无法理解std ::函数用法

Question

这就是我们使用std :: function的方式。

        std::function<void(int)> f_display = print_num;

以下确切说明的含义是什么？？？

有人会提供示例或解释如何使用此代码段..

       using genericPointAction = std::function<void (const CCVector3 &, ScalarType &)>;


       void forEach(GenericCloud::genericPointAction action) override
        {

          //there's no point of calling forEach if there's no activated scalar field!
     ScalarField* currentOutScalarFieldArray = getCurrentOutScalarField();
        if (!currentOutScalarFieldArray)
        {
            assert(false);
            return;
        }

        unsigned n = size();
        for (unsigned i = 0; i < n; ++i)
        {
            action(m_points[i], (*currentOutScalarFieldArray)[i]);
        }
    }