What is the difference between function templates and class templates?

I am confused about the strange syntax provided by C++ function templates and class templates. Take a quick look at the code below:

#include <iostream>
#include <algorithm>
#include <functional>
#include <iterator>
#include <vector>
using namespace std;

template <class op1,class op2>
class compose_fg_x_t : public unary_function<typename op2::argument_type,typename op1::result_type>{
public:
// constructor

compose_fg_x_t(const op1& arg1,const op2& arg2): p1(arg1),p2(arg2){
}
//function call
typename op1::result_type operator()(const typename op2::argument_type& x) const{
    return p1(p2(x));
  }

private:
op1 p1;
op2 p2;

};

template <class Op1,class Op2>
inline compose_fg_x_t<Op1,Op2> compose_fg_x(const Op1& p1,const Op2& p2){
    return compose_fg_x_t<Op1,Op2>(p1,p2);
}
int main(int argc, char *argv[])
{
int a[] = {1,2,3,4,5};
vector<int> IntVec(a,a+sizeof(a)/sizeof(int));
copy(IntVec.begin(),IntVec.end(),ostream_iterator<int>(cout," "));
cout<<endl;
transform(IntVec.begin(),IntVec.end(),ostream_iterator<int>(cout,"    "),compose_fg_x( bind2nd(multiplies<int>(),5),bind2nd(plus<int>(),10) ));
transform(IntVec.begin(),IntVec.end(),ostream_iterator<int>(cout," "),compose_fg_x_t(   bind2nd(multiplies<int>(),5),bind2nd(plus<int>(),10) ));
return 0;
}

So, my question is, why is the first transform is correct while the second is not? What the helper function compose_fg_x does is return an object of the underlying compose_fg_x_t type. I am trying to omit开发者_如何学运维 the indirect function call, which fails to compile. Why?

Template arguments can only be deduced for function templates, not for class templates. The whole point of helper functions such as make_pair (or your compose_fg_x) is to circumvent this limitation.

Here is a slightly less complicated example that demonstrates the problem:

#include <utility>

int main()
{
    auto x = std::make_pair(3, 4);   // function template arguments are deduced
    auto y = std::pair(3, 4);        // error: missing class template arguments
}