Pass template function to std::bind?-CodePudding

I want to use std::bind with template function. Is it somehow possible?

P.S. It is IMPORTANT to use std::bind, because I know at least one solution through lambdas and want to find out if there is std::bind solution.

#include <iostream>
#include <functional>
#include <memory>

using namespace std;

struct foo : std::enable_shared_from_this<foo>
{
    void f()
    {
        // doesn't compile, error : no matching function for call to 'bind'
        auto cb = std::bind(&foo::handle, shared_from_this(), placeholders::_1, placeholders::_2);
    }

    template <typename T, typename U>
    void handle(T, U)
    {
    }
};

int main()
{
    return 0;
}

CodePudding user response：

handle is not a template function. There are no "template functions". handle is a function template, ie it is a template, it is not a function. You cannot std::bind to a template. You can only std::bind to a callable.

The trick is to defer instantiation of the template and deduction of the template parameters to when the function is actually called:

#include <iostream>
#include <functional>
#include <memory>

using namespace std;

struct foo {

    struct handle_caller {
        template <typename T,typename U>
        void operator()(foo* f, T t,U u){
            f->handle(t,u);
        }
    };

    void f()
    {
        auto cb = std::bind(handle_caller{},this, placeholders::_1, placeholders::_2);
    }

    template <typename T, typename U>
    void handle(T, U)
    {
    }
};

int main()
{
    return 0;
}

The callable passed to bind is an object of a concrete type handle_caller. It is not a template. Only when cb is called the parameters are forwarded to handle_caller::operator() where the template arguments can be deduced.

Lambdas can do this out-of-the box, because a lambda with auto arguments is of a concrete type and only its operator() is a template:

#include <iostream>
#include <functional>
#include <memory>

using namespace std;

struct foo {
    void f()
    {
        auto cb = std::bind([](auto f,auto t,auto u){ f->handle(t,u);},this, placeholders::_1, placeholders::_2);
    }

    template <typename T, typename U>
    void handle(T, U)
    {
    }
};

int main()
{
    return 0;
}

However, once you use the lambda there is no need for std::bind anymore, because you can bind the parameters via a lambda capture. std::bind is the ancient way to bind parameters, it is convoluted and has clunky syntax. I have read of cases that can be done with std::bind but not with a lambda, but I have never encountered one.

PS: Note that I removed the shared_from_this stuff from your code, because I know it can be used wrong easily, but I am not sure how to use it correctly. As cb is only local to foo::f there is no need to worry about the lifetime of this in the example code.

CodePudding user response：

&foo::handle is not valid C , because foo::handle is not a function. foo::handle<int, int> is a function, and foo::handle<double, std::string> is a different function.

You will have to wrap it in something, so you may as well use a lambda.