如何解决如何将void模板类型用于编译时条件代码和static_assert？
我有遍历函数，该函数为每个元素调用回调，并希望在编译时使用非默认参数实现其逻辑扩展，如下所示：
#include <type_traits>
#include <iostream>
using namespace std;
template<
typename Odds = void,typename Evens = void,typename Skip = void
>
void iterate(
const auto & on_element,const auto & skip
) {
for ( int i = 0; i < 6; ++ i ) {
if constexpr ( ! is_same_v< Skip,void > ) {
if ( skip.find( i ) != skip.end() )
continue;
}
if ( i % 2 == 1 ) {
if constexpr ( is_same_v< Odds,void > )
on_element( i );
}
else {
if constexpr ( is_same_v< Evens,void > )
on_element( i );
}
static_assert(
is_same_v< Odds,void > || is_same_v< Evens,void >,"Either odds or evens are required to get traversed"
);
}
}
int main() {
const auto & on_element = []( const int & i ) {
cout << " " << i << endl;
};
cout << "Whole range:" << endl;
iterate( on_element );
cout << "Should skip specified elements:" << endl;
iterate( on_element,{ 1,2,3 } );
cout << "Should traverse only evens:" << endl;
iterate< bool >( on_element );
cout << "Should traverse only odds:" << endl;
iterate< void,bool >( on_element );
//should NOT compile if uncomment:
//iterate< bool,bool >( on_element );
}
但是它不能与g++ -std=gnu++2a -fconcepts main.cpp -o main一起编译：
main.cpp: In function ‘int main()’:
main.cpp:42:25: error: no matching function for call to ‘iterate(const main()::<lambda(const int&)>&)’
42 | iterate( on_element );
| ^
main.cpp:10:6: note: candidate: ‘template<class Odds,class Evens,class Skip,class auto:11,class auto:12> void iterate(const auto:11&,const auto:12&)’
10 | void iterate(
| ^~~~~~~
main.cpp:10:6: note: template argument deduction/substitution Failed:
main.cpp:42:25: note: candidate expects 2 arguments,1 provided
42 | iterate( on_element );
| ^
main.cpp:45:38: error: no matching function for call to ‘iterate(const main()::<lambda(const int&)>&,<brace-enclosed initializer list>)’
45 | iterate( on_element,3 } );
| ^
main.cpp:10:6: note: candidate: ‘template<class Odds,const auto:12&)’
10 | void iterate(
| ^~~~~~~
main.cpp:10:6: note: template argument deduction/substitution Failed:
main.cpp:45:38: note: Couldn’t deduce template parameter ‘auto:12’
45 | iterate( on_element,3 } );
| ^
main.cpp:48:33: error: no matching function for call to ‘iterate<bool>(const main()::<lambda(const int&)>&)’
48 | iterate< bool >( on_element );
| ^
main.cpp:10:6: note: candidate: ‘template<class Odds,const auto:12&)’
10 | void iterate(
| ^~~~~~~
main.cpp:10:6: note: template argument deduction/substitution Failed:
main.cpp:48:33: note: candidate expects 2 arguments,1 provided
48 | iterate< bool >( on_element );
| ^
main.cpp:51:39: error: no matching function for call to ‘iterate<void,bool>(const main()::<lambda(const int&)>&)’
51 | iterate< void,bool >( on_element );
| ^
main.cpp:10:6: note: candidate: ‘template<class Odds,const auto:12&)’
10 | void iterate(
| ^~~~~~~
main.cpp:10:6: note: template argument deduction/substitution Failed:
main.cpp:51:39: note: candidate expects 2 arguments,1 provided
51 | iterate< void,bool >( on_element );
| ^
有没有一种方法可以在C ++中实现？ 也许有更好的编译时技术来实现所需的行为？
解决方法
template<
bool Odds = false,bool Evens = false,bool Skip = false // or really,true: why pass skip if we won't use it?
>
void iterate(
const auto & on_element,const auto & skip
)
并编写另一个重载：
template<
bool Odds = false,bool Evens = false
> // no bool Skip; lack of arg is enough
void iterate(
const auto & on_element
)
{
iterate<Odds,Evens,false>(on_element);
}
然后将! is_same_v< Skip,void >替换为Skip，赔率/偶数类似。
也
static_assert( !Skip||!std::is_same_v<decltype(skip),const int&>,"You must pass a 2nd argument" );
对生活质量有好处。
,在我看来，在这种情况下，您应该避免使用auto类型的参数。
我提出以下代码
#include <type_traits>
#include <iostream>
#include <set>
using namespace std;
template<
bool Odds = true,bool Evens = true,typename Func,typename Skip = bool
>
void iterate(
Func const & on_element,Skip const & skip = false
) {
for ( int i = 0; i < 6; ++ i ) {
if constexpr ( ! is_same_v< Skip,bool > ) {
if ( skip.find( i ) != skip.end() )
continue;
}
if ( i % 2 == 1 ) {
if constexpr ( Odds )
on_element( i );
}
else {
if constexpr ( Evens )
on_element( i );
}
static_assert(
Odds || Evens,"Either odds or evens are required to get traversed"
);
}
}
int main() {
const auto & on_element = []( const int & i ) {
cout << " " << i << endl;
};
cout << "Whole range:" << endl;
iterate( on_element );
cout << "Should skip specified elements:" << endl;
iterate( on_element,std::set{ 1,2,3 } );
cout << "Should traverse only evens:" << endl;
iterate< false >( on_element );
cout << "Should traverse only odds:" << endl;
iterate< true,false >( on_element );
// should NOT compile if uncomment:
// iterate< false,false >( on_element );
}
也许简化
#include <type_traits>
#include <iostream>
#include <algorithm>
#include <array>
auto Odds = [](auto&& i) { return i % 2 == 0; };
auto Evens = [](auto&& i) { return i % 2 == 1; };
auto None = [](auto&&) { return false; };
template<class...Ts>
struct These
{
constexpr These(Ts... args)
: args_ { args... }
{}
template<class Arg>
bool operator()(Arg&& i) const
{
return std::any_of(
std::begin(args_),std::end(args_),[i](auto x) { return x == i; }
);
}
std::array<std::common_type_t<Ts...>,sizeof...(Ts)> args_;
};
template<
typename Skip = decltype(None)
>
void iterate(
const auto & on_element,Skip&& skip = Skip()
) {
for ( int i = 0; i < 6; ++ i ) {
if (skip(i))
continue;
std::cout << i << ",";
}
std::cout << std::endl;
}
int main() {
const auto & on_element = []( const int & i ) {
std::cout << " " << i << std::endl;
};
std::cout << "Whole range:" << std::endl;
iterate( on_element );
std::cout << "Should skip specified elements:" << std::endl;
iterate( on_element,These( 1,3 ));
std::cout << "Should traverse only evens:" << std::endl;
iterate( on_element,Evens);
std::cout << "Should traverse only odds:" << std::endl;
iterate( on_element,Odds );
}
预期输出：
Whole range:
0,1,3,4,5,Should skip specified elements:
0,Should traverse only evens:
0,Should traverse only odds:
1,如果您需要使用iterate版本的参数，则必须实现一个参数。
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