anyway, I'll look into how to do it using mpl::string in all places, maybe using a mpl::front_inserter. – Saltatory

@Code: OK, it can be done. The type has to be: typedef typename mpl::copy< typename converter<B,C,D>::value, mpl::back_inserter< typename type<A>::value > >::type value;. That will add all the characters in the type<A>::value into the back of the converter string. – Saltatory

Thanks. Looks complex but shall give it a try:) – Eastern

Answer 2

Does it help to add the typename keyword, to tell the compiler that ::value is a type?

struct converter {
    typedef mpl::push_back< typename type<A>::value, typename converter<B,C,D>::value > value ;
};

Prevost

Thanks. Plz check the updated question. The code compiles now but i again got erros when i added code to print the result. – Eastern

Answer 3

You need to use the typename keyword:

typedef mpl::push_back< typename type<A>::value, typename converter<B,C,D>>:value >::type value;

When accessing a nested typedef of a template that is instantiated with your template arguments, you need to help C++ decide whether the nested name refers to a method/field or to a nested type definition. If you don't say anything, C++ will assume it's a field name. If you say typename it will assume the nested thing is a type.

Warison

Thanks. Ur suggestion worked but there's a new error when i try to print the value, plz check edits to the question. – Eastern

Try adding ::type to the push_back line. See the example in the Boost documentation – Warison

Changed that line to typedef mpl::push_back< typename type<A>::value, typename converter<B,C,D>::value >::type value ; but now i'm getting main.cpp:41: error: type ‘boost::mpl::push_back<typename type<A>::value, typename converter<B, C, D, empty>::value>’ is not derived from type ‘converter<A, B, C, D>’ – Eastern

@Code: There's something wrong with your keyboard. "you" always comes through a "u". Also, the capital "I" seems to be broken. You might want to get a new keyboard. – Angeliqueangelis