CODEWITHSUNDEEP
c++templateslambdastd-function
Mike Weir
Mike Weir

Reputation: 3189

Deducing lambdas to std::function<T>

I'm essentially trying to have it so I can create a generic callback from any lamba with captures: GenericCallback([=]{});

Using the following class:

template<typename T> class GenericCallback : public Callback {
public:
    GenericCallback(const std::function<T>& f) {
        mFunction = f;
    }

    void Call() override {
        mFunction();
    }

    std::function<T> mFunction;
};

The problem is it expects me to define the template arguments. There's no issues doing so in general, but if I use any captures, whether it's using [=] or specific arguments, the type becomes impossible for me to shove it into this structure.

My end goal is to simply call these functions at a later time with a specified condition.

A case where this errors:

int v = 5;
GenericCallback<void()>([=]{ int x = v; });

Upvotes: 0

Views: 69

Answers (1)

Slava
Slava

Reputation: 44268

You misunderstand the whole concept of std::function<> and overcomplicated the issue. You can use std::function<void()> in this case and bind pretty match anything to it, you need to change type in <> only when you need to change calling signature. So 

class Callback {
public:
    using callback_type = std::function<void()>;

    Callback( callback_type cb ) : m_cb( cb ) {}
    void call() { m_cb(); }

private:
    callback_type m_cb;
};

int main()
{
    int v;
    Callback c( [=]{ int x = v; } );
    c.call();
}

would simple work and you do not need template there. You need to change type in std::function in this case only if you want Callback::call() to pass something to that callback or make it to return something and I doubt you plan to do that.

live example

Upvotes: 1

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