C++ 模板，默认参数作为方法

Question

对于这个选择排序的实现:

template <typename Iterator, typename Compare>
void sort(Iterator begin, Iterator end, Compare comp) {
    for (auto i = begin; i != end; ++i) {
        auto min = i;
        for (auto j = i + 1; j != end; ++j) {
            if (comp(*j, *min)) {
                min = j;
            }
        }
        std::swap(*min, *i);
    }
}

我应该如何修改它，使 Compare comp 应该是 std::less排序方法是否跳过最后一个参数？

我通过引入另一种方法来尝试函数重载:

template <typename Iterator>
void sort(Iterator begin, Iterator end) {
    sort(begin, end, std::less<std::iterator_traits<Iterator>::value_type>());
}

但是它给出了这样的错误:

In file included from ../src/selection_sort_demo.cpp:1:
../include/selection_sort.hpp:24:29: error: template argument for template type parameter must be a type; did you forget 'typename'?
        sort(begin, end, std::less<std::iterator_traits<Iterator>::value_type>());
                                   ^
                                   typename 
/usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/6.3.1/../../../../include/c++/6.3.1/bits/stl_function.h:380:21: note: template parameter is declared here
  template<typename _Tp>
                    ^
In file included from ../src/selection_sort_demo.cpp:1:
../include/selection_sort.hpp:24:2: error: call to 'sort' is ambiguous
        sort(begin, end, std::less<std::iterator_traits<Iterator>::value_type>());
        ^~~~
../src/selection_sort_demo.cpp:22:13: note: in instantiation of function template specialization 'selection::sort<__gnu_cxx::__normal_iterator<int *, std::vector<int, std::allocator<int> > > >' requested here
        selection::sort(v.begin(), v.end());
                   ^
/usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/6.3.1/../../../../include/c++/6.3.1/bits/stl_algo.h:4727:5: note: candidate function [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<int *, std::vector<int, std::allocator<int> > >, _Compare = std::less<int>]
    sort(_RandomAccessIterator __first, _RandomAccessIterator __last,
    ^
../include/selection_sort.hpp:7:6: note: candidate function [with Iterator = __gnu_cxx::__normal_iterator<int *, std::vector<int, std::allocator<int> > >, Compare = std::less<int>]
void sort(Iterator begin, Iterator end, Compare comp)
     ^
2 errors generated.
[2/4] Compiling cpp object 'test/testexe@exe/selection_sort_test.cpp.o'
FAILED: test/testexe@exe/selection_sort_test.cpp.o 
clang++  '-Itest/testexe@exe' '-Itest' '-I../test' '-I../include' '-Wall' '-Winvalid-pch' '-Wnon-virtual-dtor' '-std=c++14' '-O0' '-g' '-pthread' '-MMD' '-MQ' 'test/testexe@exe/selection_sort_test.cpp.o' '-MF' 'test/testexe@exe/selection_sort_test.cpp.o.d' -o 'test/testexe@exe/selection_sort_test.cpp.o' -c ../test/selection_sort_test.cpp
In file included from ../test/selection_sort_test.cpp:2:
../include/selection_sort.hpp:24:29: error: template argument for template type parameter must be a type; did you forget 'typename'?
        sort(begin, end, std::less<std::iterator_traits<Iterator>::value_type>());
                                   ^
                                   typename 
/usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/6.3.1/../../../../include/c++/6.3.1/bits/stl_function.h:380:21: note: template parameter is declared here
  template<typename _Tp>
                    ^
In file included from ../test/selection_sort_test.cpp:2:
../include/selection_sort.hpp:24:2: error: call to 'sort' is ambiguous
        sort(begin, end, std::less<std::iterator_traits<Iterator>::value_type>());
        ^~~~
../test/selection_sort_test.cpp:17:13: note: in instantiation of function template specialization 'selection::sort<__gnu_cxx::__normal_iterator<int *, std::vector<int, std::allocator<int> > > >' requested here
        selection::sort(v1.begin(), v1.end());
                   ^
/usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/6.3.1/../../../../include/c++/6.3.1/bits/stl_algo.h:4727:5: note: candidate function [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<int *, std::vector<int, std::allocator<int> > >, _Compare = std::less<int>]
    sort(_RandomAccessIterator __first, _RandomAccessIterator __last,
    ^
../include/selection_sort.hpp:7:6: note: candidate function [with Iterator = __gnu_cxx::__normal_iterator<int *, std::vector<int, std::allocator<int> > >, Compare = std::less<int>]
void sort(Iterator begin, Iterator end, Compare comp)
     ^
2 errors generated.

Answer 1

因为它是 c++14:

template <typename Iterator, typename Compare = std::less<> >
void sort(Iterator begin, Iterator end, Compare comp = Compare())
{
    for (auto i = begin; i != end; ++i) {
        auto min = i;
        for (auto j = i + 1; j != end; ++j) {
            if (comp(*j, *min)) {
                min = j;
            }
        }
        std::swap(*min, *i);
    }
}

C++11:

template <typename Iterator, typename Compare = std::less< typename std::iterator_traits<Iterator>::value_type > >
void sort(Iterator begin, Iterator end, Compare comp = Compare())
{
    for (auto i = begin; i != end; ++i) {
        auto min = i;
        for (auto j = i + 1; j != end; ++j) {
            if (comp(*j, *min)) {
                min = j;
            }
        }
        std::swap(*min, *i);
    }
}

解释:

我们必须为编译器提供模板参数列表中的默认类型和默认函数参数列表。

对于 std::less<> 自 c++14 的解释，请参见:

http://en.cppreference.com/w/cpp/utility/functional/less