Create Arguments via Command Line LIKE A PRO in C++

Adding magic to parameters via command line.

Create Arguments via Command Line LIKE A PRO in C++

The arguments via command line leveraged the UNIX commands making them into high power tools.

Today we are going to meet argparse a library written in Modern C++. This tool not only helps you to create arguments in a professional way, but also to create help in a practical way.



Run the commands in order:

git clone
cd argparse
mkdir build
cd build
sudo make install

The files will be installed in:

Install the project...
-- Install configuration: ""
-- Installing: /usr/local/lib64/cmake/argparse/argparseConfig.cmake
-- Installing: /usr/local/include/argparse/argparse.hpp
-- Installing: /usr/local/lib64/cmake/argparse/argparseConfig-version.cmake
-- Installing: /usr/local/lib64/pkgconfig/argparse.pc


To test a basic file would be the example provided in the documentation

#include <argparse/argparse.hpp> // header

int main(int argc, char *argv[]) { // important
   argparse::ArgumentParser program("program_name"); // change to your command name, eg "square"

     .help("display the square of a given integer") // automatically add to help
     .scan<'i', int>(); // check for parameters that are digits only

   try {
     program.parse_args(argc, argv);
   catch (const std::runtime_error& err) { // note that since it's just checking if the arguments were passed, so it's 'runtime_error'
     std::cerr << err.what() << std::endl;
     std::cerr << program;

   auto input = program.get<int>("square"); // stores the parameter to a variable so we can manipulate it more easily
   std::cout << (input * input) << std::endl; // execute the operation

   return 0;

Let’s see more options by creating a command from scratch.

Creating a Command from Scratch

Let’s create a command similar to the UNIX cat command, but let’s call it: mycat.

1. Include the header:

#include <argparse/argparse.hpp>

2. Create an instance of ArgumentParser and enter the name of our command:

Remembering that your main() function needs to receive parameters, for example: int main (int argc, char **argv).

argparse::ArgumentParser app("mycat");

3. Add your program argument to help

Instruct the command to receive at least 1 file as a parameter. And already add this to --help.

   .help("Inform the file(s) you want to see the content.");

4. Check if parameters were passed:

try {
   app.parse_args(argc, argv);
}catch (const std::runtime_error &err){ // runtime_error
   std::cerr << err.what() << '\n';
   std::cerr << app;

5. Check if FILES were passed:

  auto files = app.get<std::vector<std::string>>("files");
}catch(const std::logic_error &err){ // logic_error
    std::cerr << err.what() << '\n';
   std::cerr << app;

6. Recreate the same variable, but outside the block to use the files

auto files = app.get<std::vector<std::string>>("files");
for(auto &file : files){
   std::cout << "FILE: " << file << '\n';

7. Compile and test!

To compile, you don’t need any specific flag, just compile and run:

g++ mycat.cpp -o mycat

Before running let’s use this file as an example


Of course life is good
And joy, the only unspeakable emotion
Of course I think you're beautiful
In you I bless the love of simple things
Of course I love you
And I have everything to be happy

But I happen to be sad...

If we just run the program/command without informing any files, it already shows the type of error in the first line and then the help already with the customized data for our command:

$ ./mycat

files: 1 argument(s) expected. 0 provided.
Usage: mycat [--help] [--version] files

Positional arguments:
   files Inform the file(s) you want to see the content.

Optional arguments:
   -h, --help shows help message and exits
   -v, --version prints version information and exits

If we pass the file, it already lists:

./mycat file.txt

8. Displaying file contents

As we want to display the contents of the file instead of the list, let’s just create a function named mycat() and of type void receiving a string as a parameter:

#include <fstream>

void mycat(const std::string &name){
   std::string line{};
   std::ifstream file(name);
   while(std::getline(file, line)){
    std::cout << line << '\n';

And in our loop we will replace std::cout with the function passing the name of our file:

for(auto &file : files){

This is a basic example, but it would be right to include the <filesystem> and create another try catch to check if the files(strings) are of type file.

Now if we recompile and run it, it will already display the contents of the file:

g++ mycat.cpp -o mycat
./mycat file.txt

9. Displaying number of lines

The cat command has a -n or --number parameter which displays line numbers. Let’s implement this parameter to our mycat.

First let’s add a new argument:

There are several union functions that we can add, in this case we will use two:

  • .default_value(false) - if you want a default value;
  • .implicit_value(true); - does not require specific value;
app.add_argument("-n", "--number")
   .help("Display the number of lines")

10. Now let’s create a check if this argument is active and pass it to our function:

leave it like that

void mycat(const std::string &name, bool check){
   std::string line{};
   int number {1};
   std::ifstream file(name);
   while(std::getline(file, line)){
       std::cout << number << "." << line << '\n';
       std::cout << line << '\n';

And our check will look like this:

bool check{false};
if( app["-n"] == true ){
   check = true;

auto files = app.get<std::vector<std::string>>("files");
for(auto &file : files){
   mycat(file, check);

Recompile and test in several ways:

g++ mycat.cpp -o mycat
./mycat file.txt
./mycat --number file.txt
./mycat -n file.txt
./mycat file.txt --number
.mycat file.txt -n

11. We can improve more things like:

  • Change version: app("mycat", "2.3.0");
  • Change help and version display text:
argparse::ArgumentParser app("mycat", "2.3.0", argparse::default_arguments::none);
app.add_argument("-h", "--help")
  .help("Show this help");

app.add_argument("-v", "--version")
  .help("Show the version");
  • Add a description:
app.add_description("A minimalist alternative to the 'cat' command.");
  • Read multiple files:
  .help("Enter the file(s) you want to see the content.")

And use:

/.mycat file1.txt file2.txt file3.txt
/.mycat -n file1.txt file2.txt file3.txt

And among several possibilities that can be found in the documentation.

Useful links

cpp command