This is a text-only version of the following page on https://raymii.org:
---
Title       : 	C++ create and write to a CSV file with a variadic template
Author      : 	Remy van Elst
Date        : 	17-06-2019
URL         : 	https://raymii.org/s/snippets/Cpp_create_and_write_to_a_CSV_file_with_a_variadic_template.html
Format      : 	Markdown/HTML
---



In this snippet I'll show you a [variadic template][1] to write to a file. In
line with my [other experiments][3] to get a better grasp at templates, this
example improves on my [earlier attempt][2] by using a variadic template, thus
allowing you to provide an infinite number of columns to the csv file of any
type that has  the overloaded `<<` operator.

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In [the first example][2] I wrote to a CSV file with a set amount of columns and
types. After experimenting some more with templates and variadic templates, I 
tried to create this function again but with templates. A simple example,
without any checking if a file already exists or is successfully opened. It
would probably be better to pass the CSV data via a `struct` since most of the
time the data format is going to be the same.

### The code

    #include <iostream>
    #include <mutex>
    #include <fstream>

    std::mutex logMutex;

    bool fileExists(std::string& fileName) {
        return static_cast<bool>(std::ifstream(fileName));
    }

    template <typename filename, typename Arg>
    void writeFile(filename &fileName, Arg arg) {
        std::lock_guard<std::mutex> csvLock(logMutex);
        std::fstream file;
        file.open (fileName, std::ios::out | std::ios::app);
        if (file)
            file << arg;
    }


    template <typename filename, typename First, typename... Args>
    void writeFile(filename &fileName, First first, Args... args) {
        writeFile(fileName, first);
        writeFile(fileName, args...);
    }

    int main() {
        std::string theFile = "a";
        if(!fileExists(theFile))
            writeFile(theFile, "\"header1\",", "\"header2\",", "\"header3\"", "\n");
        writeFile(theFile, "\"data1\",", "\"data2\",", "\"data3\"", "\n");
        writeFile(theFile, "\"second1\",", "\"second2\",", "\"second3\"", "\n");
        return 0;
    }


[1]: https://raymii.org/s/snippets/Cpp_variadic_template_recursive_example.html
[2]: https://raymii.org/s/snippets/Cpp_create_and_write_to_a_csv_file.html
[3]: https://raymii.org/s/blog/Cooking_with_Cpp_templates_and_stronger_types.html

---

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