Question

Please take a look at this example:

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

class mySubContainer
{
public:
    string val;
};

class myMainContainer
{
public:
    mySubContainer sub;
};

void doSomethingWith( myMainContainer &container )
{
    container.sub.val = "I was modified";
}

int main( )
{
    vector<myMainContainer> vec;


    /**
     * Add test data
     */
    myMainContainer tempInst;

    tempInst.sub.val = "foo";
    vec.push_back( tempInst );

    tempInst.sub.val = "bar";
    vec.push_back( tempInst );


    // 1000 lines of random code here

    int i;
    int size = vec.size( );
    myMainContainer current;

    for( i = 0; i < size; i ++ )
    {
        cout << i << ": Value before='" << vec.at( i ).sub.val << "'" << endl;

        current = vec.at( i );
        doSomethingWith( current );

        cout << i << ": Value after='" << vec.at( i ).sub.val << "'" << endl;
    }
    system("pause");//i suck

}

A hell lot of code for an example, I know.

Now so you don’t have to spend years thinking about what this [should] do[es]: I have a class myMainContainer which has as its only member an instance of mySubContainer. mySubContainer only has a string val as member.

So I create a vector and fill it with some sample data.

Now, what I want to do is: Iterate through the vector and make a separate function able to modify the current myMainContainer in the vector. However, the vector remains unchanged as the output tells:

0: Value before='foo'
0: Value after='foo'
1: Value before='bar'
1: Value after='bar'

• What am I doing wrong?

doSomethingWith has to return void, I can’t let it return the modified myMainContainer and then just overwrite it in the vector, that’s why I tried to pass it by reference as seen in the doSomethingWith definition above.

Answer 1

You’re making a copy of the vector here:

current = vec.at( i );

and modifying current, but printing the original, vec.at(i).

Instead, modify the object directly, e.g.

doSomethingWith(vec[i]);  // or vec.at(i) for checked access.