templates without arguments

[cambalinho]

see these sample:

template <typename a>
class test
{
  test(a argument)
  {
     cout << argument;
  }
};

//declare it:
int main()
{
   test<string> a("hello");
}

how can i do it:

test a("hello");

[deceptikon]

The closest you can get without using the preprocessor is providing a default type:

template <typename a = string>
class test
...

test<> a("hello");

[cambalinho]

why here don't use the template arguments?
see page 3('codigo-fonte'): https://dl.dropboxusercontent.com/u/28818654/Community/templates.pdf

[deceptikon]

You didn't point out exactly where you're talking about in that document, but I'm guessing you mean the template functions. If a parameter in a template function uses one of the template type parameters, calls to that function will deduce the type of the template parameter based on the type of the argument:

template <typename T>
void foo(T arg)
{
    cout << arg << '\n';
}

int main()
{
    foo(123);     // T deduced as int
    foo(123.456); // T deduced as double
}

This doesn't work for template classes because there's nothing to deduce. Of course, if you're okay with using a strict template type parameter, you could use typedef or a C++11 alias:

typedef test<string> test_t;

test_t a("hello");

[mike_2000_17]

What deceptikon said. In addition, I want to mention that this deduction of the template arguments is also the reason for having the make_foo functions to create class templates in a way that avoids explicitely listing the template arguments. What you would do is this:

template <typename A>
class test
{
  test(A argument)
  {
     cout << argument;
  }
};

template <typename A>
test<A> make_test(A argument) {
  return test<A>(argument);
};


//declare it:
int main()
{
   make_test("hello");

   // or, in C++11, with the 'auto' type deduction:
   auto a = make_test("hello");

   return 0;
};

This is a classic technique, used in many places, for example, in the STL, with class templates like std::pair, std::tuple, std::shared_ptr, etc...

[cambalinho]

thanks, these classic technique have a term name?
(for google search: i want read more about it)

[cambalinho]

i belive theres another way.
see my code:

#ifndef events_H_INCLUDED
#define events_H_INCLUDED
#include <functional>
#include <vector>

template <typename ... b>
class event
{
public:
    typedef std::function<void(b...argx )> OnSomethingHandler;

    event(OnSomethingHandler Handler)
    {
        handlers_=Handler;
    }

    void operator ()(b... args)
    {
        handlers_(args...);
    }

    event& operator = (OnSomethingHandler Handler)
    {
        handlers_ = Handler;
        return *this;
    }
private:
    OnSomethingHandler handlers_;
};
#endif // events_H_INCLUDED

how i use it:

event<int,int> Soma{[](int a, int b) { ; }};

someone tell me that i can change the my template for not repeat the same type, can you advice me?

[cambalinho]

#define events(eventname, ... )     event<__VA_ARGS__> eventname{[](__VA_ARGS__ ) { ; }};

what isn't right with these macro?