Ambiguous C++ compiler error

问题:

The following bit of code fails to compile. The error seems to be some kind of ambigous call to the merge routine. My understanding is that STL has a merge routine found in the std namespace, but as far as I can tell the name merge in the code below should be unique.

If I rename merge to xmerge, everything works. What could the problem be? where is the name clash coming from?

http://codepad.org/uAKciGy5

#include <iostream>
#include <iterator>
#include <vector>

template<typename InputIterator1,
         typename InputIterator2,
         typename OutputIterator>
void merge(const InputIterator1 begin1, const InputIterator1 end1,
           const InputIterator2 begin2, const InputIterator2 end2,
           OutputIterator out)
{
   InputIterator1 itr1 = begin1;
   InputIterator2 itr2 = begin2;
   while ((itr1 != end1) && (itr2 != end2))
   {
      if (*itr1 < *itr2)
         *out = *itr1, ++itr1;
      else
         *out = *itr2, ++itr2;
      ++out;
   }
   while (itr1 != end1) *out++ = *itr1++;
   while (itr2 != end2) *out++ = *itr2++;
}

int main()
{
   std::vector<int> l1;
   std::vector<int> l2;
   std::vector<int> merged_list;

   merge(l1.begin(),l1.end(),
         l2.begin(),l2.end(),
         std::back_inserter(merged_list));

   return 0;
}

回答1:

Compiler is getting confused between your merge function and the std::merge defined in the algorithm. Use ::merge to remove this ambiguity. This call is ambiguous as compiler is using Argument Dependendent Lookup to search for the function when unqualified function name is used.