[英]c++17 : lambda to std::function conversion failure
我目前正在探索c ++ 17的新增功能。 在使用 std::variant
,也想使用std::optional
,具有相同的示例。 當前看到編譯由於以下錯誤而失敗:
error: no viable conversion from returned value of type
'(lambda at ./html_parser.hpp:53:9)' to function return type 'Parser<char>' (aka
'std::__1::function<std::__1::optional<std::__1::pair<char, std::__1::basic_string<char> > >
(std::__1::basic_string<char>)>')
return [=](std::string& input) -> ParserResult<char> {
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/acid/tools/include/c++/v1/functional:1627:5: note: candidate constructor not viable: no known conversion
from '(lambda at ./html_parser.hpp:53:9)' to 'std::nullptr_t' (aka 'nullptr_t') for 1st argument
function(nullptr_t) _NOEXCEPT : __f_(0) {}
^
/home/acid/tools/include/c++/v1/functional:1628:5: note: candidate constructor not viable: no known conversion
from '(lambda at ./html_parser.hpp:53:9)' to 'const
std::__1::function<std::__1::optional<std::__1::pair<char, std::__1::basic_string<char> > >
(std::__1::basic_string<char>)> &' for 1st argument
function(const function&);
^
/home/acid/tools/include/c++/v1/functional:1629:5: note: candidate constructor not viable: no known conversion
from '(lambda at ./html_parser.hpp:53:9)' to 'std::__1::function<std::__1::optional<std::__1::pair<char,
std::__1::basic_string<char> > > (std::__1::basic_string<char>)> &&' for 1st argument
function(function&&) _NOEXCEPT;
^
/home/acid/tools/include/c++/v1/functional:1631:5: note: candidate template ignored: requirement
'__callable<(lambda at ./html_parser.hpp:53:9)>::value' was not satisfied [with _Fp =
(lambda at ./html_parser.hpp:53:9)]
function(_Fp);
^
1 error generated.
要解析HTML以提供DOM,首先要聲明一些解析器組合器,如下所示:
#pragma once
#include <string>
#include <utility>
#include <functional>
#include <optional>
namespace dragon {
namespace html {
namespace parser {
template <typename ParserOutput, typename ParserInput = std::string>
using ParserResult = std::optional<std::pair<ParserOutput, ParserInput>>;
template<typename ParserOutput, typename ParserInput = std::string>
using Parser = std::function<ParserResult<ParserOutput, ParserInput>(ParserInput)>;
template <typename ParserOutput, typename ParserInput = std::string>
auto parse(Parser<ParserOutput, ParserInput> p, ParserInput i) -> ParserResult<ParserOutput, ParserInput>{
return p(i);
}
// few parser combinators.
// thenP combinator: applies the first parser, if it succeeds apply the second to the rest of
// the input left over by the first parser.
// currently just fails and returns empty!! does not provide any debugging info/msg
// as to why the parsing failed.
template<typename FirstParser, typename SecondParser>
auto thenP(FirstParser f, SecondParser s) {
return [=](std::string input) -> decltype(parse(s, std::string())) {
auto fv = parse(f, input);
if (fv) {
auto fvv = *fv;
return parse(s, fvv.second);
}
else {
return {};
}
};
}
template<typename FirstParser, typename SecondParser>
auto choiceP(FirstParser f, SecondParser s) {
return [=](std::string input) {
auto fv = parse(f, input);
if (!fv) return parse(s, input);
return fv;
};
}
auto charP(char match) -> Parser<char> {
return [=](std::string& input) -> ParserResult<char> {
if ((input.empty() == false) && (input[0] == match)) {
return std::make_pair(input[0], input.substr(1));
}
return {};
};
}
}
}
}
嘗試編譯如下所示的簡單用法時,我看到上述錯誤:
int main()
{
auto less = Parser::parser::charP('<');
auto greater = Parser::parser::charP('>');
auto lag = Parser::parser::thenP(less, greater);
auto log = Parser::parser::choiceP(less, greater);
auto lagv = lag("<>");
auto logv = log("|>");
return 0;
}
使用Visual Studio 2017(std = c ++-latest)可以很好地進行編譯。 但是Clang給出了以上錯誤。 試圖找出這兩個編譯器之間的差異。 以及如何使用Clang解決此問題。
這是不正確的形式:
auto charP(char match) -> Parser<char> {
return [=](std::string& input) -> ParserResult<char> { ... };
}
出於同樣的原因,它格式不正確:
std::function<void(int)> f = [](int& ){};
右側的lambda 不能用int
調用,只能用int&
function<void(int)>
,因此您不能從中構造一個function<void(int)>
。 MSVC具有某種允許模式,該模式允許從右值構造非常量左值引用,這可能就是它起作用的原因。
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