Generating class datamembers from a Vector

Question

Please consider this C++ question:

#include <cstdio>
#include <iostream>
#include <vector>
using namespace std;

class ParseURL{
    private:
        int qualify; // 0 means we qualify
    public:
        string node;
        string service;
        bool primary;
        bool health;
        string epoch;
        // methods
        ParseURL(string URL);
        ~ParseURL();
};


ParseURL::ParseURL(string URL){
    vector<string> g2 = {"node", "service", "primary", "health", "epoch"};

    for (string tag : g2){
        auto found = URL.find(tag);
        if ( found != string::npos ){
            auto cut_from = found + 1 + tag.size() ;
            auto meh = URL.substr(cut_from, URL.substr(cut_from).find("&") );
            // is there a way we can avoid these lines below?
            if (tag.find("node",0) == 0){
                this->node = meh;
            } else if (tag.find("service",0) == 0 ){
                this->service = meh;
            } else if (tag.find("epoch",0) == 0) {
                this->epoch = meh;
            } else if (tag.find("health",0) == 0){
                this->health = (meh == "OK");
            } else if (tag.find("primary",0) == 0 ){
                this->primary == (this->node == meh);
            }
        }
    }

}

ParseURL::~ParseURL(){
    cout << "Tearing Down the class\n";
}
int main(){
    char req[] = "GET /register?node=hostrpi3&service=potatoservice&primary=node1&health=OK&epoch=1559345106 HTTP";
    string Request = req;
    Request = Request.substr(Request.find_first_of(" ") );
    Request = Request.substr(0, Request.find(" HTTP"));
    ParseURL *a = new ParseURL(Request);

    cout.width(12);
    cout << "a->node: " << a->node << endl;
    cout.width(12);
    cout << "a->service: " << a->service << endl;
    cout.width(12);
    cout << "a->epoch: " << a->epoch << endl;
    cout.width(12);
    cout << "a->health: " << a->health << endl;
    cout.width(12);
    cout << "a->primary: " << a->primary << endl;

    delete(a);

    return 0;
}

I need to represent a object based on URL Query Strings, the tags I need are represented in vector g2, and as you can see, I make datamembers for ParseURL out of those tags.

The o/p looks like:

   a->node: hostrpi3
a->service: potatoservice
  a->epoch: 1559345106
 a->health: 1
a->primary: 0
Tearing Down the class

Although this version works, it feels this can be improved much. Despite having it all in the Vector, I am repeatedly doing tag.find for each of those attributes. There must be a better way, I hope. Could you please point in the right direction? Thanks!

Edit 1

Thanks, I updated the constructor following the suggestions:

ParseURL::ParseURL(string URL){
    char tags[10][100] = {"node", "service", "primary", "health", "epoch"};

    int i = 0;
    for (auto tag : tags){
        auto found = URL.find(tag);
        if ( found != string::npos ){
            auto cut_from = found + 1 + strlen(tag);
            auto tag_info = URL.substr(cut_from, URL.substr(cut_from).find("&") );
            switch (i){
                case 0:
                    this->node = tag_info; 
                    break;
                case 1:
                    this->service = tag_info; 
                    break;
                case 2:
                    this->primary = (this->node == tag_info); 
                    break;
                case 3:
                    this->health = (tag_info == "OK"); 
                    break;
                case 4:
                    this->epoch = tag_info; 
                    break;
            }
        }
    i++;
    }
}

bash for example, offers indirect reference of variables - that is, one variable may contain the name of another variable:

$ cat indirect.sh 
#!/bin/bash 

values="value1 value2"
value1="this is value1"
value2="this is value2"

for i in $values; do 
    echo "$i has value ${!i}"
done

$ ./indirect.sh 
value1 has value this is value1
value2 has value this is value2

I was hoping something similar exists for cases like this, nevertheless, I learned valuable lessons here. thank you!

Answer 1

Assuming that you want to keep the distinct class members (rather than one key-value store), you can use a static table of strings and pointers to members:

vector<pair<string,string ParseURL::*>> keys={{"node",&ParseURL::node},…};

But that doesn’t work here because the types are different and because the processing isn’t identical for each. Instead, for this sort of almost-parallelism, the syntactically optimal choice is often a lambda:

const auto get=[&URL](const string &tag) {
  auto found = URL.find(tag);
  if(found == string::npos) return "";
  auto cut_from = found + 1 + tag.size() ;
  return URL.substr(cut_from, URL.substr(cut_from).find("&") );
};
node=get("node");
service=get("service");
epoch=get("epoch");
health=get("health")=="OK";
primary=get("primary")==node;

This style has the advantage of definitely initializing health and primary. With sufficient contortions with a private constructor, it can be put into a member initializer list, which is even better practice.

Answer 2

I didn't exactly understand what you want to use that indirection for in you example, but here's how it can be done:

vector<string> values{"value1", "value2"}
map<const string, const string> fields_and_values{{"value1", "this is value1"}, {"value2", "this is value2"}};
for (value: values)
     cout << value << " have value " << fields_and_values[value] << endl;

This code allows you to dynamically add more fields. If you have a predefined number of fields, this is an overkill, and a simple array would do. i.e.

array<const string, 2> values{"this is value1", "this is value2"};
for (int i = 0; i < 2; ++i)
    cout << "value" << i << " have value " << values[i] << endl;

Hope that answers your question.

In general about your code:

Why did you go from vector<string> to char[10][100]? (esp. when you only have 5 strings, and none of them take up 100 chars?) array<string, 5> is the way to go for your case.

Also, don't use new and delete.. There's no reason to do a heap allocation, just use ParseURL parse(request) (and even if there was a good reason - use unique_ptr instead).

And finally, if you don't want to go over the string multiple times looking for different keywords, you could use regex.

Answer 3

I believe that question can be decomposed:

• URL parsing and then
• assigning results to object data members.

regex gives compact and robust solution for extracting URL key value pairs (parameters) into map:

#include <iostream>
#include <iterator>
#include <map>
#include <string>
#include <regex>

using namespace std;

int main()
{
    map<string, string> keyValues;
    string s = "GET /register?node=hostrpi3&service=potatoservice&primary=node1&health=OK&epoch=1559345106 HTTP";
    regex keyValueRegex("[?&](\\w+)=(\\w+)");
    auto pairsBegin = std::sregex_iterator(s.begin(), s.end(), keyValueRegex);
    auto pairsEnd = std::sregex_iterator();

    for (std::sregex_iterator i = pairsBegin; i != pairsEnd; ++i)
    {
        std::smatch match = *i;
        keyValues[match[1]] = match[2];
    }

    for (auto p : keyValues)
    {
        cout << p.first << " " << p.second << endl;
    }
}

gives output:

epoch 1559345106
health OK
node hostrpi3
primary node1
service potatoservice

Another approach (for very granular URL parsing) is using cpp-netlib library (proposed for Boost inclusion).