Question

我试图了解可变参数模板的工作原理。在下面的示例中，我想将变量参数传递给类的构造函数，并将其存储到稍后可以使用的元组中。

template<typename... Args>
class CompoundOrs
{
public:
    CompoundOrs(){}

    CompoundOrs(Args... args) {
        m_tuple_args = std::tuple<Args...>(args...);
    }

    virtual bool eventPredicate() {
        return unpack_tuple(m_tuple_args, std::index_sequence_for<Args...>());
    }

    bool iterativeOr(bool evt) {
        return evt;
    }

    bool iterativeOr(bool evt, Args... args) {
        return (evt || iterativeOr(args...));
    }

    template<std::size_t... Is>
    bool unpack_tuple(const std::tuple<Args...>& args, std::index_sequence<Is...>) {
        return iterativeOr(std::get<Is>(args)...);
    }

private:
    std::tuple<Args...> m_tuple_args;
};
int main()
{
    bool a = true;
    bool b = false;
    bool c = true;
    CompoundOrs<bool> c_or(a, b, c);

    return 0;
}

这将引发指示参数不匹配的编译错误。我在某处读到，基本函数的声明顺序也很重要，这就是为什么我将空构造函数添加为第一个函数的原因，但这也无济于事。

main.cpp: In function 'int main()':
main.cpp:64:33: error: no matching function for call to 'CompoundOrs::CompoundOrs(bool&, bool&, bool&)'
     CompoundOrs<bool> c_or(a,b,c);
                                 ^
main.cpp:28:5: note: candidate: CompoundOrs::CompoundOrs(Args ...) [with Args = {bool}]
     CompoundOrs(Args... args)
     ^
main.cpp:28:5: note:   candidate expects 1 argument, 3 provided
main.cpp:19:1: note: candidate: CompoundOrs::CompoundOrs() [with Args = {bool}]
 CompoundOrs()
 ^
main.cpp:19:1: note:   candidate expects 0 arguments, 3 provided
main.cpp:15:7: note: candidate: constexpr CompoundOrs::CompoundOrs(const CompoundOrs&)
 class CompoundOrs
       ^
main.cpp:15:7: note:   candidate expects 1 argument, 3 provided
main.cpp:15:7: note: candidate: constexpr CompoundOrs::CompoundOrs(CompoundOrs&&)
main.cpp:15:7: note:   candidate expects 1 argument, 3 provided

Answer 1

这可以简化为

#include <tuple>
template<typename... Args>
class CompoundOrs
{
    private: std::tuple<Args...> m_tuple_args;

    // template constructor accepting another set of arguments
    // and forwarding them to tuple field constructor.
    public: template<typename... InnerArgs>
    CompoundOrs(InnerArgs &&... args) : m_tuple_args{::std::forward<InnerArgs>(args)...} {}
};
int main()
{
    bool a = true;
    bool b = false;
    bool c = true;
    CompoundOrs<bool, bool, bool> c_or(a, b, c);

    return 0;
}

online compiler

可变参数模板类的构造函数无法接受变量参数

1 个答案: