如何在推导上下文中将 std::bind(或 lambda)转换为 std::function?
How to convert std::bind (or lambda) to std::function in deduced context?
我正在尝试将指向成员函数的指针(使用 std::bind
或 lambda)转换为 std::function
。我的尝试(根据 的回答)看起来像这样:
#include <functional>
template<typename T>
struct AsFunction :
AsFunction<decltype(&T::operator())>
{};
template<class ReturnType, class... Args>
struct AsFunction<ReturnType(Args...)> {
using type = std::function<ReturnType(Args...)>;
};
template<class ReturnType, class... Args>
struct AsFunction<ReturnType(*)(Args...)> {
using type = std::function<ReturnType(Args...)>;
};
template<class Class, class ReturnType, class... Args>
struct AsFunction<ReturnType(Class::*)(Args...) const> {
using type = std::function<ReturnType(Args...)>;
};
template<class F>
auto toFunction( F f ) -> typename AsFunction<F>::type {
return {f};
}
struct MyStruct {
int x,y;
void f(int){};
};
int main(){
MyStruct m;
{
// this works
auto f = std::bind(&MyStruct::f, &m, std::placeholders::_1);
f(2);
}
{
// this doesn't
auto f = toFunction(std::bind(&MyStruct::f, &m, std::placeholders::_1));
f(2);
}
{
// .. neither does this
auto f = toFunction([m](int x) mutable { m.f(x); });
f(2);
}
}
但我从编译器收到以下错误消息:
// first not working
main.cpp:24:6: note: substitution of deduced template arguments resulted in errors seen above
main.cpp: In instantiation of ‘struct AsFunction<std::_Bind<std::_Mem_fn<void (MyStruct::*)(int)>(MyStruct*, std::_Placeholder<1>)> >’:
main.cpp:24:6: required by substitution of ‘template<class F> typename AsFunction<F>::type toFunction(F) [with F = std::_Bind<std::_Mem_fn<void (MyStruct::*)(int)>(MyStruct*, std::_Placeholder<1>)>]’
main.cpp:44:75: required from here
main.cpp:4:8: error: decltype cannot resolve address of overloaded function
struct AsFunction :
^~~~~~~~~~
main.cpp: In function ‘int main()’:
main.cpp:44:75: error: no matching function for call to ‘toFunction(std::_Bind_helper<false, void (MyStruct::*)(int), MyStruct*, const std::_Placeholder<1>&>::type)’
auto f = toFunction(std::bind(&MyStruct::f, &m, std::placeholders::_1));
^
main.cpp:24:6: note: candidate: template<class F> typename AsFunction<F>::type toFunction(F)
auto toFunction( F f ) -> typename AsFunction<F>::type {
^~~~~~~~~~
main.cpp:24:6: note: substitution of deduced template arguments resulted in errors seen above
// second non working braces with lambda
main.cpp: In instantiation of ‘struct AsFunction<void (main()::<lambda(int)>::*)(int)>’:
main.cpp:4:8: required from ‘struct AsFunction<main()::<lambda(int)> >’
main.cpp:24:6: required by substitution of ‘template<class F> typename AsFunction<F>::type toFunction(F) [with F = main()::<lambda(int)>]’
main.cpp:50:55: required from here
main.cpp:5:23: error: ‘operator()’ is not a member of ‘void (main()::<lambda(int)>::*)(int)’
AsFunction<decltype(&T::operator())>
^~
main.cpp:50:55: error: no matching function for call to ‘toFunction(main()::<lambda(int)>)’
auto f = toFunction([m](int x) mutable { m.f(x); });
^
main.cpp:24:6: note: candidate: template<class F> typename AsFunction<F>::type toFunction(F)
auto toFunction( F f ) -> typename AsFunction<F>::type {
^~~~~~~~~~
main.cpp:24:6: note: substitution of deduced template arguments resulted in errors seen above
我认为你有点矫枉过正了。如果你真的想要一个明确的 std::function
类型作为你的 f
,你可以直接用一些技巧来指定它:
int main(){
MyStruct m;
{
std::function<decltype(std::declval<MyStruct>().f(int()))(int)> f(std::bind(&MyStruct::f, &m, std::placeholders::_1));
f(2);
}
}
话虽如此,到目前为止最简单的事情就是简单地使用 auto f
而不指定类型。我假设您已经想到了这一点,但在您的情况下不会这样做,但我还是以防万一。
很难从 lambda 或 std::bind
return 类型中推断出参数,因此您可能希望延迟 toFunction
内的绑定,例如有:
template<typename C, typename Ret, typename... Args, typename... BArgs>
auto toFunction(Ret (C::*f)(Args...), BArgs&&... bargs) {
return std::function<Ret(Args...)>(std::bind(f, std::forward<BArgs>(bargs)...));
}
这样,您可以使用以下方法检索 std::function
:
