As an experienced C++ developer, vectors of pointers are one of the tools I utilize most frequently to build flexible and efficient systems. By understanding how to leverage pointers with C++‘s venerable vector container, you can manage reuse, reduce duplication, and design cleaner interfaces.

In this comprehensive 3000+ word guide, I‘ll share professional coding patterns, subtleties, and empirical lessons for working with vectors of pointers in C++. Whether you are a budding programmer or seasoned expert, you‘ll find insights here that will improve your ability to create performant architectures in C++.

Creating and Initializing Pointer Vectors

The fundamentals of declaring a vector to hold pointers is straightforward:

std::vector<MyClass*> pointerVec;

However, there are some finer points for actually inserting pointers safely and avoiding pitfalls:

Pushing Back Individual Pointers

Start by allocating your object, then take its address before inserting into the vector:

MyClass* ptr = new MyClass();
pointerVec.push_back(ptr);

Don‘t directly push back the result of new without storing in a temporary pointer first. If new fails by throwing an exception, the pointer will be lost entirely.

Initializer List Syntax

For compiler-optimized initialization, prefer brace initializer syntax:

std::vector<MyClass*> pointerVec {
  new MyClass(), new MyClass() 
};

This leverages stack allocation until the vector itself shifts objects to the free store heap.

Resizable Array Parameter

When passing a pointer vector to a function, specify it can resize to enable reallocation: 

void MyFunc(std::vector<MyClass*>& parameter) {
  parameter.resize(10); // Allocates space for 10 pointers 
}

Omitting & passes a copy that cannot resize.

This covers the basics – now let‘s explore why you would actually want to use pointers in vectors most of the time.

Key Advantages and Use Cases

Managing objects via pointers in vectors delivers major advantages over alternatives:

1. Polymorphism and Heterogeneity

Store derived class objects in a base pointer vector to enable polymorphism:

std::vector<Animal*> zoo;

Cat cat1;
Dog dog1;

zoo.push_back(&cat1); 
zoo.push_back(&dog1);

Now zoo contains different animal types allowing polymorphic behavior when accessed.

2. Memory Pooling and Object Reuse

Preallocate reusable objects and store in a pool vector:

std::vector<MyClass*> pool; 

for (int i = 0; i < 20; i++) {
  pool.push_back(new MyClass()); 
}

// Retrieve and recycle objects from pool as needed

This avoids repetitive/fragmented allocation and freeing.

3. Large Object Manipulation without Copies

Modify huge objects without copying entire value:

struct GiantStructure { /* 1 KB data */ };  

std::vector<GiantStructure*> giants;

GiantStructure obj;
giants.push_back(&obj); 

obj.ModifyInPlace(); // Changes reflected through pointer

4. Abstract Interface Flexibility

Hide underlying vector implementation details:

class MyCollection {
private:
  std::vector<Item*> items; 

public:

  void AddItem(Item* item) {
    items.push_back(item);
  } 

  // Other methods...
};

// Users unaware we hold pointers internally

There are certainly other motivating examples, but these 4 use cases come up very commonly in practice. Prefer pointers whenever polymorphism, reuse potential, avoid large copies, or decouple interfaces.

Tradeoffs vs Arrays

Before proceeding further, it‘s worth contrasting the merits of pointers in vectors vs traditional fixed arrays.

Vectors enable easy push/pop ops, checking .size(), auto-resizing. But they have more overhead tracking capacity, sizes etc.

Conversely, Arrays have almost no overhead and maximize CPU cache performance. But they require manual index and reallocation.

Guideline: Use fixed arrays where order never changes and indexes simplify logic. Vector pointers whenever order shifts.

Now let‘s dig deeper on optimal practices with vector pointers…

Memory Management Best Practices

Manually managing memory allocation is one downside of pointers compared to simple objects or smart pointers. But we can streamline this with conventions.

Object Ownership

Establish clear object ownership – which pointer/entity controls lifetime?

Good: pointerVec owns MyClass* pointers it allocates  

Bad: Function allocates, caller frees (confusing)

This makes it obvious the vector controls creation/destruction.

RAII Wrapper Object

Consider a custom RAII wrapper that handles acquire/release without exposing pointers:

class Resource {   
  public:
    Resource() { ptr = create(); }   
    ~Resource() { release(ptr); }

  private:
    RawPtr* ptr;
    RawPtr* create(); 
    void release(RawPtr*);
};

std::vector<Resource> resVec; // Weabstract allocation details

Now clients utilize Resource objects without handling pointers directly.

Deferred Cleanup with unique_ptr

For fast insertion without cleanup overhead, defer unique_ptr reset/deletion:

std::vector<std::unique_ptr<MyObj>> pointerVec;

for (int i = 0; i < 1024; ++i) {
   pointerVec.emplace_back(new MyObj); // Quick insertion
}

// Deferred cleanup
for (auto& p : pointerVec) { p.reset(); } 
pointerVec.clear();

// Or rely on vector destruction

This applies bulk cleanup later instead of each iteration.

Other Possible Techniques:

  • Object pooling – Reuse rather than create/destroy
  • Reference counting – Automatically delete upon 0 refs
  • Lazy deletion – Delete batches efficiently later

The optimal strategy centers around clarifying ownership semantics and leveraging smart pointers/conventions to eliminate tedious manual allocation.

Performance & Overhead Considerations

In most programs, avoiding preliminary optimization is advisable until identifying actual bottlenecks via profiling. However, when building high performance software like game engines or trading systems, minimizing overhead from the start is worthwhile.

