How to pass a template function in a template argument list

c++, c++11, function-templates, templates

Solution

I suspect the answer is "you cannot do this".

Yes, that is the case, you cannot pass a function template as a template argument. From 14.3.3:

A template-argument for a template template-parameter shall be the name of a class template or an alias template, expressed as id-expression.

The template function needs to be instantiated before you pass it to the other template. One possible solution is to pass a class type that holds a static `produce_5_function` like so:

template<typename T>
struct Workaround {
  static T produce_5_functor() { return T(5); }
};
template<template<typename>class F>
struct client_template {
  int operator()() const { return F<int>::produce_5_functor(); }
};
int five = client_template<Workaround>()();

Using alias templates, I could get a little closer:

template <typename T>
T produce_5_functor() { return T(5); }

template <typename R>
using prod_func = R();

template<template<typename>class F>
struct client_template {
  int operator()(F<int> f) const { return f(); }
};

int five = client_template<prod_func>()(produce_5_functor);

Problem

Suppose I have a `template` function: ``` template<typename T> T produce_5_function() { return T(5); } ``` How can I pass this entire `template` to another `template`? If `produce_5_function` was a functor, there would be no problem: ``` template<typename T> struct produce_5_functor { T operator()() const { return T(5); } }; template<template<typename T>class F> struct client_template { int operator()() const { return F<int>()(); } }; int five = client_template< produce_5_functor >()(); ``` but I want to be able to do this with a raw function template: ``` template<??? F> struct client_template { int operator()() const { return F<int>(); } }; int five = client_template< produce_5_function >()(); ``` I suspect the answer is "you cannot do this".

Original source