This is a text-only version of the following page on https://raymii.org:
---
Title       : 	C++ create and write to a CSV file
Author      : 	Remy van Elst
Date        : 	07-06-2019
Last update : 	16-06-2019
URL         : 	https://raymii.org/s/snippets/Cpp_create_and_write_to_a_csv_file.html
Format      : 	Markdown/HTML
---



In this quick snippet I'll show you how to create and write to a  csv file. It
includes checking if the file is writable, and if it's not there, creates it
with a different first row as header. It's a quick example, I've  used it to log
some test data. It can probably be improved. It does use a mutex and `guard_lock`
so it should be thread safe.

I've updated this snippet to use a variadic template. [Click here to view the
new snippet][1].

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### write_csv.cpp

The first method checks if the file exists. The second method is a template, but 
with a set amount of columns (since that was enough in my case). The types could
differ which is why I opted for a template. 

Note that this is a cut-down simplified example, in the next section I list some
possible improvements. Since this was a one-off user story for some tests, the 
time allocated wasn't much, thus making the code bare.

In `main()` there is an example with different types and a loop to show how you
could use the function. 

    #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 T1, typename T2, typename T3>
    bool writeCsvFile(filename &fileName, T1 column1, T2 column2, T3 column3) {
        std::lock_guard<std::mutex> csvLock(logMutex);
        std::fstream file;
        file.open (fileName, std::ios::out | std::ios::app);
        if (file) {
            file << "\"" << column1 << "\",";
            file << "\"" << column2 << "\",";
            file << "\"" << column3 << "\"";
            file <<  std::endl;
            return true;
        } else {
            return false;
        }
    }

    int main() {
        std::string csvFile = "logfile2.csv";
        std::string naam = "Hallo";
        if(!fileExists(csvFile))
            writeCsvFile(csvFile, "header1", "header2", "header3");

        for (int i = 1; i < 10; ++i) {
            if (!writeCsvFile(csvFile, i, naam, static_cast<float>(i * 3.5))) {
                std::cerr << "Failed to write to file: " << csvFile << "\n";
            }
        }
        return 0;
    }

The example `logfile.csv` looks like this after two runs:

    "header1","header2","header3"
    "1","Hallo","3.5"
    "2","Hallo","7"
    "3","Hallo","10.5"
    "4","Hallo","14"
    "5","Hallo","17.5"
    "6","Hallo","21"
    "7","Hallo","24.5"
    "8","Hallo","28"
    "9","Hallo","31.5"
    "1","Hallo","3.5"
    "2","Hallo","7"
    "3","Hallo","10.5"
    "4","Hallo","14"
    "5","Hallo","17.5"
    "6","Hallo","21"
    "7","Hallo","24.5"
    "8","Hallo","28"
    "9","Hallo","31.5"
    "1","Hallo","3.5"
    "2","Hallo","7"
    "3","Hallo","10.5"
    "4","Hallo","14"
    "5","Hallo","17.5"
    "6","Hallo","21"
    "7","Hallo","24.5"
    "8","Hallo","28"
    "9","Hallo","31.5"


### Possible improvements

* Use a `struct` with the column data (making the amount of columns expandable)
* Check if the folder exists
* Check if the file exists but is empty (then write the header)
* Checking and escaping special characters (like `"` or newlines)

[1]: https://raymii.org/s/snippets/Cpp_create_and_write_to_a_CSV_file_with_a_variadic_template.html

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