在 c++ 中获取"is not a static member of class"错误

我正试图为类FBLPost创建一个静态int变量currentId，以便每次创建FBLPost对象时，都会为新实例分配该id，并且为创建的下一个新实例将静态currentId增加1。我的构造函数是在FBLPost.cpp中实现的。这在java中是一项微不足道的任务，但C++给我带来了困难。我已经在谷歌上搜索并查看了SO上的其他帖子，所以如果我错过了什么，请原谅我。这是我的代码和错误：

C:UsersDevDesktopmyProjectsAssignment_3_220Assignment3FBLPost.h|31|error: 'int FBLPost::currentId' is not a static data member of 'class FBLPost'|
C:UsersDevDesktopmyProjectsAssignment_3_220Assignment3FBLPost.cpp||In constructor 'FBLPost::FBLPost()':|
C:UsersDevDesktopmyProjectsAssignment_3_220Assignment3FBLPost.cpp|5|error: 'currentId' was not declared in this scope|
||=== Build failed: 3 error(s), 3 warning(s) (0 minute(s), 0 second(s)) ===|

当然currentId没有声明，我试图在这里声明它。

此外，如果我尝试删除static关键字，我仍然会收到错误，如果我试图在FBLPost中声明静态变量，我会收到错误。

FBL成本h

#ifndef FBLPOST_H_INCLUDED
#define FBLPOST_H_INCLUDED
#include <stdio.h>
#include <string>
#include <stdlib.h>
class FBLUser;
using namespace std;
class FBLPost {
private:
string Post = "";
FBLPost* next = 0;
int likes = 0;
FBLUser* author;
int id;
public:
FBLPost();
string getPost();
void setPost(string p);
FBLPost* getNext();
void setNext(FBLPost* n);
int getLikes();
void incrementLikes();
void setAuthor(FBLUser* user);
FBLUser* getAuthor();
};
static int FBLPost::currentId = 0; //This line give me an error
#endif // FBLPOST_H_INCLUDED

FBLPost.cpp

#include "FBLPost.h"
using namespace std;
FBLPost::FBLPost() {
id = ++currentId;
}

string FBLPost::getPost() {
return Post;
}
void FBLPost::setPost(string p) {
Post = p;
}
FBLPost* FBLPost::getNext() {
return next;
}
void FBLPost::setNext(FBLPost* n) {
next = n;
}
int FBLPost::getLikes() {
return likes;
}
void FBLPost::incrementLikes() {
likes++;
}
void FBLPost::setAuthor(FBLUser* user) {
author = user;
}