using boost::bind with a map

Question

    bool takeInt(int a,int b) 
    {
      cout << a << " " << b << endl;   return true;
    } 

    int main()
    { 
      map<int,int> m;
      m.insert(make_pair(1,2));
      m.insert(make_pair(2,5));
      m.insert(make_pair(4,8));
      m.insert(make_pair(5,6));
for_each(m.begin(),m.end(),boost::bind(&takeInt,42,boost::bind(&pair<int,int>::first,_1)));
     return 0;
    }

I want an output of

Fails compilation with loong errors. Any clue on what I am missing?

pmr · Accepted Answer · 2012-03-13 14:49:08Z

1

The type of map<T, U>::value_type is pair<const T, U>. Use this:

for_each(m.begin(),m.end(),
         boost::bind(&takeInt, 42, boost::bind(&pair<const int,int>::first,_1)));
                                                     ^^^^^

answered Mar 13, 2012 at 14:49

pmr

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4 Comments

Carsten Greiner Over a year ago

out of curiosity: where did find the definition of the map::value_type ? @myself: just looked up definition on cpp.com and it cleary says value_type : 'pair<const Key,T>'

pmr Over a year ago

@CarstenGreiner In this case I was pretty sure, otherwise I look it up in the standard (usually the draft N3337 open-std.org/JTC1/SC22/WG21/docs/papers/2012/n3337.pdf) or the library implementation itself.

balki Over a year ago

From this, I learn to always use the typedef unless its not possible. boost::bind(&Map::value_type::first,_1)

pmr Over a year ago

@balki Excellent conclusion. Your code also became considerably more robust to change. Essentially, every time you specify template parameters manually to access a nested type, you are making it harder to change your code.

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