C++ vector only save the last push_back value

Question

I am a newbie, I have a problem which trouble me a lot. Please help me out.

I am trying to save char array values in a vector. The size of the vector is the same as the number I push_back. However, all the values in the vector is the last value I push_back.

Here are the code:

vector<char*> qField;

int index = 0;

for (int i =1;i <=q_size-seedLength;i++)
{
    index++;
    for (int j=0;j<seedLength-1;j++)
    {
        seed[j] = seed[j+1];
    }
    seed[seedLength-1] = *q_ptr;
    ++q_ptr;                
    seed[seedLength] = '\0';    
    qField.push_back(seed);
    cout << "Insert Seed"<<index<<"\t\t: " <<qField[i]<<"\n";       

} 

/* 
for (int x=0; x<qField.size();x++)
    {
        cout << " Result Query"<<x<<": " << qField[x]<<"\n";
    }

*/  

int x = 0;
for(nuc_vector::iterator iter=qField.begin();iter!=qField.end();++iter)
    {
        cout << x <<"\n";
        x++;
        cout << "Query " << *iter<<"\n";

    }

The strange thing is that the value seems to be stored in the vector right after the push_back. However, when I use another loop to output all the values in the vector qField, all the values are same as the last value pushed back.

Here are part of my results:

    Insert Seed7313     : 00133310
    Insert Seed7314     : 01333100
    Insert Seed7315     : 13331001
    Insert Seed7316     : 33310013
    Insert Seed7317     : 33100130
    Insert Seed7318     : 31001303
    Insert Seed7319     : 10013033
    0
    Query 10013033
    1
    Query 10013033
    2
    Query 10013033
    3
    Query 10013033
    4
    Query 10013033
    5
    Query 10013033
    6
    Query 10013033

Why is that ? Is there any thing wrong.

`seed` is a `char*` that you keep modifying over and over. All of the vector elements are pointing to the same memory. — Cory Kramer, May 21 '15 at 14:36
You add `seed` but print `qField[i]` so your test output doesn't prove much. — Benjamin Bannier, May 21 '15 at 14:36
@BenjaminBannier: It proves that the pointee of each element of `qField` has an equivalent value which, from the prior output, is clearly not the intention. — Lightness Races in Orbit, May 21 '15 at 14:48

Lightness Races in Orbit · Accepted Answer · 2015-05-21T14:52:19.887

I invite you over to my house, giving you my address which you hastily note down on paper.
You come over and see that my house has green walls.
The next day, you copy the address into your phone's address book for safekeeping.
A few months later, I repaint the walls blue.
A few years after that, you want to come over again so you take my address from your phone's address book and show up at my doorstep.
Do you express surprise that my walls are now blue? They weren't when you first wrote down my address! How can they be now? How did copying my address into your phone magically change the colour of my walls?

It didn't.

A char* is not a string, or a container, or anything like that. It's not the "value" that you think it is. It is a pointer. It is an object that points to something else: in this case, apparently, a buffer of chars.

Your vector qField contains copies of the same pointer (seed), over and over again. So yes of course when you dereference each copy, the result will be the same! Even though you modified the thing that the pointer points to in the meantime.

Copying a pointer does not magically copy the thing it points to then update the pointer's value to now point to the new copy of the pointee.

You want an object that semantically contains the data you're trying to store. You want it to have value semantics.

You want to store std::strings instead.

std::vector<std::string> qField;

Hah I was literally sat here waiting for someone to say that \o/ — Lightness Races in Orbit, May 21 '15 at 14:53
=) Well, if you having blue walls can help someone learn about pointers then I think it might be worth it! — sunefred, May 21 '15 at 14:55

score 1 · Answer 2 · answered May 21 '15 at 15:08

1

Try by wrapping seed inside a std::string. Just few lines of code should be changed:

#include<string>

vector<std::string> qField;
...
...
qField.push_back(std::string(seed));

answered May 21 '15 at 15:08

l__LG__l

101
1
4

Thank you very much. This time the result is correct as expected. :) – xxhaxin May 21 '15 at 19:51

C++ vector only save the last push_back value

2 Answers2