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#include <iostream>
#include <string>
#include <functional>
using namespace std;
template <typename Class>
class Filter
{
Class obj;
public:
template <typename Arg>
typename std::result_of<Class(Arg)>::type
operator()(Arg const &arg) const
{
return obj(arg);
}
};
struct Convert // if there's just one ReturnType operator()(...), simply
{ // specify `using type = ReturnType'
template <typename Signature>
struct result;
double operator()(int x) const
{
return 12.5;
}
template <typename Class>
struct result<Class(int)>
{
using type = double;
};
string operator()(double x) const
{
return "hello world"s;
}
template <typename Class>
struct result<Class(double)>
{
using type = string;
};
};
int main()
{
Filter<Convert> fc;
cout << fc(5.5).length() << '\n';
cout << fc(5) << '\n';
}
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