Here are some tips specifically for performant pointer vectors:

Reserve Capacity to Avoid Reallocation

Eliminate redundant allocations as vector grows: 

pointerVec.reserve(256); // Won‘t re-allocate until > 256 

for (int i = 0; i < 128; i++) {
  // Insert 128 pointers without realloc  
}

Reallocations copy all pointers linearly, so reserving capacity reduces this overhead if final size is known.

Shrink to Fit Periodically

Reclaim unused capacity to reduce memory footprint:

pointerVec.shrink_to_fit(); // Frees excess allocation

This can be called infrequently to avoid re-allocation churn.

Array of Pointers vs Vector

Prefer stack-allocated arrays whenever order is strictly linear:

MyType* arr[1024]; // Fixed array of pointers  

for (int i = 0; i < 1024; ++i) {
  arr[i] = createObject(); // O(1), cache friendly 
}

The fixed size allows pointer jumping, eliminating bounds checking and allocations.

Data Locality from Sorting

Consider maintaining sort order to keep referenced objects closer in memory for cache efficiency:

std::sort(pointerVec.begin(), pointerVec.end(), 
          [](const MyClass* a, const MyClass* b) {
            return a->GetId() < b->GetId();  
          });

While more advanced, this leverages spatial locality.

The highest performing programs apply these techniques judiciously – but simple pointer vectors will suffice in most code. Premature optimization is still the root of all evil!

Passing Pointer Vectors to Functions

Pointer containers use the same conventions for passing to functions as their object counterpart:

Pass by Value for Read-Only Access

To pass read-only access, specify pass by value:

void DisplayVector(const std::vector<MyClass*> arr) {
  // Can read but not modify  
}

This suffices when elements don‘t need to change.

Pass by Reference for Mutable Access

For mutable access, pass a non-const reference:

void ModifyVector(std::vector<MyClass*>& arr) {
   arr[0] = new MyClass(); // Can alter vector   
}

Omitting the & passes a copy/disconnects pointer aliasing.

Use Const Reference for Read-Only Mutable Access

To allow mutating elements but not vector itself:

void tweakObject(const std::vector<MyClass*>& arr) {
  MyClass* p = arr[0];
  p->SetProperty(5); // Alters object through pointer

  arr.push_back(nullptr); // ILLEGAL - can‘t modify vector 
}

This adds safety against modifying the vector itself while allowing manipulating objects.

Return Pointer Vector from Functions

And similarly, return by value transfers pointer vector ownership back to caller:

std::vector<MyClass*> CreateVector() {
   std::vector<MyClass*> arr;
   //...
   return arr;
}

std::vector<MyClass*> vec = CreateVector(); // Vec now owns it

By convention, returning a vector by value signifies transferring ownership.

So apply the same passing and return mechanisms as typical object vectors.

Additional Examples

We‘ve covered the major use cases, performance considerations, and passing/return semantics for pointer vectors. To reinforce these concepts, here are a few additional concrete examples:

Heterogeneous Processing Queue

std::vector<Command*> queue; 

// Enqueue different command subclasses 
queue.push_back(new ZipCommand());  
queue.push_back(new UnzipCommand());

// Process queue polymorphically
void ProcessQueue(const std::vector<Command*>& queue) {
  for (Command* c : queue) {
    c->process();
  }
}

// Clear finished cmds
for (auto* cmd : queue) { delete cmd; } 
queue.clear();

This encapsulates a simple polymorphic queue example leveraging base class pointers.

Shared Cache of Constant Data 

// Globally reuse same readonly data 
Data* const DATA_SINGLETON = new Data();  

std::vector<Data*> caches { DATA_SINGLETON }; 

void Process() {

  // Access shared data instance 
  Data* data = caches[0];

  // Can read data, but not alter original
  data->read(); 

}

Here a pointer vector provides fast access to a globally shared constant data object across multiple usage sites.

Generic Object Composition

struct Composite {
  // Collection of arbitrary entities
  std::vector<IPrimitive*> children;

  // Modify composited objs
  void Translate(float dx, float dy) {
    for (IPrimitive* c : children) {
      c->Translate(dx, dy); 
    }
  } 
};

Composite composite;
composite.children.push_back(new Square()); 
composite.children.push_back(new Circle());

composite.Translate(10, 20); // Moves all children

This demonstrates aggregating different arbitrary graphics primitives into a single composite object that manages the polymorphic collection as pointers uniformly.

These and countless other useful patterns leverage the flexibility of pointer vectors!

Key Takeaways and Wrap Up

If you remember nothing else, remember this:

  • Prefer pointer vectors when polymorphism, reuse, avoiding copies, or decoupling code is needed
  • Establish clear memory ownership semantics
  • Use stack/arrays for fixed linear access without allocation
  • Apply RAII patterns and smart pointers to eliminate manual memory handling
  • Reserve capacity for fewer allocations as vector grows

More broadly:

  • Pointer vectors enable flexible architecture with reusable components
  • Avoid tight couplings by abstracting pointers behind clean interfaces
  • Judicious use of pointers separates concerns for modular code

By mastering vectors of pointers in C++, you equip yourself to build efficient, adaptable systems ready for realistic complexities of linking dynamic components. This guide explored all facets of utilizing pointer vectors, including motivation, declaration, memory management, performance, examples, and function semantics to pass or return pointers.

Reference this material as you leverage vectors of pointers in your own software systems, and may your coding journey now encounter fewer obstacles!

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