Vector - Expression must have class type

[binary3926]

Hello,

I am receiving an error: "Expression must have class type".

I am supposed to "Create a table which is a vector of 'Essay', which is dynamically created with the 'new' operator".

However, when reading online, I guess it's not good to use the 'new' operator with vector?


HEADER FILE:

#include <vector>
#include <string>

struct Essay {
    std::string word;
    std::string paragraph;

    Entry () { word = "---"; }
};

class AB {
  private:
      std::vector<Essay>* table;
      int table_size;

  public:
      AB(int size);
      ~AB();
};

MAIN FILE:

AB::AB(int s = 10)
{
    //Essay *table = new Essay[s]  //This will work but I 
                                   //believe it is incorrect

    table.reserve(s);   //ERROR: Expression must have class type

    table_size = 0;
}

AB::~AB()
{
 
}

[Peter87]

table is a pointer. To be able to call the reserve function on the vector that the pointer points to you have to first dereference the pointer.

(*table).reserve(s);

Or you can use the shorter "arrow" syntax.

table->reserve(s);

If you don't have a very good reason for using pointers and new you should stay away from them because they complicate the code and it's easy to make mistakes.

[coder777]

I would guess that not the vector should be created with new, but an object of the type AB. Or Essay. Then you would need a vector of pointer.
And yes: creating a vector dynamically is not usefull since itself manages the data dynamically.

By the way: you don't need table_size since vector has a size() member.

[binary3926]

Peter87 -

(*table).reserve(s); or table->reserve(s);
produces a segmentation fault.


coder777 -

Could you explain a little more? Do you mean like...

Essay* table = new Essay;
table.resize(s);                      //This code does not work 

Thanks for your replies

[coder777]

Create a table which is a vector of 'Essay', which is dynamically created with the 'new' operator

The wording is unclear.

'which' refers to 'table' or 'Essay'?


A table usually has 2 dimensions.

'is a' usually means inherits from (vector).
'has a' usually means a member variable.

What are the exact words for your assignment?

[binary3926]

'table' is a hash table. The 'word' string is actually a 'key' which is always in the form of.. "letter-letter-int" The 'table' is a container of type vector<Essay> and is used to store entries.

In the constructor, the hash table 'table' which is a vector of 'Essay' is supposed to be created dynamically (with the new operator) for a given size 's'.

I hope this helps,

Thank you very much!

[binary3926]

So here is another thought I had:

vector<Entry>* table = new vector<Entry>(s);
int a = (*table).size();

cout << a;

The output is '11' which is what it should be. Does this way of creating the hash table work?

Thanks!

[binary3926]

For those that might look at this later, below is the correct implementation:


HEADER FILE:

#include <vector>
#include <string>

struct Essay {
    std::string word;
    std::string paragraph;

    Entry () { word = "---"; }
};

class AB {
  private:
      std::vector<Essay>* table;
      int table_size;

  public:
      AB(int size);
      ~AB();
};

MAIN FILE:

AB::AB(int s = 10)
{
    table = new vector<Entry>(s);

    table_size = s;
}

AB::~AB()
{
     delete table;
}