Get class pointer from a vector via function argument

Question

I have a class

class MyClass {
   public:
     string name;
};

I have a class manager, with a vector of said classes.

class MyManager { 
 public:
   vector<MyClass> classes;
};

I want to pull out a class from the vector by looping and matching with the name

int MyManager::FindMyClass(string name, MyClass *clazz){
    vector<MyClass>::iterator it;
    for(it = classes.begin(); it != classes.end(); ++it){
        if(it->name == name){
            // if i cout here, i see the cout, so i did find it..
            *clazz = *it;
            return 1;
        }
    }
    return 0;
}

int main() {
    MyClass *myClass;
    int result = myManager.FindMyClass("bob", *myClass);
}

I know for a fact that the object is matched properly (see the comment). I've tried every combination of pointers, double pointers, references, and get every error from unknown conversions to invalid pointers. I am just not sure what I have to do to get a reference to the class stored in the vector.

`myManager.FindMyClass("bob", *myClass);` is a type error. `*myClass` is a `MyClass`, not a `MyClass *`. If it weren't a type error, it would have undefined behavior: You're trying to read from an uninitialized variable. — melpomene, Oct 26 '17 at 21:15

R Sahu · Accepted Answer · 2017-10-27T03:53:26.627

Change the argument type to MyClass*& so you can return a pointer to the object found in the vector.

int MyManager::FindMyClass(string name, MyClass* &foundPointer)

MyClass *myClass;
int result = myManager.FindMyClass("bob", myClass);

Once you find the match, you can get a pointer to the object using

foundPointer = &(*it);

I would also recommend changing the return type to bool.

Here's an updated version of the function:

bool MyManager::FindMyClass(string name, MyClass* &foundPointer)
{
   vector<MyClass>::iterator it = classes.begin();
   for(; it != classes.end(); ++it)
   {
      if(it->name == name)
      {
         foundPointer = &(*it);
         return true;
      }
   }
   return false;
}

Suggested second improvement:

Change the return type to MyClass*. Return a pointer to the found object or nullptr if none is found.

MyClass* MyManager::FindMyClass(string name)
{
   ...
}

Suggested third improvement:

Use the standard library function std::find_if and a lambda expression to simplify your function.

auto it = std::find_if(classes.begin(),
                       classes.end(),
                       [&](const MyClass &c) { return (c.name == name); });

The updated function:

MyClass* MyManager::FindMyClass(string name)
{
   auto it = std::find_if(classes.begin(),
                          classes.end(),
                          [&](const MyClass &c) { return (c.name == name); });
   if ( it != classes.end() )
   {
      return &(*it);
   }
   return nullptr;
}

Thank you. This works perfectly. Would you be able to explain it a bit more.. I am struggling very hard with pointers and just don't understand what is going on with `MyClass*&` as well as `foundPointer = &(*it)` — J Doe., Oct 26 '17 at 21:19
@JDoe., those are fundamental aspects of the language. You will be better off picking a text book and reading through it to understand the basics. Here's a [starting point](http://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list). — R Sahu, Oct 26 '17 at 21:21
Instead of using an output parameter, I would `return &(*it);` the matching object, and `return nullptr;` if no match is found: `MyClass *myClass = myManager.FindMyClass("bob");` And consider using `std::find_if()` instead of a manual loop: `auto it = std::find_if(classes.begin(), classes.end(), [&](const MyClass &c) { return (c.name == name); });` — Remy Lebeau, Oct 26 '17 at 22:39
Thanks but I should have specified im stuck pre-c++ 11, i think c++ 98 — J Doe., Oct 27 '17 at 13:46

Get class pointer from a vector via function argument

1 Answers1