c++ parameter pack specification in constructor rather than template

Question

Unlike function declarations with parameter packs, I've found that classes require the type for each argument in the angle brackets...

Component<IntegerPair, int, int> temp(40, 5);

...which seems redundant. Here's how I defined Component:

template<typename T, class... T_Args>
class Component
{
public:
  Component(T_Args... args)
    : m_data(args...)
  {}

  T m_data;
};

Is there a way to remove int, int from the above statement?
If so, is it ok to remove it?
Also, is my way of instantiation m_data safe? When using std::forward<T_Args>(args)... my compiler told me I didn't have a constructor that could convert all of the argument types.

You're basically asking if it's possible to deduce class template arguments from a constructor call - to which the asnwer is *no*. What you'll need is a "maker" function - like for example `std::make_pair` is, or a constructor template. — jrok, Jul 19 '15 at 07:13
3: you need to take in a universal reference `T_Args&&` to use perfect forwarding — tkausl, Jul 19 '15 at 07:14
Relevant: http://stackoverflow.com/questions/29677505/why-cant-constructors-deduce-template-arguments — cpplearner, Jul 19 '15 at 07:17

score 3 · Accepted Answer · answered Jul 19 '15 at 07:24

3

One way is to make the constructor a template:

#include <utility>

struct IntegerPair {
    IntegerPair(int, int) {}
};

template<typename T>
class Component
{
public:
  template<typename... T_Args>
  Component(T_Args&&... args)
    : m_data(std::forward<T_Args>(args)...)
  {}

  T m_data;
};

int main()
{
    Component<IntegerPair> c {1,2};
}

This is functionally equivalent to std::vector and its member function emplace_back. It's perfectly ok, IMO. The error messages are pretty cryptic, as usual in template constructs like this, but this can be mitigated with an appropriate static_assert.

answered Jul 19 '15 at 07:24

jrok

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That's exactly what I was looking for, and you also fixed the forwarding. Why the curly braces for the arguments? – user3097902 Jul 19 '15 at 07:30
1

@user3097902 For no particular reason, it's newer syntax :) Parentheses would br equivalent in this case. – jrok Jul 19 '15 at 07:48

score 0 · Answer 2 · answered Jul 19 '15 at 07:20

template parameter deduction only work for function calls so the basic pattern to achieve what you want looks like this:

template<typename T, class... T_Args>
Component<T, T_Args...> makeComponent(T_Args&&... args) {
   return Component<T, T_Args...>(std::forward<T_Args>(args)...);
}

Usage:

auto c = makeComponent<IntegerPair>(1, 1)

c++ parameter pack specification in constructor rather than template

2 Answers2