I am trying to write a class that uses a std::tuple to store a bunch of objects with potentially different types. If the elements all have ctors that take some parameter, I can initialize the tuple from the container’s ctor correctly. However, if one of the elements only has a default ctor and the others do not, I am unable to construct the tuple. I get a compilation error.

Here’s the code:

struct Op1WithArg
{
    Op1WithArg(int){}
};

struct Op2WithArg
{
    Op2WithArg(std::string){}
};

struct OpWithoutArg
{
    OpWithoutArg(){}
};

template <class... Ops>
struct OpsSequence {
    using OpTuple = std::tuple<Ops...>;

    template<typename... Args>
    OpsSequence(Args... args)
    :   mOps{args...}
    {}

    OpTuple mOps;
};

void TestSequenceWithOpWithoutArg()
{
    // This works
    using SequenceType = OpsSequence<OpWithoutArg>;
    SequenceType sequence{};
}

void TestSequenceWithTwoOpsWithArg()
{
    // This works
    using SequenceType = OpsSequence<Op1WithArg, Op2WithArg>;
    SequenceType sequence{1, "Hello"};
}

void TestSequenceWithOneOpWithArgAndOneWithout()
{
    // This fails to compile
    using SequenceType = OpsSequence<Op1WithArg, OpWithoutArg>;
    SequenceType sequence{1,{}};
}

The compilation failure for the final function is as follows:

|| [ 50%] Building CXX object CMakeFiles/tsconv_tests.dir/tests/tsconv_tests.cpp.o
|| /home/anders/src/tsconv/tests/tsconv_tests.cpp: In function ‘void tsconv::TestSequenceWithOneOpWithArgAndOneWithout()’:
tests/tsconv_tests.cpp|1014 col 31| error: no matching function for call to ‘tsconv::OpsSequence<tsconv::Op1WithArg, tsconv::OpWithoutArg>::OpsSequence(<brace-enclosed initializer list>)’
||  1014 |     SequenceType sequence{1,{}};
||       |                               ^
tests/tsconv_tests.cpp|989 col 5| note: candidate: ‘tsconv::OpsSequence<Ops>::OpsSequence(Args ...) [with Args = {}; Ops = {tsconv::Op1WithArg, tsconv::OpWithoutArg}]’
||   989 |     OpsSequence(Args... args)
||       |     ^~~~~~~~~~~
tests/tsconv_tests.cpp|989 col 5| note:   candidate expects 0 arguments, 2 provided
tests/tsconv_tests.cpp|985 col 8| note: candidate: ‘constexpr tsconv::OpsSequence<tsconv::Op1WithArg, tsconv::OpWithoutArg>::OpsSequence(const tsconv::OpsSequence<tsconv::Op1WithArg, tsconv::OpWithoutArg>&)’
||   985 | struct OpsSequence {
||       |        ^~~~~~~~~~~
tests/tsconv_tests.cpp|985 col 8| note:   candidate expects 1 argument, 2 provided
tests/tsconv_tests.cpp|985 col 8| note: candidate: ‘constexpr tsconv::OpsSequence<tsconv::Op1WithArg, tsconv::OpWithoutArg>::OpsSequence(tsconv::OpsSequence<tsconv::Op1WithArg, tsconv::OpWithoutArg>&&)’
tests/tsconv_tests.cpp|985 col 8| note:   candidate expects 1 argument, 2 provided
gmake[2]: *** [CMakeFiles/tsconv_tests.dir/build.make|76| CMakeFiles/tsconv_tests.dir/tests/tsconv_tests.cpp.o] Error 1
gmake[1]: *** [CMakeFiles/Makefile2|83| CMakeFiles/tsconv_tests.dir/all] Error 2
gmake: *** [Makefile|101| all] Error 2

Is it possible to modify the code to get the failing function to compile?