论C++的lambda是函数还是对象

int n = 100;
auto foo = [n](int a) {
    return a > n;
};
cout<< foo(99);

struct Foo {
    Foo(int i) {n=i;}
    bool operator()(int a) {
        return a > n;
    }
private:
    int n;
};
...
int n = 100;
Foo foo(n);
cout<< foo(99);

set<int> ns = {100, 200, 300};
auto foo = [&ns](int a) {
    return ns.find(a);
};
cout<< foo(99);

struct Foo {
    Foo(set<int>* p) {p_ns = p;}
    bool operator()(int a) {
        auto &ns = *p-ns;
        return ns.find(a);
    }
private:
    set<int>* p_ns;
};
...
set<int> ns = {100, 200, 300};
Foo foo(&ns);
cout<< foo(99);

using namespace std;
struct A {
    void* operator()(void*) {
        cout<<"xxxx"<<endl;
        return nullptr;
    }
};
int main() {
    A a;
    a(NULL);
    pthread_t t;
    //pthread_create(&t, NULL, a, NULL); // 编译失败
    auto cb = [](void*)->void* {
        cout<<"xxxx"<<endl;
        return nullptr;
    };
    pthread_create(&t, NULL, cb, NULL); // 编译通过
    pthread_join(t, NULL);
    return 0;
}

auto cb = [&](void*)->void* {
        cout<<"xxxx"<<endl;
        return nullptr;
    };
    pthread_create(&t, NULL, cb, NULL);

cb.cpp: In function ‘int main()':
cb.cpp:23:30: error: cannot convert ‘main()::<lambda(void*)>' to ‘void* (*)(void*)'
   23 |     pthread_create(&t, NULL, cb, NULL);
      |                              ^~
      |                              |
      |                              main()::<lambda(void*)>
In file included from /usr/include/x86_64-linux-gnu/c++/9/bits/gthr-default.h:35,
                 from /usr/include/x86_64-linux-gnu/c++/9/bits/gthr.h:148,
                 from /usr/include/c++/9/ext/atomicity.h:35,
                 from /usr/include/c++/9/bits/iOS_base.h:39,
                 from /usr/include/c++/9/ios:42,
                 from /usr/include/c++/9/ostream:38,
                 from /usr/include/c++/9/iostream:39,
                 from cb.cpp:1:
/usr/include/pthread.h:200:15: note:   initializing argument 3 of ‘int pthread_create(pthread_t*, const pthread_attr_t*, void* (*)(void*), void*)'
  200 |       void *(*__start_routine) (void *),
      |       ~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~