fx-converter

The sole objective of this project is to learn the practical use of various C++ features.
Besides serving as a handy reference for myself, the motive is to present these usages out in the wild. I hope these usages can be easily understood by new programmers.

How to use?

git clone https://github.com/trunc8/fx-converter.git
cd fx-converter
# To run the project
make test

# To debug the project
make debug

Notes

• Command-line instructions are designed for Linux users
• #include <iostream>; using namespace std; applies to all snippets below

Table of Contents

• Makefile
• Fetch data using web API
• Fetch data from file
• Parse json
• Operator Overloading
• Function Template
• Lambda Function
• Debug

Makefile

For a quick guide on make and Makefile, please refer to my blog. The Makefile in this project presents an easier example:

build: clean
  g++ -Wall -o fx main.cpp utils.cpp -lcurl

clean:
  rm -rf fx

test: build
  ./fx 

For some intuition of the above code, running make test on the terminal:

• first runs clean, i.e., the command rm -rf fx
• then runs build, i.e., the command g++ -Wall -o fx main.cpp utils.cpp -lcurl
• finally runs ./fx

So in a single terminal command, we removed the stale output file, compiled the source code, and ran the executable.

Fetch data using web API

Ensure that curl library is installed in your system. We ultimately want to include the curl.h header file in our code. You can verify if the header file is available by checking if the directory /usr/include/x86_64-linux-gnu/curl is present. While compiling, add -lcurl at the end of the command.

Relevant lines from Makefile:

build: clean
	g++ -Wall -o fx main.cpp utils.cpp -lcurl

Relevant snippets from the project code:

#include <curl/curl.h>

size_t WriteCallback(void *contents, size_t size, size_t nmemb, void *userp)
{
    ((string*)userp)->append((char*)contents, size * nmemb);
    return size * nmemb;
}

double getFxRate(string curr1, string curr2) {
  ...
  string url_string = "https://free.currconv.com/api/v7/convert?q="
    +curr1+"_"+curr2+"&compact=ultra&apiKey=b0591706cb6a10f406d5";

  CURL *curl;
  string readBuffer;

  curl = curl_easy_init();

  if(curl) {
    curl_easy_setopt(curl, CURLOPT_URL, url_string.c_str());
    curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, WriteCallback);
    curl_easy_setopt(curl, CURLOPT_WRITEDATA, &readBuffer);
    curl_easy_perform(curl);
    curl_easy_cleanup(curl);
  }
  ...
}

Fetch data from file

We have currency_codes.txt as a text file in the same directory.
Relevant snippets from the project code:

#include <fstream>

void fetchValidCurrency(set<string> &valid_curr) {
  if (ifstream input{"currency_codes.txt"}) {
    
    while (input) {
      string line;
      getline(input, line);
      
      if (line.empty())
        continue;
      
      valid_curr.insert(line);
    }
  }
}

Parse json

We shall use nlohmann's json library. Head to their GitHub releases. Scroll down to their latest assets. Download the json.hpp file to the include directory of your project.
Relevant snippets from the project code:

#include "json.hpp"
using json = nlohmann::json;

double getFxRate(string curr1, string curr2) {
  double _fx_rate = 0;
  
  ... // string readBuffer contains json text in string format

  // Using nlohmann json
  json fx_json = json::parse(readBuffer);
  // Pretty print in json format
  // cout << setw(4) << fx_json << std::endl;
  _fx_rate = fx_json.at(curr1+"_"+curr2);

  return _fx_rate;
}

Operator Overloading

Relevant snippets from the project code:

ostream &operator<<(ostream &os, const set<string> &s) {
  for (auto const &i: s) {
    os << i << " ";
  }
  os << "\n";
  return os;
}

Function Template

Relevant snippets from the project code:

template<typename T>
void printSet(set<T> const &s) {
  // for(auto i : s) makes a copy of each element of s
  for (auto const &i: s) {
    cout << i << " ";
  }
  cout << endl;
}

Lambda Function

Relevant snippets from the project code:

int main(void) {
  ...

  // Using lambda expression
  auto validateCurrency = [valid_curr](string curr) -> bool {
    return valid_curr.find(curr) != valid_curr.end();
  };

  if (!validateCurrency(curr1) || !validateCurrency(curr2)) {
    cout << "Please use valid currency code. Exiting.\n";
  }
  else {
    ...
  }
  ...
}

Debug

Ensure that gdb is installed in your system. We need to pass an additional flag (-g) while compiling in order to use gdb later.

Relevant lines from Makefile:

debug: clean
	g++ -Wall -g -o fx main.cpp utils.cpp -lcurl
	gdb -tui fx

Useful gdb shortcuts:

• q: quit
• b: breakpoint (on line number)
• r: run (program)
• l: list (print source code till that point)
• p: print (variable)
• c: continue (until next breakpoint)
• n: next (line in function)
• s: step (next innermost instruction)
• fin: finish (continue until just after current function returns)
• ENTER: repeat previous command
• bt: backtrack (backtrace stack frames)

Acknowledgement

• gdb debugging

Author(s)

• Siddharth Saha - trunc8

Created with ❤️ by Siddharth