Thread: Problem in template that uses "decltype", "typename", and "std::remove_reference"

Afternoon all,

I have got a problem here that I cannot get sorted out.

The attached code runs fine as coded. If, however, you uncomment the alternate return lines 82 and 89 (in UnorderedMap::c0val and UnorderedMap::c1val), compile time errors result.

This bugs me, because those commented out return lines show the approach I would really prefer to use.

I have just started learning how to use the new C++11 "decltype" and "std::remove_reference" features in the past few days, so my mistake almost certainly lies at lines 42 to 45, in Sheet::data<I>(), where these are both being used.

Anyways, here's the code:

Code:

#include <tuple>
#include <unordered_map>
#include <vector>
#include <type_traits>


#include <cstdio>


// Compiler used: gcc version 4.6.3 on Ubuntu, via g++ Typename.C -std=c++0x


template<class C0, class C1>
class Sheet
{
 public:


  // Simple "spreadsheet" class with an embedded 2-tuple.
  // Here std::get<0>(embed) is an std::vector<C0>, the "first column",
  //  and std::get<1>(embed) is an std::vector<C1>, the "second column".


  std::tuple<std::vector<C0>, 
             std::vector<C1>>   embed;


  // Template function to return the std::vector::data() pointers of the
  //   C0 and C1 vectors in the embedded object.
  // Since std::get<I>() returns an std::vector<C0 or C1>&, but we want a
  //   (C0*) or (C1*), we need to strip away the reference, and then use
  //   std::vector::value_type to get (C0) or (C1), and add a pointer.
  // I have little experience with "decltype", so this is almost certainly
  //   where the mistake lies.
  // Notice that both std::remove_reference::type and std::vector::value_type
  //   are typenames, but I've got only one "typename" listed here; that
  //   bugs me. (I did try using remove_reference_t, but couldn't figure out
  //   the correct syntax for its use.)


  template<int I>
  auto data() -> typename std::remove_reference
                          < decltype(std::get<I>(embed))
                          > ::type
                          ::value_type *
  {
    return std::get<I>(embed).data();
  }


  // Forwarders to the data<I>() template.
  // You'll see in a minute why we need these.


  C0* c0data() { return data<0>(); }
  C1* c1data() { return data<1>(); }
};


template<class Key, class C0, class C1>
class UnorderedMap
{
  // Now, a class with an embedded unordered_map of Keys to Sheet<C0,C1>s.


 public:
  std::unordered_map<Key, Sheet<C0, C1>> uomap;


  // These forwarders take a Key argument, and return the data() pointers of
  //   the corresponding Sheet's std::vectors in the embedded unordered_map.
  // PROBLEM pops up at this point:
  // These *only* work when the Sheet::c0data/c1data forwarders are used.
  // Compilation fails if you try to use Sheet::data<I>, with this error:
  // "69:34: error: expected primary-expression before ‘)’ token".
  // Since the only thing Sheet::c0data/c1data do is to call Sheet::data<I>,
  //   I'm at a loss to explain this.


  C0* c0val(Key key)
  {
  //return uomap.at(key).data<0>(); // Compile time error
    return uomap.at(key).c0data();
  }


  C1* c1val(Key key)
  {
  //return uomap.at(key).data<1>(); // Compile time error
    return uomap.at(key).c1data();
  }
};


// Test program.


int main()
{
  // Create a simple three-row Sheet<float, double>.


  Sheet<float,double> s;
  std::get<0>(s.embed) = std::vector<float>  {1., 3., 5.};
  std::get<1>(s.embed) = std::vector<double> {2., 4., 6.};


  // Print out the addresses of the embedded float and double vectors,
  //   and their first elements (1. and 2.).


  printf("In original Sheet:\n");
  printf("Vectors are at  %p and %p\n", s.data<0>(),    s.data<1>());
  printf("First values:   %f and %f\n", s.data<0>()[0], s.data<1>()[0]);


  // Now make up an UnorderedMap whose embedded object contains only one
  //   element: the above-defined Sheet, at integer Key value 3.


  UnorderedMap<int, float, double> x;


  x.uomap[3] = s;


  // Here in main(), there is no problem with using Sheet::data<I>.


  printf("In UnorderedMap:\n");
  printf("Vectors are at  %p and %p\n", x.uomap[3].data<0>(),    
                                        x.uomap[3].data<1>());
  printf("First  values:  %f and %f\n", x.uomap[3].data<0>()[0], 
                                        x.uomap[3].data<1>()[0]);
  printf("Second values:  %f and %f\n", x.c0val(3)[1],
                                        x.c1val(3)[1]);
}

Thanks
Grump

Here's your solution: https://stackoverflow.com/questions/...template-class
Why you do use printf instead of std::cout?