使用lambda表达式和std :: function没有匹配的函数调用错误。

Question

I am trying to use this lambda expression in my code. And looks like something's wrong with definition of functor or lambda.

I believe the lambda expression is true. but I can't satisfy the prototype of function I defined.

Function definition:

    template<typename E, typename container> 
void for_each(Iterator<E,container> begin, std::function<void(E&)> fun)
{
    do // does the given unary functor to elements 
    {  //from starting iterator 'til the end.
        fun(*begin);
        begin = begin.next();
    }while(begin.hasNext());

    fun(*begin);
}

And caller:

  for_each(c.iterator(), [&](E& e){add(e);});

I except achieve this function call with lambda expression. But compiler says "error: no matching function for call to .."

Answer 1

lambda is not std::function , so deduction fails.

You might simply pass generic Functor :

template<typename E, typename container, typename F> 
void for_each(Iterator<E, container> begin, F&& fun)
{
    // ...
}

Answer 2

Issues with the code and Possible solution approaches below:

1. A Lambda expression is not std::function , it can only be stored in an object of type std::function .

Approach: (With your existing signature):

std::function<void()> f = []() -> void { // do something };
for_each(vec.begin(), vec.end(), f);

2. Better solution would be to use a Callable as a parameter (Go to end of the answer).

3. If you want to pull up your own version of for_each , why not follow the implementation that is consistent with other algorithms or for that matter with the implementation of for_each in the algorithm header.

Does all you want and is also consistent with other standard algorithms: en.cppreference.com/algorithms/for_each :

    template<class InputIt, class UnaryFunction>
    constexpr UnaryFunction for_each(InputIt first, InputIt last, UnaryFunction f)
    {
      for (; first != last; ++first) {
        f(*first);
      }
      return f;
    }