std :: function （和它的灵感， boost :: function ）不仅存储函数指针。它还可以存储函数对象。在这个意义上，传递函数签名作为模板参数类似于智能指针通常采用 pointee 作为模板参数，而不是指针类型！

std::function (and its inspiration, boost::function) does not only store function pointers. It can also store function objects. In that sense, passing a function signature as a template parameter is similar to how a smart pointer usually take the type of the pointee as a template parameter, not a pointer type!

对比度：

int* p; // indirection to an object of type int
std::unique_ptr<int> q; // indirection to an object of type int

和

typedef void signature_type(); // a function type

// indirection to something callable with signature_type as a signature
// i.e. f() has type void
// only work for freestanding functions however
signature_type* f;

// indirection to something callable with signature_type as a signature
// i.e. g() has type void
// not restricted to function pointers!
std::function<signature_type> g;

这是一个有用的约定。