Creating function to read variables from file into variable quantity of parameters

Question

How would I create a function that can read values from a file into multiple numbers of variables varying from 1 to many?

Here's what I have so far, working from a similar question.

Note: I cannot use fold expressions (file >> ... >>> x) because the code uses C++ 14.

test_stream_file Contents:

teststring_a    teststring_b

Code:

template<typename... Args>
void fileread(std::fstream& file, Args...args)
{
    using expander = int[];
    expander{ (file >> (std::forward<Args>(args)), 0)... };
}

int main() {

    std::fstream teststream;
    teststream.open("test_stream_file", std::ios::in);
    std::string a, b;

    fileread(teststream, a, b);
    std::cout << a << b;
}

When I run this I get error C2679: "binary '>>': no operator found which takes a right hand operator of type '_Ty' (or there is no acceptable conversion)". I'm a bit lost here. I read the documentation and another answer on std::forward but am still not seeing what is going wrong.

I'm not knowledgeable enough to explain why, but you are missing universal reference on `Args` - it should be `Args&&... args)`: https://godbolt.org/z/GGao57YK9. I though your version (without `&&`) gets arguments by copy, but compiler states it's an r-value: https://godbolt.org/z/n5bKbq3dP — Yksisarvinen, Apr 08 '22 at 15:01

score 0 · Answer 1 · answered Apr 08 '22 at 15:06

0

This is the working code for you:

#include<iostream>
#include<sstream>

template<typename... Args>
void fileread(std::istream& file, Args&...args)
{
    (file>> ...>>args);
}

int main() {

    std::stringstream teststream("hallo welt");
    //teststream.open("test_stream_file", std::ios::in);
    std::string a, b;

    fileread(teststream, a, b);
    std::cout << a << b;
}

Why do you try to do perfect forwarding, you want to change the variables that are given to you function, so it should be regular references.

answered Apr 08 '22 at 15:06

gerum

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The code I'm working in is the C++ 14 standard so it does not allow fold expressions. I cannot change this unfortunately. Have added this to the question. – wand_swe Apr 08 '22 at 16:25
If you can not use fold expressions then you can not use any syntax with "...". If you can not use fold expressions you must implement it recursive. – gerum Apr 09 '22 at 09:07

score 0 · Answer 2 · answered Apr 08 '22 at 16:32

You don't have to use std::forward to forward args since they must be lvalue references. This should be enough

template<typename... Args>
void fileread(std::istream& file, Args&...args)
{
    using expander = int[];
    expander{ ((file >> args), 0)... };
}

Demo

Creating function to read variables from file into variable quantity of parameters

2 Answers2