Dynamic array of structs

Question

I have such piece of code:

typedef struct reader
{
    char name[50];
    char card_num[50];
    char title[100];
}reader_t;
int main()
{
    vector<reader> vec;
    ifstream input_file("D:\\lab.txt", ios::binary);
    reader_t master[1];
    input_file.read((char*)&master, sizeof(master));

    for (size_t idx = 0; idx < 1; idx++)
    {
        reader temp;
        strcpy(temp.name, master[idx].name);
        strcpy(temp.card_num, master[idx].card_num);
        strcpy(temp.title, master[idx].title);
        vec.push_back(temp);

        cout << "Name: " << master[idx].name << endl;
        cout << "Card num: " << master[idx].card_num << endl;
        cout << "Title: " << master[idx].title<<endl;

    }
    cout << vec.size();

    getchar();
}

What is does: it reads structures from binary file into an array of structures,copies them into vector and displays structure.And yes, I do need to do like this - I need to store structures from file in vector and this is the only working way to do it I could find(if you can tell, how to read structures to vector directly from file - you are welcome).

So,everything works fine, but the problem is that I need to create a function which would be able to do the same, but with dynamic array.I wrote something like this:

void read_structs(int vec_size)
{
     ifstream input_file("D:\\lab.txt", ios::binary);

     //Here I commented 2 ways how I tried to create a dynamic array of structs
     //reader* master = new reader[vec_size];
     //reader* master = (reader*)malloc(sizeof(reader) * vec_size);

    input_file.read((char*)&master, sizeof(master));
    for (size_t idx = 0; idx < vec_size; idx++)
    {

        reader temp;
        strcpy(temp.name, master[idx].name);
        strcpy(temp.card_num, master[idx].card_num);
        strcpy(temp.title, master[idx].title);
        vec.push_back(temp);

        cout << "Name: " << master[idx].name << endl;
        cout << "Card num: " << master[idx].card_num << endl;
        cout << "Title: " << master[idx].title<<endl;

    }
 }

And that worked fine too unless I tried to run it.VS wasn't higlighting error in my code, it just was throwing an exception right as the moment when the program tried to access master[0].name.

No, what you've written in the post is that you don't understand how to use std::vector for a certain purpose. That's the easiest limitation to overcome, read the documentation. — StoryTeller - Unslander Monica
– StoryTeller - Unslander Monica, Commented Apr 24, 2017 at 10:59
unrelated, struct reader {...}; is enough in C++, typedef version is C legacy. You even mess with reader and reader_t names in your example — Andriy Tylychko
– Andriy Tylychko, Commented Apr 24, 2017 at 11:13

Jan Hudec · Accepted Answer · 2017-04-24 13:41:53Z

4

There is absolutely no point in the temp struct. See, the
```
vec.push_back(temp);
```
is already using copy constructor, so copy constructor must work and then the set of strcpy is not doing anything different from that, so just go with
```
vec.push_back(master[0]).
```
You can't read into vector directly. You do need to read into temporary. So that is correct. Except I suppose you want to read all entries from the file no matter how many of them there are, so you need to put the read itself also into the loop.

There is not much point in creating an array of one element.

reader_t master[1];
input_file.read((char*)master, sizeof(master));
//                     ^ you *don't* need & here, arrays degrade to pointers automatically

and

reader_t master;
input_file.read((char *)&master, sizeof(master));
//                      ^ but you do need & here.

are equivalent. I would go with the later.

So we are basically down to:

reader temp; // calling it temp; the master name makes no sense.
while (input_file.read((char*)&temp, sizeof(temp)))
// read returns input_file and input_file is false if last operation failed
{
    vec.push_back(temp);

    // verify the stored values by reading back vfrom vec.back().
    cout << "Name: " << vec.back().name << endl;
    cout << "Card num: " << vec.back().card_num << endl;
    cout << "Title: " << vec.back().title<<endl;
}

In the second example, you didn't initialize master, so it obviously crashed.
There is a more C++ approach though. First, you define a read operator for the structure:
```
std::istream &operator>>(std::istream &in, reader &r) {
    return in.read((char *)&r, sizeof(r));
}
```
and then you simply read the vector using the istream_iterator:
```
vec.assign(std::istream_iterator<reader>(input_file),
           std::istream_iterator<reader>());
```
and the standard library will generate the above loop for you.

edited Apr 24, 2017 at 13:41

answered Apr 24, 2017 at 11:05

Jan Hudec

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7 Comments

rinn2883 Over a year ago

yep that is what i said in comment, ty for typing it out in steps. Idk if you initialized master somewhere else. Else you need to print with temp.name etc.. and you dont need master anymore.

rinn2883 Over a year ago

or print with the vector like the edit is also an option.

Jan Hudec Over a year ago

@NightStallion, you'll have to do some debugging. Does the file actually have all the bytes? Remember, it is exactly 50 bytes for name, exactly 50 bytes for card_num and exactly 100 bytes for title. If you meant newline-terminated or something, you'll need to do correct formatted input in the operator>>.

CommanderPrice Over a year ago

@JanHudec yep,exactly 200 bytes

CommanderPrice Over a year ago

@JanHudec well,I tried to do like you wrote in 6th paragraph,but I get this error:C2664: "std::_Vector_iterator<std::_Vector_val<std::_Simple_types<reader>>> std::vector<reader,std::allocator<_Ty>>::insert(std::_Vector_const_iterator<std::_Vector_val<std::_Simple_types<reader>>>,unsigned int,const _Ty &)": can not convert argument 1 from "std::istream_iterator<reader,char,std::char_traits<char>,ptrdiff_t>" into "std::_Vector_const_iterator<std::_Vector_val<std::_Simple_types<reader>>>"

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