CODEWITHSUNDEEP
c++templatesboostboost-mpl
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Reputation: 9502

Boost mpl for each and free functions

Why this code does not compile:

#include <boost/mpl/vector.hpp>
#include <boost/mpl/for_each.hpp>
#include <iostream>

using namespace std;
using namespace boost;

template <class T>   // specific visitor for type printing
static void print_type(T t)
    {
        std::cout << typeid(T).name() << std::endl;
    }


typedef mpl::vector<int, long, char*> s;
int main ()
{
    mpl::for_each<s>(print_type());
}

I wonder - how to make boost mpl for_each work with free functions from same class?

Upvotes: 3

Views: 3243

Answers (2)

mark
mark

Reputation: 7667

As stated you need a functor.

The code below includes an additional wrap template that allows the print functor to cope with references.

#include <iostream>
#include <typeinfo>
#include <boost/mpl/vector.hpp>
#include <boost/mpl/for_each.hpp>
#include <boost/mpl/placeholders.hpp>

using namespace std;
using namespace boost;

template <typename T>
struct wrap {};

struct print_type
{
    template< typename T>
    void operator()( wrap<T> ) const
    {
        cout << typeid(T).name() << "\n";
    }
};

typedef mpl::vector<int, long&, char*> s;

int main ()
{
    mpl::for_each<s, wrap<mpl::placeholders::_1> >(print_type());
    return 0;
}

Note: this code based on examples in the book 'C++ Template Metaprogramming by David Abrahams and Aleksy Gurtovoy

Upvotes: 4

Nicol Bolas
Nicol Bolas

Reputation: 473437

    mpl::for_each<s>(print_type());

This is wrong in a couple of ways.

First, print_type is a function that returns void. print_type() is an attempt to call that function. Since it returns nothing, you can't stick its non-existent return value into something else.

Second, print_type is a template function. You cannot call a template function without specifying the template parameters. So print_type() is ill-formed.

Third, even mpl::for_each<s>(print_type) doesn't work, because print_type is not a value (nor is it convertible to a value); it is a template. You cannot pass a template as the argument of a function. That's why visitors (for many things, not just MPL) are objects, which can have template operator() members.

Upvotes: 3

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