C++ Vector of objects

[mathlete]

Hello,

I'm just starting to learn C++, coming from a Java / bit of C background. I have some questions about how vectors of objects and pointers work in C++.

Say I want to create a vector of SomeClass objects, and the SomeClass constructor takes one argument. However, SomeClass is not copyable, so I can't do something like:

vector<SomeClass> someVector;
for (int i=0; i<args.size(); i++){
    someVector.push_back( SomeClass(args[i]) )
}
 

So instead, I used a temporary variable and stored pointers inside the vector instead:

vector<SomeClass *> someVector;
for (int i=0; i<args.size(); i++){
    SomeClass temp(args[i]);
    someVector.push_back(&temp);
}
 

However, all the pointers in someVector ended up pointing to temp(args[args.size()-1]). I've resolved this by using boost::shared_ptr instead, but I wanted to understand why the above code didn't work as expected. I thought that in each iteration of the for loop, a new temp will get created, and a reference to it will be stored in someVector (so that it won't garbage collected or w/e since there's still a reference to it). The same thing also seems to happen when I make a vector of a struct...

From general Googling, I get the feeling that in general, it's probably a good idea to get into the habit of storing pointers instead of actual objects and structs in vectors - so I should probably get used to using vectors of shared_ptr?

Also, any other general comments, tips, common gotchas, etc. will be greatly appreciated (trying to learn all I can!).

Thanks!

[JosAH]

vector<SomeClass *> someVector;
for (int i=0; i<args.size(); i++){
    SomeClass temp(args[i]);
    someVector.push_back(&temp);
}
 

However, all the pointers in someVector ended up pointing to temp(args[args.size()-1]).

Yep, 'temp' is just a local variable and no matter how many times your evaluate
'&temp' it'll always be the address of that same local variable. Do this instead:

vector<SomeClass *> someVector;
for (int i=0; i<args.size(); i++){
    SomeClass* temp= new SomeClass(args[i]);
    someVector.push_back(temp);
}
 

See the difference?

kind regards,

Jos

[Darryl]

Yep, 'temp' is just a local variable and no matter how many times your evaluate
'&temp' it'll always be the address of that same local variable. Do this instead:

vector<SomeClass *> someVector;
for (int i=0; i<args.size(); i++){
    SomeClass* temp= new SomeClass(args[i]);
    someVector.push_back(temp);
}
 

See the difference?

kind regards,

Jos

and since now you are using new, you can drop the temp and go back to your original design someVector.push_back(new SomeClass(args[i]))

[oscarin43]

Hi. Respect to the example. Could you please tell me the right way to delete all the memory allocated from this piece of code.

i believe that i have to do something like this.

vector<SomeClass *> someVector;

for (int i=0; i<args.size(); i++){
SomeClass* temp= new SomeClass(args[i]);
someVector.push_back(temp);

delete temp; // Is necessary??
}

//and next;

int VecSize = someVector.size();
for (int i=0; i<VecSize; i++)
{
delete someVector[i]; //Is correct?
}

thanks for your help

[abcde456]

don't delete temp within assignment for loop

//do all the work you wanted to do with someVector
// then delete like you showed in the last forloop, before you go out of scope