c – 模板参数作为仿函数模板

#include <iostream>

struct null_t { };

typedef void (*thread_func_t)();
typedef void (*thread_func_2_t)(int);

template <typename F,typename P = null_t>
class adapter
{
public:
    adapter(F f,P p = null_t()) : _f(f),_p(p) {}

    void operator ()() 
    {
        _f(_p);
    }
private:
    F _f;
    P _p;
};

template <typename T>
class thread
{
public:
    explicit thread(T f) : _f(f) { }

    void run()
    {
        _f();
    }
private:
    T _f;
};

void show_hello()
{
    std::cout << "hello" << std::endl;
}

void show_num(int x)
{
    std::cout << "show_num: " << x << std::endl;
}

int main()
{
    thread<adapter<thread_func_t> > t_a(adapter<thread_func_t>(&show_hello));

    t_a.run();

    int i = 666;
    thread<adapter<thread_func_2_t,int> > t_b(adapter<thread_func_2_t,int>(&show_num,i));
    t_b.run();
}

编译错误：

$/usr/bin/g++-4.4 func.cpp -o func
func.cpp: In function ‘int main()’:
func.cpp:51: error: request for member ‘run’ in ‘t_a’,which is of non-class type ‘thread<adapter<void (*)(),null_t> >(adapter<void (*)(),null_t>&)’

1)适配器不准备调用没有参数的函数. (我不知道该怎么做).

2)我已经尝试过线程接收模板模板参数但没有成功.

我试图做的几乎与下面的示例相同(此适配器不适用于没有参数的函数)：

typedef void (*WorkerFunPtr)(const std::string&);

template<typename FunT,typename ParamT>
struct Adapter {
    Adapter(FunT f,ParamT& p) : f_(f),p_(&p) {}

    void operator( )( ) {
        f_(*p_);
    }
    private:
    FunT f_;
    ParamT* p_;
};

void worker(const std::string& s) { std::cout << s << 'n'; }

int main( ) {
    std::string s1 = "This is the first thread!";
    boost::thread thr1(Adapter<WorkerFunPtr,std::string>(worker,s1));

    thr2.join( );
}