How to identify the typename type in template?

Question

template<typename T>
ostream& operator <<(ostream& os,const T &r_list)
{
 //Function body
}

In the above code T might be all ,But one at a time (vector, map, vector< vector >,map< primitive type,vector >,map< primitive type,map >).

I want to write the above stl content values into a file. I think, I could do it by find the T type(vector,map).So wrote like this typeid(r_list).name ,But I Could't find the type. pls show the way how can I do it.

When you say "single function" do you mean using overloading and `template template` params to be able to deal with say `int` and `list`? i.e. Having more than one function? — doctorlove, Jun 24 '15 at 12:55

score 1 · Answer 1 · edited May 23 '17 at 10:26

I understand you want to implement a generic stream output independent of the container type. Doing this -- and particularly doing this in a nice way -- is not that easy and requires to cover several aspects, see for example the thread here on SO: Pretty-print C++ STL containers.

Yet, already with a simple first approach you can acquire at least the basic output ability:

//for std::map
template<typename ... Args>
std::ostream& operator<<(std::ostream& os, std::pair<Args ...> const& t)
{
    return os<<t.first<<"  "<<t.second;
}

template<template <typename ...> class Container, typename ... Args>
std::ostream& operator<<(std::ostream& os, Container<Args ...> const& t)
{
    for(auto const& i : t)
    {
        os<<i<<" ";
    }
    os<<"\n";
    return os;
}

Here, the stream operator calls itself recursively until it arrives at a type for which an overload exists. (For this you usually have to provide a stream output of your custom classes).

Note that the container needs to be range iterable here (one could abstract that further, but to get the idea I think it's sufficient).

With this, the following at least prints something:

int main()
{
    std::set<std::vector<std::list<double> > > a{ {{1.0, 2.0}, {3.0}}, {{4.0}} };
    std::cout<<a<<std::endl;


    std::map<int, std::vector<int> > b{{1,{2,3}}};
    std::cout<<b<<std::endl;
}

DEMO ... or, "hey, I didn't say it's nice".

It's questionable, however, whether the formatting suits your needs ... I'd suggest to at least adjust it accordingly -- or better to use the library mentioned above.

How to identify the typename type in template?

1 Answers1