C++: Using fully qualified name with sort function - inconsistent behaviour

I have the following C++ code that computes the median of a container taken from Accelerated C++ by Koenig.

median.h

#ifndef GUARD_median_h
#define GUARD_median_h

#include <stdexcept>
#include <algorithm>
#include <cstddef>

template<class T, class Iterator>
T median (Iterator begin, Iterator end)
{
    size_t size = end - begin;
    if (size == 0)
        throw std::domain_error("median of an empty vector");

    sort(begin, end);

    size_t mid = size/2;

    return size%2 == 0 ? (begin[mid] + begin[mid-1])/2 : begin[mid];
}
#endif

median_test.cpp

#include <vector>
#include <iostream>
#include "median.h"

using std::vector;  using std::cin;
using std::cout;    using std::endl;

int main()
{
    vector<double> myVec;

    cout << "Please enter integers: ";

    double val;
    while (cin >> val) {
        myVec.push_back(val);
    }

    cout << "The median is: " << median<double>(myVec.begin(), myVec.end()) << endl;

    return 0;
}

This code compiles and runs just fine. But if I make a slight modification to test the median function with an array rather than a vector like so...

median_test_array.cpp

#include <iostream>
#include "median.h"

using std::cin;
using std::cout;    using std::endl;

int main()
{
    double myVec[1000];

    cout << "Please enter integers: ";

    double val;
    size_t i = 0;
    while (cin >> val) {
        myVec[i++] = val;
    }

    cout << "The median is: " << median<double>(myVec, myVec+i) << endl;

    return 0;
}

I get the following compile error:

| => g++-6 -I. median_test_array.cpp
In file included from median_test_array.cpp:2:0:
median.h: In instantiation of 'T median(Iterator, Iterator) [with T = double; Iterator = double*]':
median_test_array.cpp:19:60:   required from here
median.h:15:9: error: 'sort' was not declared in this scope
     sort(begin, end);
     ~~~~^~~~~~~~~~~~
median.h:15:9: note: suggested alternative:
In file included from /usr/local/Cellar/gcc/6.2.0/include/c++/6.2.0/algorithm:62:0,
                 from median.h:5,
                 from median_test_array.cpp:2:
/usr/local/Cellar/gcc/6.2.0/include/c++/6.2.0/bits/stl_algo.h:4727:5: note:   'std::sort'
     sort(_RandomAccessIterator __first, _RandomAccessIterator __last,
     ^~~~

This error goes away if I give the fully qualified name for sort (std::sort), but what I would like to know is why I need a fully qualified name in this instance, but not in the vector example.

It's an ADL issue.

For your 1st code sample, it seems that the implementation declare std::vector::iterator as class inside the namespace std, then ADL takes effect, std::sort is found for the calling with arguments of type std::vector::iterator.

For your 2nd code sample, the argument passed to sort is just double*, ADL doesn't take effect anymore, then the name sort can't be found.

Note that such behavior is not guaranteed; the standard doesn't specify where std::vector::iterator should be implemented, it even doesn't have to be a class, so you'd better to specify qualifier std::, or use it with using std::sort;.