不能将typeof(std :: endl)作为模板参数? [英] Cannot have typeof(std::endl) as template parameter?
问题描述
所以,我试图写一个这样的函数:
So, I was trying to write a function like this:
void append_to_stream(std::ostream &stream)
{ }
template <typename T, typename... Args>
void append_to_stream(std::ostream &stream, T first, Args&&... rest)
{
stream << first;
append_to_stream(stream, rest...);
}
并调用它:
append_to_stream(stream,
std::endl,
std::endl);
但这不起作用。我得到一个错误,说'太多的参数'的函数。我把它缩小到我知道 std :: endl
是有罪的 - 可能是因为它是一个函数。我设法通过声明一个 endl
的结构并定义<
std :: endl
。这工作,但不觉得特别好。是不是可以接受std :: endl作为模板参数?该功能适用于其他类型。
But this doesn't work. I get an error that says 'too many arguments' to the function. I've narrowed it down to the point I know that the std::endl
is guilty - probably because it's a function. I managed to 'solve' this by declaring a struct called endl
and define the <<operator
for it so that it simply calls std::endl
. This works but doesn't feel particularly good. Is it not possible to accept std::endl as a template argument? The function works for other types.
编辑:这是错误:
src/log/sinks/file_sink.cpp:62:21: error: too many arguments to function ‘void log::sinks::append_to_stream(std::string&, Args&& ...) [with Args = {}, std::string = std::basic_string<char>]’
<更新
尝试让编译器推导出正确的模板参数@MooingDuck建议使用以下形式的函数:
Trying to get the compiler to deduce the correct template arguments @MooingDuck suggested that a function of the following form could be used:
template<class e, class t, class a>
basic_ostream<e,t>&(*)(basic_ostream<e,t>&os) get_endl(basic_string<e,t,a>& s)
{
return std::endl<e,t>;
}
但这不会编译。
错误:
src/log/sinks/file_sink.cpp:42:28: error: expected unqualified-id before ‘)’ token
src/log/sinks/file_sink.cpp:42:53: error: expected initializer before ‘get_endl’
任何想法为什么?为了编译这个,我添加了 using namespace std;
Any ideas why? For the sake of compiling this, I've added using namespace std;
推荐答案
std :: endl
是一个模板,不是函数,编译器无法解析 endl
。
std::endl
is a template, not a function, and the compiler cannot resolve which endl
to use.
尝试:
append_to_stream(std::cout,
std::endl<char, std::char_traits<char>>,
std::endl<char, std::char_traits<char>>);
或者,MooingDuck的解决方案(更正):
Or, MooingDuck's solution (corrected):
template<class e, class t, class a> //string version
std::basic_ostream<e, t>& (*get_endl(const std::basic_string<e, t, a>&))
(std::basic_ostream<e, t>& )
{ return std::endl<e,t>; }
template<class e, class t> //stream version
std::basic_ostream<e, t>& (*get_endl(const std::basic_ostream<e, t>&))
(std::basic_ostream<e, t>& )
{ return std::endl<e,t>; }
int main () {
std::ostream& stream = std::cout;
append_to_stream(stream,
get_endl(stream),
get_endl(stream));
}
这里是get_endl解决方案,由C ++ 11简化 decltype
feature:
Here is get_endl solution, simplified by C++11 decltype
feature:
template<class e, class t, class a> //string version
auto get_endl(const std::basic_string<e, t, a>&)
-> decltype(&std::endl<e,t>)
{ return std::endl<e,t>; }
template<class e, class t> //stream version
auto get_endl(const std::basic_ostream<e,t>&)
-> decltype(&std::endl<e,t>)
{ return std::endl<e,t>; }
int main () {
std::ostream& stream = std::cout;
append_to_stream(stream,
get_endl(stream),
get_endl(stream));
}
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