boost::multi_array 示例的漂亮打印失败

Question

我有这个聪明又酷的example：

#include <iostream>
#include "boost/multi_array.hpp"
#include "boost/array.hpp"
#include "boost/cstdlib.hpp"

template <typename Array>
void print(std::ostream& os, const Array& A)
{
  typename Array::const_iterator i;
  os << "[";
  for (i = A.begin(); i != A.end(); ++i) {
    print(os, *i);
    if (boost::next(i) != A.end())
      os << ',';
  }
  os << "]";
}
void print(std::ostream& os, const double& x)
{
  os << x;
}
int main()
{
  typedef   boost::multi_array<double, 2> array;
  double values[] = {
    0, 1, 2,
    3, 4, 5 
  };
  const int values_size=6;
  array A(boost::extents[2][3]);
  A.assign(values,values+values_size);
  print(std::cout, A);
  return boost::exit_success;
}

但是如果我尝试编译它：g++ -I/usr/include/boost/b.cpp我得到这个错误：

b.cpp: In function ‘void print(std::ostream&, const Array&) [with Array = double]’:
b.cpp:12:   instantiated from ‘void print(std::ostream&, const Array&) [with Array = boost::detail::multi_array::const_sub_array<double, 1u, const double*>]’
b.cpp:12:   instantiated from ‘void print(std::ostream&, const Array&) [with Array = main()::array]’
b.cpp:32:   instantiated from here
b.cpp:9: error: ‘double’ is not a class, struct, or union type
b.cpp:11: error: request for member ‘begin’ in ‘A’, which is of non-class type ‘const double’
b.cpp:9: error: ‘double’ is not a class, struct, or union type
b.cpp:9: error: request for member ‘end’ in ‘A’, which is of non-class type ‘const double’
b.cpp:9: error: ‘double’ is not a class, struct, or union type
b.cpp:9: error: ‘double’ is not a class, struct, or union type
b.cpp:9: error: ‘double’ is not a class, struct, or union type
b.cpp:13: error: request for member ‘end’ in ‘A’, which is of non-class type ‘const double’
b.cpp:9: error: ‘double’ is not a class, struct, or union type

shell returned 1

出了什么问题？似乎不明白第一个和第二个 print 函数的区别。也许我错过了一些编译器选项？

编辑： 如果我使用 template 作为 Craig H 的答案，我会在一个孤立的 example.cpp 中解决问题。但是如果我把这2个函数放在我项目中的一个单独的.h文件中，错误又会出现！

Answer 1

在此示例中，您创建了一个模板函数作为您的第一个打印函数，然后声明了另一个函数，该函数实现了该模板的特定版本而无需模板专门化。我想如果你改变以下行

void print(std::ostream& os, const double& x)

到

template<> void print<double>(std::ostream& os, const double& x)

您的问题应该会消失。