如何初始化在 header 文件中声明为 const 的 object？

Question

I want to initialize std::ifstream object only in the main() function after declare it in the header.

Is there any way to do it in C++?

I wrote this but it's not compiling

//header.h
#include <iostream>
#include <fstream>

class class1{
        static const std::ifstream fs;
};

//proj.cpp
#include "header.h"

void main(){
        class1::fs("Employee.txt")
}

Answer 1

static variables need to be defined at global scope, not inside a function.

main should also return int not void .

A const std::ifstream doesn't make much sense as most of the methods you would need to use are non- const so wouldn't be callable on your const stream.

Fixing these issues gives:

//header.h
#include <iostream>
#include <fstream>

class class1{
        static std::ifstream fs;
};

//proj.cpp
std::ifstream class1::fs("Employee.txt");
int main(){
    return 0;
}

If you want to open the stream in main then you need to do:

const std::ifstream class1::fs;
int main(){
    class1::fs.open("Employee.txt");
    return 0;
}