Char * to std::string

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• What is the complexity of reverse() function in c++?

  I guess it's complexity maybe o(nlogn) but i am confused. so, I want to know it

    reverse(v.begin(), v.begin());
  
• I'm trying to Dequeue. It works! But is it the right way to do it?

  I wrote the following code in C++ to for deQueue function. My professor used two new nodes in his dequeue function for some reason. This code works just fine, but i need to know if something is wrong at the back.

  #include<iostream>
  using namespace std;
  class node{
     public:
        node *nxt;
        int data;   
        };
  class queue{
          node *head, *tail;
          public:
             queue(){
              head=tail=NULL;
                    }
      void q(int var)
      {
          node *n=new node();
          n->data=var;
          n->nxt=NULL;
  
          if(head==NULL)
          {
              head=tail=n;
          }
          else{
              tail->nxt=n;
              tail=n;
          }
      }
      void dQ()
      {
          node *tmp=new node;
          tmp=head;
          cout<<tmp->data<<" Deleted"<<endl;
          head=head->nxt;
      }
  
  
  
   };
  int main()
  {
  queue obj;
  obj.q(13);
  obj.q(14);
  obj.q(18);
  obj.display();
  obj.dQ();
  cout<<"After DQ"<<endl;
  obj.display();
  return 0;
  }
  
• How to convert old connect syntax

  I can't convert QEventLoop connect statement to a new syntax... Compile error: no matching member function to call 'connect'

  QNetworkAccessManager manager;    
  QNetworkReply *reply = manager.get(QNetworkRequest(QUrl("request")));   
  QEventLoop loop;
  loop.connect(&manager, SIGNAL(finished(QNetworkReply*)), SLOT(quit()));
  //loop.connect(&manager, &QNetworkReply::finished, &QEventLoop::quit); //ERROR
  loop.exec();
  
• Segmentation error on string affectation within a tagged union

  I have the following simplified code that models a Variable that can be an integer or a string:

  //tagged union
  union VariableValueType {
      std::int64_t  i;
      std::string   s;
  
      VariableValueType(void): i(0) {}
      ~VariableValueType(void) {}
  };
  enum class VariableType {Undefined, Integer, String};
  
  class Variable
  {
      VariableValueType _value;
      VariableType _type;
  
      //constructor from a string
      Variable(const VariableType type, const std::string& value)
       : _type(VariableType::Undefined)
      {
          set(type,value);
      }
  
      // setter
      void set(const VariableType type, const std::string& value)
      {
          switch(type)
          {
              case VariableType::Integer :
                  _value.i = std::stoi(value);
                  _type = VariableType::Integer;
                  break;
              case VariableType::String :
                  _value.s = value; //<-- crashes here!!!
                  _type = VariableType::String;
                  break;
          }
      }
  };
  

  When I call set from another point in the code with type=String, I get a segmentation fault on the string affectation line.

  #0  0x00007ffff7b70c73 in std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::_M_assign(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&) () from /usr/lib/x86_64-linux-gnu/libstdc++.so.6
  #1  0x00007ffff7b70f89 in std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::operator=(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&) () from /usr/lib/x86_64-linux-gnu/libstdc++.so.6
  #2  0x00005555556c3fdd in Variable::set (this=0x5555559d7ae0, type=VariableType::String, value="1") at /home/.../Variable.cpp:118
  

  I can't figure what is going on here...

  Fun fact: if I add std::string foo = value; as the first line of the set method, then the segmentation fault disapears...

  ---

  Edit:

  When I ask gdb the value of _value.s, I get the following message:

  error reading variable: Cannot create a lazy string with address 0x0, and a non-zero length.
  

  Which I do not understand...
• Reading a file into memory C++: Is there a getline() for std::strings

  I was asked to update my code that reads in a text file and parses it for specific strings.

  Basically instead of opening the text file every time, I want to read the text file into memory and have it for the duration of the object.

  I was wondering if there was a similar function to getline() I could use for a std::string like i can for a std::ifstream.

  I realize I could just use a while/for loop but I am curious if there is some other way. Here is what I am currently doing:

  file.txt: (\n represents a newline )

  file.txt

  My Code:

  ifstream file("/tmp/file.txt");
  int argIndex = 0;
  std::string arg,line,substring,whatIneed1,whatIneed2;
  if(file)
  {
      while(std::getline(file,line))
      {
          if(line.find("3421",0) != string::npos)
          {
              std::getline(file,line);
              std::getline(file,line);
              std::stringstream ss1(line);
              std::getline(file,line);
              std::stringstream ss2(line);
              while( ss1 >> arg)
              {
                  if( argIndex==0)
                  {
                      whatIneed1 = arg;
                  }
                  argIndex++;
               }
               argIndex=0;
              while( ss2 >> arg)
              {
                  if( argIndex==0)
                  {
                      whatIneed2 = arg;
                  }
                  argIndex++;
               }
               argIndex=0;
           }
       }
   }
  

  Where at the end whatIneed1=="whatIneed1" and whatIneed2=="whatIneed2".

  Is there a way to do this with storing file.txt in a std::string instead of a std::ifstream asnd using a function like getline()? I like getline() because it makes getting the next line of the file that much easier.
• Is there library to manipulate std::string of C++ from C?

  I am thinking of manipulating C++ std::string objects from C. (Get string, and concatenate them.)

  Are there some libraries or some common code snippets to do this ?

  If there are not such things, I am thinking of implementing it by sharing pointers to the std::string objects between C and C++.

  Thanks !