variable 'std:ofstream' has initializer but incomplete type

variable 'std:ofstream’ has initializer but incomplete type

或者是variable 'std:ifstream’ has initializer but incomplete type

其原因是因为没有包含fstream这个头文件。

#include<fstream>
#include<iostream>
#include<string> 
using namespace std;
int main()
{
fstream f("filename");
f << 20;
f.close(); 
} 

上面这段代码如果没有那个<fstream>就会有标题的错误。