How do I build a vector<> of search results from a source vector<>?

Question

Considering this example:

std::vector<Student> students;
//poplate students from a data source
std::vector<Student> searched(students.size());
auto s = std::copy_if(students.begin(), students.end(), searched.begin(),
    [](const Student &stud) {
        return stud.getFirstName().find("an") != std::string::npos;
    });
searched.resize(std::distance(searched.begin(), s));

I have the following questions:

1. Is it ok to allocate memory for searched vector equals to the initial vector? There may be 500 not small objects and maybe none satisfying the search criteria? Is there any other way?
2. When copying to the searched vector it is called the copy assignment operator and ..obviously a copy is made. What if from those 500 objects 400 satisfying the search criteria? Isn't just memory wasting?

I am a c++ noob so I may say something stupid. I don't see why to ever use vector<T> where T is a object. I would always use vector<shared_ptr<T>>. If T is a primitive type like an int i guess it's kinda straight forward to use vector<T>.

I considered this example because I think it's very general, you always have to pull some data out of a database or xml file or any other source. Would you ever have vector<T> in your data access layer or vector<shared_ptr<T>>?

Answer 1

Concerning your first question:

1 - Is it ok to allocate memory for searched vector equals to the initial vector? There may be 500 not small objects and maybe none satisfying the search criteria? Is there any other way?

You could use a back inserter iterator, using the std::back_inserter() standard function to create one for the searched vector:

#include <vector>
#include <string>
#include <algorithm>
#include <iterator> // This is the header to include for std::back_inserter()

// Just a dummy definition of your Student class,
// to make this example compile...
struct Student
{
    std::string getFirstName() const { return "hello"; }
};

int main()
{
    std::vector<Student> students;

    std::vector<Student> searched;
    //                   ^^^^^^^^^
    //                   Watch out: no parentheses here, or you will be
    //                   declaring a function accepting no arguments and
    //                   returning a std::vector<Student>

    auto s = std::copy_if(
        students.begin(),
        students.end(),
        std::back_inserter(searched),
    //  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    //  Returns an insert iterator
        [] (const Student &stud) 
        { 
            return stud.getFirstName().find("an") != std::string::npos; 
        });
}

Concering your second question:

2 - When copying to the searched vector it is called the copy assignment operator and ..obviously a copy is made. What if from those 500 objects 400 satisfying the search criteria? Isn't just memory wasting?

Well, if you have no statistical information on the selectivity of your predicate, then there is not much you can do about it. Of course, if your purpose is to process somehow all those students for which a certain predicate is true, than you should use std::for_each() on the source vector rather than create a separate vector:

std::for_each(students.begin(), students.end(), [] (const Student &stud) 
{ 
    if (stud.getFirstName().find("an") != std::string::npos)
    {
        // ...
    }
});

However, whether this approach satisfies your requirements depends on your particular application.

I don't see why to ever use vector<T> where T is a object. I would always use vector<shared_ptr<T>>.

Whether or not to use (smart) pointers rather than values depends on whether or not you need reference semantics (apart from possible performance considerations about copying and moving those objects around). From the information you provided, it is not clear whether this is the case, so it may or may not be a good idea.