c++ vectors and pointers

As i understand if i dont store pointers, everything in c++ gets copied, which can lead to bad performance (ignore the simplicity of my example). So i thought i store my objects as pointers instead of string object inside my vector, thats better for performance right? (assumining i have very long strings and lots of them).

The problem when i try to iterate over my vector of string pointers is i cant extract the actual value from them

string test = "my-name";
vector<string*> names(20);
names.pu开发者_Python百科sh_back(&test);

vector<string*>::iterator iterator = names.begin();
while (iterator != names.end())
{
    std::cout << (*iterator) << ":" << std::endl;
    // std::cout << *(*iterator); // fails

    iterator++;
}

See the commented line, i have no problem in receiving the string pointer. But when i try to get the string pointers value i get an error (i couldnt find what excatly the error is but the program just fails).

I also tried storing (iterator) in a new string variable and but it didnt help?

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You've created the vector and initialized it to contain 20 items. Those items are being default initialized, which in the case of a pointer is a null pointer. The program is having trouble dereferencing those null pointers.

One piece of advice is to not worry about what's most efficient until you have a demonstrated problem. This code would certainly work much better with a vector<string> versus a vector<string*>.

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No, no, a thousand times no.

Don't prematurely optimize. If the program is fast, there's no need to worry about performance. In this instance, the pointers clearly reduce performance by consuming memory and time, since each object is only the target of a single pointer!

Not to mention that manual pointer programming tends to introduce errors, especially for novices. Sacrificing correctness and stability for performance is a huge step backwards.

The advantage of C++ is that it simplifies the optimization process by providing encapsulated data structures and algorithms. So when you decide to optimize, you can usually do so by swapping in standard parts.

If you want to learn about optimizing data structures, read up on smart pointers.

This is probably the program you want:

vector<string> names(20, "my-name");

for ( vector<string>::iterator iterator = names.begin();
      iterator != names.end();
      ++ iterator )
{
    std::cout << *iterator << '\n';
}

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Your code looks like you're storing a pointer to a stack-based variable into a vector. As soon as the function where your string is declared returns, that string becomes garbage and the pointer is invalid. If you're going to store pointers into a vector, you probably need to allocate your strings dynamically (using new).

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Have a look at your initialization:

string test = "my-name";
vector<string*> names(20);
names.push_back(&test);

You first create a std::vector with 20 elements.

Then you use push_back to append a 21st element, which points to a valid string. That's fine, but this element is never reached in the loop: the first iteration crashes already since the first 20 pointers stored in the vector don't point to valid strings.

Dereferencing an invalid pointer causes a crash. If you make sure that you have a valid pointers in your vector, **iterator is just fine to access an element.

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Try

if (*iterator)
{
  std::cout << *(*iterator) << ":" << std::endl;
}

Mark Ransom explains why some of the pointers are now

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string test = "my-name";
vector<string*> names(20);

The size of vector is 20, meaning it can hold 20 string pointers.

names.push_back(&test);

With the push_back operation, you are leaving out the first 20 elements and adding a new element to the vector which holds the address of test. First 20 elements of vector are uninitialized and might be pointing to garbage. And the while loop runs till the end of vector whose size is 21 and dereferencing uninitialized pointers is what causing the problem. Since the size of vector can be dynamically increased with a push_back operation, there is no need to explicitly mention the size.

vector<string*> names; // Not explicitly mentioning the size and the rest of
                       // the program should work as expected.