Question

对于选择排序的这种实现：

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);
    }
}

如果为排序方法跳过最后一个参数，我该如何修改它以使Compare comp成为std::less方法？

我通过引入另一种方法尝试了函数重载：

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

但它出现了错误：

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);
    }
}

说明：

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。

有关std :: less＆lt;＆gt;的说明因为c ++ 14见：

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

Answer 2

你是对的，但忘记了typename关键字。检查一下：

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

可能你想要默认模板参数，但这也有效。

C ++模板，默认参数作为方法

2 个答案: