Is there a standalone implementation of std::function?

问题

I'm working on an embedded system, so code size is an issue. Using the standard library ups my binary size by about 60k, from 40k to 100k. I'd like to use std::function, but I can't justify it for 60k. Is there a standalone implementation that I can use, or something similar? I'm using it to implicitly cast lambdas in member functions with bound variables in c++ 11.

回答1:

Here is simple implementation of std::function-like class template without inclusion of any headers. You can customize the behavior as you wish(like move/forward, empty call response, etc):

live_demo

// Scroll down for example of usage
namespace bicycle
{
    template<typename Result,typename ...Args>
    struct abstract_function
    {
        virtual Result operator()(Args... args)=0;
        virtual abstract_function *clone() const =0;
        virtual ~abstract_function() = default;
    };

    template<typename Func,typename Result,typename ...Args>
    class concrete_function: public abstract_function<Result,Args...>
    {
        Func f;
    public:
        concrete_function(const Func &x)
            : f(x)
        {}
        Result operator()(Args... args) override
        {
            return f(args...);
        }
        concrete_function *clone() const override
        {
            return new concrete_function{f};
        }
    };

    template<typename Func>
    struct func_filter
    {
        typedef Func type;
    };
    template<typename Result,typename ...Args>
    struct func_filter<Result(Args...)>
    {
        typedef Result (*type)(Args...);
    };

    template<typename signature>
    class function;

    template<typename Result,typename ...Args>
    class function<Result(Args...)>
    {
        abstract_function<Result,Args...> *f;
    public:
        function()
            : f(nullptr)
        {}
        template<typename Func> function(const Func &x)
            : f(new concrete_function<typename func_filter<Func>::type,Result,Args...>(x))
        {}
        function(const function &rhs)
            : f(rhs.f ? rhs.f->clone() : nullptr)
        {}
        function &operator=(const function &rhs)
        {
            if( (&rhs != this ) && (rhs.f) )
            {
                auto *temp = rhs.f->clone();
                delete f;
                f = temp;
            }
            return *this;
        }
        template<typename Func> function &operator=(const Func &x)
        {
            auto *temp = new concrete_function<typename func_filter<Func>::type,Result,Args...>(x);
            delete f;
            f = temp;
            return *this;
        }
        Result operator()(Args... args)
        {
            if(f)
                return (*f)(args...);
            else
                return Result{};
        }
        ~function()
        {
            delete f;
        }
    };
}

// ___________________[ Example of usage ]___________________ //

int func1(double)
{
    return 1;
}
struct Functor2
{
    int operator()(double)
    {
        return 2;
    }
};

double func3(bool,int)
{
    return 3.0;
}
struct Functor4
{
    double operator()(bool,int)
    {
        return 4.0;
    }
};

int main()
{
    int res = 10;
    {
        bicycle::function<int(double)> f{func1};

        res -= f(1.0);
        f = Functor2{};
        res -= f(2.0);
    }
    {
        bicycle::function<double(bool,int)> f1;
        f1 = func3;

        bicycle::function<double(bool,int)> f2{f1};
        res -= f2(true,1);

        f1 = Functor4{};
        f2 = f1;
        res -= f2(false,2);
    }
    return res;
}

回答2:

The 60k came from exception handling being added by the compiler, because exceptions were required for std::function. std::function only throws one exception, "bad_function_call". So I removed the code that threw the exception, now it seg faults if an empty function is called, and I saved myself 60k.

来源：https://stackoverflow.com/questions/14739902/is-there-a-standalone-implementation-of-stdfunction