CODEWITHSUNDEEP
c++c++11functional-programmingtr1
Nicola Bonelli
Nicola Bonelli

Reputation: 8287

tr1::mem_fn and tr1::bind: on const-correctness and overloading

What's wrong with the following snippet ?

#include <tr1/functional>
#include <functional>
#include <iostream>

using namespace std::tr1::placeholders;

struct abc
{
    typedef void result_type;

    void hello(int)
    { std::cout << __PRETTY_FUNCTION__ << std::endl; }

    void hello(int) const
    { std::cout << __PRETTY_FUNCTION__ << std::endl; }

    abc()
    {}
};

int
main(int argc, char *argv[])
{
    const abc x;
    int a = 1;

    std::tr1::bind(&abc::hello, x , _1)(a);
    return 0;
}

Trying to compile it with g++-4.3, it seems that cv-qualifier overloaded functions confuse both tr1::mem_fn<> and tr1::bind<> and it comes out the following error:

no matching function for call to ‘bind(<unresolved overloaded function type>,...

Instead the following snippet compiles but seems to break the const-correctness:

struct abc
{
    typedef void result_type;

    void operator()(int)
    { std::cout << __PRETTY_FUNCTION__ << std::endl; }

    void operator()(int) const
    { std::cout << __PRETTY_FUNCTION__ << std::endl; }

    abc()
    {}
};

...

    const abc x;
    int a = 1;
    std::tr1::bind( x , _1)(a);

Any clue?

Upvotes: 6

Views: 3238

Answers (3)

coppro
coppro

Reputation: 14526

This question has been answered, but I find the best way to specify an overload with bind is to specify it on the template:

std::tr1::bind<void(foo::*)(int)>(&foo::bar);

This method is just as explicit, but shorter than casting (with static_cast anyway. But it's cleaner than the C-cast, which is the same length.

Upvotes: 1

Nicola Bonelli
Nicola Bonelli

Reputation: 8287

As John suggested, the problems arisen in those snippets are the following:

  1. When passing a member-function-pointer it's necessary to specify its signature (if overloaded)
  2. bind() are passed arguments by value.

The first problem is solved by casting the member function pointer provided to bind:

    std::tr1::bind(static_cast< void(abc::*)(int) const >(&abc::hello), x, _1)(a);

The second can be solved by passing the callable object by address (as John suggested), or by means of TR1 reference_wrapper<> -- otherwise it will be passed by value, making the const-correctness breaking hallucination.

Given x a callable object:

std::tr1::bind( std::tr1::ref(x) , _1)(a);

bind() will forward a to the proper operator() in accordance to the x constness.

Upvotes: 1

John Zwinck
John Zwinck

Reputation: 249592

The lookup is done at a time when the constness of this is not known. You just have to give it a hint via casting. Try this:

typedef void (abc::*fptr)(int) const; // or remove const
std::tr1::bind((fptr)&abc::hello, x , _1)(a);

You may also notice here that removing the const still works. This is because you should be passing x by pointer (because the first argument to a C++ member function, the implicit this parameter, is always a pointer). Try this instead:

typedef void (abc::*fptr)(int) const; // won't compile without const (good!)
std::tr1::bind((fptr)&abc::hello, &x , _1)(a);

As discovered during within my comments below, if you omit the & as you originally did, you'll be passing x by value, which is usually not what you want (though it makes little practical difference in your particular example). This actually seems like an unfortunate pitfall for bind.

Upvotes: 5

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