Here's an example of doing both:

  $ cat test.cpp
  #include <iostream>
  #include <string>

  template <typename F>
  auto pass_func(F func) {
    return func();
  }

  auto return_func() {
    std::string test = "Hello World!";
    return [=](){ return test; };
  }

  int main() {
    auto func = return_func();
    std::cout << pass_func(func) << std::endl;
  }

  $ clang test.cpp -lstdc++ -std=c++14 -o test
  $ ./test
  "Hello World!"

What's hard about it?

You didn't pass a function to std::cout, you called it and passed its return value.

The way I interpret the GP, they're talking about passing closures as a function argument. In the example I pass func (a closure) to pass_func. Not sure what std::cout has to do with it?

Sorry, I misread the code.