功能模板参数包出现问题

Question

Why does the following fail to compile?

inline Obj init_output_string() { return open_output_string(); }

template<typename... Args>
Obj init_output_string(Args... prev, int last)
{
    Obj port(init_output_string(prev...));
    write_char(port, last);
    return port;
}

int ch1 = ...;
int ch2 = ...;
Obj port = init_output_string(ch1, ch2);

(error is 'init_output_string': no overloaded function takes 2 arguments for MSVC, g++ gives a similar error).

But the following variation does compile

inline Obj init_output_string() { return open_output_string(); }

template<typename... Args>
Obj init_output_string(int first, Args... rest)
{
    Obj port(init_output_string(rest...));
    write_char(port, first);
    return port;
}

int ch1 = ...;
int ch2 = ...;
Obj port = init_output_string(ch1, ch2);

The difference being the order in which the characters are written. I can work around this easily enough, but I'm curious to know what rule my first example is breaking.

Answer 1

Your construction is illformed. see https://en.cppreference.com/w/cpp/language/parameter_pack

In a function template, the template parameter pack may appear earlier in the list provided that all following parameters can be deduced from the function arguments, or have default arguments:

the example given is(the latter in your case):

template<typename... Ts, typename U>
struct Invalid; // Error: Ts.. not at the end 

template<typename ...Ts, typename U, typename=void>
void valid(U, Ts...);  // OK: can deduce U

// void valid(Ts..., U); 
// Can't be used: Ts... is a non-deduced context in this position

the problem is "how to deduce where to stop the int fro the pack"? is your last int part of the pack or not?

Imagine the compiler is just going thru the list of parameters and should directly know where to stop the pack.