CODEWITHSUNDEEP
c++templatestypesvariadic-templatessfinae
Drake Johnson
Drake Johnson

Reputation: 664

Initialize std::tuple with variadic arguments in constructor

I have a class that creates an std::function. For simplicity, I'll say the std::function returns a boolean for this example. The std::function needs to take in a variadic tuple. Currently, I have

template<class... FuncArgs>
class Function
{
public:
    // Type that std::function returns
    using func_ret_ty = bool;

private:
    std::function<func_ret_ty(std::tuple<FuncArgs...>)> m_Function;
    std::tuple<FuncArgs...> m_Args;  // Stores m_Function's arguments

public:
    Function(
        std::function<func_ret_ty(std::tuple<FuncArgs...>)> function,
        FuncArgs... args)
        : m_Function(function)
        , m_Args(std::make_tuple(std::forward<FuncArgs>(args)...))
    {}
};

My question is simple: will this work?

More specifically, I'm concerned because there seems to be a circular dependency in declaring the type for function. Another thought for implementation I had was:

template<class FuncTy, class FuncArgs...>
class Function
{
public:
    using func_ret_ty = bool;

private:
    FuncTy m_Function;
    std::tuple<FuncArgs...> m_Args;

public:
    Function(
        FuncTy function,
        FuncArgs... args)
        : m_Args(std::make_tuple(std::forward<FuncArgs>(args)...))
    {
        static_assert(
            std::is_same_v<FuncTy, std::function<func_ret_ty(std::tuple<FuncArgs...>)>>,
            "FuncTy invalid type!"
        );

        m_Function = std::move(function);
    }
};

Is the second implementation better? Is there a better way to go about doing this?

Upvotes: 2

Views: 707

Answers (1)

Jan Schultke
Jan Schultke

Reputation: 39774

The first implementation looks better to me, because you don't have to repeat yourself by providing the function arguments twice. Because you set the return type to bool, I would recommend changing the name of the class to Predicate, which is a well known term to describe functions that return boolean values.

Note that std::function can take parameter packs as template parameters too, so you can also do this:

std::function<func_ret_ty(FuncArgs...)> m_Function;

Upvotes: 2

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