CODEWITHSUNDEEP
c++c++11objectpointersvector
DEEPANSH NAGARIA
DEEPANSH NAGARIA

Reputation: 113

pointer attribute changes when reexamined

I am new to C++, currently working on a networking project, faced an unusual error with a vector of object pointers.

State class:

struct State
  {
    public:
      int reject_percent_;
      int fill_percent_;
      int partial_fill_;
      bool is_logged_in_;
      struct order
      {
        long id;
        long price;
        int quantity;
        bool is_filled = false;
        bool is_partially_filled = false;
      };
      std::vector<order *> orders;
  };

pushing into vector: (here state is an object of State struct)

State::order* o;

o->id = (obj->ClOrdID);   // obj->ClOrdID = 1
o->price = (obj->Price);  // obj->Price = 1
o->quantity = (obj->OrderQty); // obj->OrderQty = 1
std::cout<<o->id<<"\n";           //outputs 1
state->orders.push_back(o);

in other function:

State::order* ord = NULL;
    for (int i = 0; i < state->orders.size(); ++i) 
    {
      std::cout<<((state->orders).at(i)->id)<<"\n";  //outputs :: 93893845689152
      std::cout<<((state->orders).at(i)->price)<<"\n"; //outputs :: 93893845689184
      std::cout<<((state->orders).at(i)->quantity)<<"\n"; //outputs :: 869246848
      if(obj->ClOrdID==(state->orders).at(i)->id)
      {
        ord=(state->orders).at(i);
        break;
      }
    }

I know this is not a minimal reproducible example, but I think this might be a trivial error that I don't see, the code is big and will take a long time shortening, so please bear with me, can you just point out what might cause this problem, as the values seem to be junk values of the datatypes.

Upvotes: 1

Views: 100

Answers (2)

Thomas Sablik
Thomas Sablik

Reputation: 16454

You didn't allocate memory for the order and you didn't initialize the pointer

State::order* o;

Dereferencing this pointer to write into it

o->id = (obj->ClOrdID);   // obj->ClOrdID = 1
o->price = (obj->Price);  // obj->Price = 1
o->quantity = (obj->OrderQty); // obj->OrderQty = 1

to read from it

std::cout<<o->id<<"\n";           //outputs 1

or copying it

state->orders.push_back(o);

causes undefined behavior. Your program could crash, everything could seem correct or your computer could order a pizza.

It's difficult to say what's the best way to solve this problem with just some code snippets. One way is to change std::vector<order *> orders; to std::vector<order> orders;. Another way is to use smart pointers.

Upvotes: 4

eerorika
eerorika

Reputation: 238311

State::order* o;

o->id = (obj->ClOrdID);

The pointer has an indeterminate value. The behaviour of indirecting through this uninitialised pointer is undefined.

You should probably use this instead:

std::vector<order> orders;

So that the vector contains order instances.

Upvotes: 2

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