CODEWITHSUNDEEP
c++vector
Jannat Arora
Jannat Arora

Reputation: 2989

Resizing vectors c++

struct struct1
{
    vector<unsigned> vec1;
};

void main()
{
    vector<struct1> vecStruct1;
    struct1 obj1Struct1;
    struct1 obj2Struct1;
    vecStruct1.push_back(obj1Struct1);
    vecStruct1.push_back(obj2Struct1);
}

Now if I resize vec1 of struct1 of object vecStruct1[0], then is it also the case that vecStruct1 also gets resized i.e. will it cause reallocation in vecStruct1 i.e. will &vecStruct[0] and &vecStruct[1] not be different after resizing vec1 of vecStruct1[0]?

Upvotes: 0

Views: 300

Answers (5)

Fabio says Reinstate Monica
Fabio says Reinstate Monica

Reputation: 5726

Maybe this example will clarify the situation:

#include <iostream>
#include <vector>
using namespace std;

struct struct1
{
    vector<unsigned> vec1;
};

int main() {

    vector<struct1> vecStruct1;
    struct1 obj1Struct1;
    struct1 obj2Struct1;
    vecStruct1.push_back(obj1Struct1);
    vecStruct1.push_back(obj2Struct1);

    cout << "Before resizing the vector:" << endl;
    cout << "Address of vecStruct1[0]:" << &vecStruct1[0] << endl;
    cout << "Address of vecStruct1[1]:" << &vecStruct1[1] << endl;
    cout << "Capacity of the vector in vecStruct1[0]:" << vecStruct1[0].vec1.capacity() << endl;
    cout << "Capacity of the vector in vecStruct1[1]:" << vecStruct1[1].vec1.capacity() << endl;
    cout << endl;


    vecStruct1[0].vec1.resize(1000);

    cout << "After resizing vecStruct1[0]:" << endl;
    cout << "Address of vecStruct1[0]:" << &vecStruct1[0] << endl;
    cout << "Address of vecStruct1[1]:" << &vecStruct1[1] << endl;
    cout << "Capacity of the vector in vecStruct1[0]:" << vecStruct1[0].vec1.capacity() << endl;
    cout << "Capacity of the vector in vecStruct1[1]:" << vecStruct1[1].vec1.capacity() << endl;
    cout << endl;


    vecStruct1[1].vec1.resize(2000);

    cout << "After resizing vecStruct1[1]:" << endl;
    cout << "Address of vecStruct1[0]:" << &vecStruct1[0] << endl;
    cout << "Address of vecStruct1[1]:" << &vecStruct1[1] << endl;
    cout << "Capacity of the vector in vecStruct1[0]:" << vecStruct1[0].vec1.capacity() << endl;
    cout << "Capacity of the vector in vecStruct1[1]:" << vecStruct1[1].vec1.capacity() << endl;
    cout << endl;

    return 0;
}

The output (you can check it on ideone: http://ideone.com/JVaiqg) is:

Before resizing the vector:
Address of vecStruct1[0]:0x8f33018
Address of vecStruct1[1]:0x8f33024
Capacity of the vector in vecStruct1[0]:0
Capacity of the vector in vecStruct1[1]:0

After resizing vecStruct1[0]:
Address of vecStruct1[0]:0x8f33018
Address of vecStruct1[1]:0x8f33024
Capacity of the vector in vecStruct1[0]:1000
Capacity of the vector in vecStruct1[1]:0

After resizing vecStruct1[1]:
Address of vecStruct1[0]:0x8f33018
Address of vecStruct1[1]:0x8f33024
Capacity of the vector in vecStruct1[0]:1000
Capacity of the vector in vecStruct1[1]:2000

And as you can see, the address of vecStruct1[0] and vecStruct1[1] never changes. That's because each element of the vector has a fixed size, and internally it contains a pointer to the area where the resized vectors are stored. When you resize the vectors (the "internal" ones, that is, each vec1 inside each struct1) it's possible that they have to be moved to a new address, but the "external" vector vecStruct1 stays where it is.

Upvotes: 2

Some programmer dude
Some programmer dude

Reputation: 409482

No, the two vectors are totally unrelated. Adding elements to one vector, or resizing it some other way, will not cause any other vector to become resized.

Also, resizing vecStruct1[0].vec1 will not resize vecStruct1[1].vec1.

Clarifications:

If you have a pointer to the first element in vecStruct1 (i.e. &vecStruct1[0]) that pointer will be invalidated if you resize vecStruct1. What you do with vecStruct1[0].vec1 doesn't matter here, and pointer to element in vecStruct1[0].vec1 will not change.

And if you have a pointer &(vecStruct1[0].vec1[0]) (parentheses not needed but added for clarity) then if you resize vecStruct1 that pointer will not change. You need to resize vecStruct1[0].vec1 for that pointer to become invalid.

Upvotes: 1

aslg
aslg

Reputation: 1974

The vector handles its memory outside of the structure, in the heap as mentioned in another answer. So resizing or reallocating the vector inside the structure won't change the structure's size at all, not even the vector containing that structure.

http://www.cplusplus.com/reference/vector/vector/

The vector handles its own memory dynamically ( in the heap ).

Upvotes: 0

Drax
Drax

Reputation: 13318

Nope, vector stores its element on the heap not inside itself in the structure, so the vector per se never grows when you resize it, what grows is the data buffer pointed by vector.

Upvotes: 3

Jarod42
Jarod42

Reputation: 218323

As vecStruct1 contain copy of obj1Struct1, once the object is added, the 2 object are distinct.

so modify obj1Struct1 doesn't modify vecStruct1[0].

Upvotes: 2

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