头文件包含的问题求解

代码如下，Function.h文件，如果没有#include <vector>这句的话会报错，但是不包含string头文件却没错，为什么?
Function.h
#include <vector>
#define MAX_CHARACTER_NUMBER 9
class CQueue{
public:
CQueue();
~CQueue();
void InputString();
void DivisionString();
void OutputString();
private:
std::string strList;
std::vector<std::string> m_vecList;
int m_nListCount;
char m_aCharater[MAX_CHARACTER_NUMBER];
};
Function.cpp
#include "stddef.h"
#include <stdio.h>
#include <iostream>
#include <vector>
#include <string>
#include <set>
#include "Function.h"
using std::set;
using std::iterator;
using std::cin;
using std::cout;
using std::endl;
using std::vector;
using std::string;