auto f = toFunction(&MyStruct::f, &m, std::placeholders::_1);
f(2);
我正在尝试将指向成员函数的指针(使用 std::bind
或 lambda)转换为 std::function
。我的尝试(根据
#include <functional>
template<typename T>
struct AsFunction :
AsFunction<decltype(&T::operator())>
{};
template<class ReturnType, class... Args>
struct AsFunction<ReturnType(Args...)> {
using type = std::function<ReturnType(Args...)>;
};
template<class ReturnType, class... Args>
struct AsFunction<ReturnType(*)(Args...)> {
using type = std::function<ReturnType(Args...)>;
};
template<class Class, class ReturnType, class... Args>
struct AsFunction<ReturnType(Class::*)(Args...) const> {
using type = std::function<ReturnType(Args...)>;
};
template<class F>
auto toFunction( F f ) -> typename AsFunction<F>::type {
return {f};
}
struct MyStruct {
int x,y;
void f(int){};
};
int main(){
MyStruct m;
{
// this works
auto f = std::bind(&MyStruct::f, &m, std::placeholders::_1);
f(2);
}
{
// this doesn't
auto f = toFunction(std::bind(&MyStruct::f, &m, std::placeholders::_1));
f(2);
}
{
// .. neither does this
auto f = toFunction([m](int x) mutable { m.f(x); });
f(2);
}
}
但我从编译器收到以下错误消息:
// first not working
main.cpp:24:6: note: substitution of deduced template arguments resulted in errors seen above
main.cpp: In instantiation of ‘struct AsFunction<std::_Bind<std::_Mem_fn<void (MyStruct::*)(int)>(MyStruct*, std::_Placeholder<1>)> >’:
main.cpp:24:6: required by substitution of ‘template<class F> typename AsFunction<F>::type toFunction(F) [with F = std::_Bind<std::_Mem_fn<void (MyStruct::*)(int)>(MyStruct*, std::_Placeholder<1>)>]’
main.cpp:44:75: required from here
main.cpp:4:8: error: decltype cannot resolve address of overloaded function
struct AsFunction :
^~~~~~~~~~
main.cpp: In function ‘int main()’:
main.cpp:44:75: error: no matching function for call to ‘toFunction(std::_Bind_helper<false, void (MyStruct::*)(int), MyStruct*, const std::_Placeholder<1>&>::type)’
auto f = toFunction(std::bind(&MyStruct::f, &m, std::placeholders::_1));
^
main.cpp:24:6: note: candidate: template<class F> typename AsFunction<F>::type toFunction(F)
auto toFunction( F f ) -> typename AsFunction<F>::type {
^~~~~~~~~~
main.cpp:24:6: note: substitution of deduced template arguments resulted in errors seen above
// second non working braces with lambda
main.cpp: In instantiation of ‘struct AsFunction<void (main()::<lambda(int)>::*)(int)>’:
main.cpp:4:8: required from ‘struct AsFunction<main()::<lambda(int)> >’
main.cpp:24:6: required by substitution of ‘template<class F> typename AsFunction<F>::type toFunction(F) [with F = main()::<lambda(int)>]’
main.cpp:50:55: required from here
main.cpp:5:23: error: ‘operator()’ is not a member of ‘void (main()::<lambda(int)>::*)(int)’
AsFunction<decltype(&T::operator())>
^~
main.cpp:50:55: error: no matching function for call to ‘toFunction(main()::<lambda(int)>)’
auto f = toFunction([m](int x) mutable { m.f(x); });
^
main.cpp:24:6: note: candidate: template<class F> typename AsFunction<F>::type toFunction(F)
auto toFunction( F f ) -> typename AsFunction<F>::type {
^~~~~~~~~~
main.cpp:24:6: note: substitution of deduced template arguments resulted in errors seen above
我认为你有点矫枉过正了。如果你真的想要一个明确的 std::function
类型作为你的 f
,你可以直接用一些技巧来指定它:
int main(){
MyStruct m;
{
std::function<decltype(std::declval<MyStruct>().f(int()))(int)> f(std::bind(&MyStruct::f, &m, std::placeholders::_1));
f(2);
}
}
话虽如此,到目前为止最简单的事情就是简单地使用 auto f
而不指定类型。我假设您已经想到了这一点,但在您的情况下不会这样做,但我还是以防万一。
很难从 lambda 或 std::bind
return 类型中推断出参数,因此您可能希望延迟 toFunction
内的绑定,例如有:
template<typename C, typename Ret, typename... Args, typename... BArgs>
auto toFunction(Ret (C::*f)(Args...), BArgs&&... bargs) {
return std::function<Ret(Args...)>(std::bind(f, std::forward<BArgs>(bargs)...));
}
这样,您可以使用以下方法检索 std::function
:
auto f = toFunction(&MyStruct::f, &m, std::placeholders::_1);
f(2);