rvalue参数无法解析功能超载

rvalue argument cannot resolve in function overload

本文关键字：功能 超载 参数 rvalue      更新时间：2023-10-16

如何在main()编译中制作最后一行？

#include <initializer_list>
#include <type_traits>
#include <functional>
template <typename T>
struct foo {
    foo(std::initializer_list<T>) { }
    template <typename C> struct is_foo : std::false_type { };
    template <typename U> struct is_foo<foo<U>> : std::true_type { };
    template <typename Compare>
    std::enable_if_t<!is_foo<Compare>::value> bar(foo&, Compare comp) {
        bool b = comp(T(), T());  // This line compiles thanks to is_foo<Compare>.
    }
    void bar(foo&& f) { bar(std::forward<foo>(f), std::less<T>()); }
    template <typename... Foos>
    void bar(Foos&&...) { }
};
int main() {
    foo<int> f = {1,2,3};
    f.bar({4,5,6});  // Compiles fine
    f.bar(f,f);  // Compiles fine (thanks to is_foo<Compare>)
    f.bar(f,{4,5,6});  // Won't compile
}

应该调用foo<T>::bar(Foos&&...)。

Test.cpp:27:17: error: no matching function for call to 'foo<int>::bar(foo<int>&, <brace-enclosed initializer list>)'
  f.bar(f,{4,5,6});  // Won't compile
                 ^
Test.cpp:13:44: note: candidate: template<class Compare> std::enable_if_t<(! foo<T>::is_foo<C>::value)> foo<T>::bar(foo<T>&, Compare) [with Compare = Compare; T = int]
  std::enable_if_t<!is_foo<Compare>::value> bar(foo&, Compare comp) {
                                            ^~~
Test.cpp:13:44: note:   template argument deduction/substitution failed:
Test.cpp:27:17: note:   couldn't deduce template parameter 'Compare'
  f.bar(f,{4,5,6});

不确定要确切想要什么，但是，如果您的打算获得了一个variadic bar()函数，该函数接收零或更多 foo<T>参数（或可用于初始化 foo<T>的参数），，好吧...如果您接受参数数量的限制（例如63，在下面的示例中），则有一个技巧。显示一次我想可以适应您的情况。

如果定义了typer模板别名

template <typename T, std::size_t>
using typer = T;

和递归struct proBar 

template <typename T, std::size_t N = 64U,
          typename = std::make_index_sequence<N>>
struct proBar;
template <typename T, std::size_t N, std::size_t... Is>
struct proBar<T, N, std::index_sequence<Is...>> : public proBar<T, N-1U>
 {
   using proBar<T, N-1U>::bar;
   void bar (typer<T, Is>... ts)
    { }
 };
template <typename T>
struct proBar<T, 0U, std::index_sequence<>>
 {
    void bar ()
     { }
 };

允许定义您需要的bar()功能，模板struct foo成为

template <typename T>
struct foo : public proBar<foo<T>>
 {
   using proBar<foo<T>>::bar;
   foo (std::initializer_list<T>)
    { }
   template <typename Compare>
   auto bar (foo &, Compare comp) -> decltype(comp(T(), T()), void())
    { bool b = comp(T(), T()); }
 };

以下是完整的编译示例

#include <initializer_list>
template <typename T, std::size_t>
using typer = T;
template <typename T, std::size_t N = 64U,
          typename = std::make_index_sequence<N>>
struct proBar;
template <typename T, std::size_t N, std::size_t... Is>
struct proBar<T, N, std::index_sequence<Is...>> : public proBar<T, N-1U>
 {
   using proBar<T, N-1U>::bar;
   void bar (typer<T, Is>... ts)
    { }
 };
template <typename T>
struct proBar<T, 0U, std::index_sequence<>>
 {
    void bar ()
     { }
 };
template <typename T>
struct foo : public proBar<foo<T>>
 {
   using proBar<foo<T>>::bar;
   foo (std::initializer_list<T>)
    { }
   template <typename Compare>
   auto bar (foo &, Compare comp) -> decltype(comp(T(), T()), void())
    { bool b = comp(T(), T()); }
 };

int main()
 {
   foo<int> f = {1, 2, 3};
   f.bar({4, 5, 6});
   f.bar(f, f);
   f.bar(f, {4, 5, 6}); // now compile
 }

这有效。

#include <initializer_list>
#include <type_traits>
#include <functional>
template <typename T>
struct foo {
    foo(std::initializer_list<T>) { }
    template <typename C> struct is_foo : std::false_type { };
    template <typename U> struct is_foo<foo<U>> : std::true_type { };
    template <typename Compare>
    std::enable_if_t<!is_foo<Compare>::value> bar(foo&, Compare comp) {
        bool b = comp(T(), T());  // This line compiles thanks to is_foo<Compare>.
    }
    void bar(foo&& f) { bar(std::forward<foo>(f), std::less<T>()); }
    template <typename... Foos>
    void bar(Foos&&...) { }
};
int main() {
    foo<int> f = { 1,2,3 };
    f.bar({ 4,5,6 });  // Compiles fine
    f.bar(f, f);  // Compiles fine (thanks to is_foo<Compare>)
    f.bar(f, foo<int>({ 4,5,6 }));  // Won't compile
}
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