Add constructors.md

Author: sneaky-potato

content/cpp/constructors.md

@@ -1,3 +1,101 @@
 There are several ways of constructing objects in C++, I shall summarize some of them.
 
+## Const and Refs
+Const data types and references cannot be assigned once declared. If the class uses a `const` or a reference they must be initialized using the initializer list.
+```cpp
+class CustomMap {
+private:
+    const int M;
+public:
+    CustomMap(int modulo) : M(modulo) {}
+};
+
+int main() {
+    CustomMap map(5);
+    return 0;
+}
+```
+
+## Brace initialization
+This is different from constructor initializer list. Use `std::initializer_list<T>` to assign arrays.
+
+```cpp
+class Graph {
+    vector<array<int, 2>> edges;
+
+public:
+    // Constructor that accepts { ... } initializer
+    Graph(initializer_list<array<int, 2>> edgeList) : edges(edgeList) {
+        cout << "Initializer-list constructor called" << endl;
+    }
+
+    void showEdges() const {
+        for (int x : edges) cout << x << "";
+        cout << endl;
+    }
+};
+
+int main() {
+    Graph g = {{1, 2}, {2, 3}, {3, 1}};  // uses initializer_list ctor
+    return 0;
+}
+
+```
+
+All the other ways are summarized in the below snippet
+```cpp
+class Order {
+private:
+    int qty;
+    int prc;
+    char side;      // buy or sell
+    char symbol[6]; // ticker symbol name
+
+public:
+    // Paramterized constructor
+    Order(int q, int p, char s, const char* sym) 
+        : qty(q), prc(p), side(s) {
+        strncpy(symbol, sym, 5);
+        symbol[5] = '\0'; // ensure null-terminated
+        cout << "Parameterized constructor called" << endl;
+    }
+
+    // Copy constructor
+    Order(const Order& order) 
+        : qty(order.qty), prc(order.prc), side(order.side) {
+        strcpy(symbol, order.symbol);
+        cout << "Copy constructor called" << endl;
+    }
+
+    // Move constructor
+    Order(Order&& order) noexcept 
+        : qty(order.qty), prc(order.prc), side(order.side) {
+        strcpy(symbol, order.symbol);
+
+        // Reset source
+        order.qty = 0;
+        order.prc = 0;
+        order.side = '\0';
+        order.symbol[0] = '\0';
+
+        cout << "Move constructor called" << endl;
+    }
+
+    void displayTick() {
+        cout << symbol << "\n";
+    }
+};
+
+int main() {
+    Order appleOrder(100, 50, 'B', "AAPL");
+    Order appleOrderCopy = appleOrder; // copy
+
+    Order appleOrderMove = std::move(appleOrder); // move
+    appleOrderMove.displayTick();
+    appleOrder.displayTick(); // after move
+
+    return 0;
+}
+```
+