Constructors and Member Initialization

Constructors put a new object into a valid state. C++ gives you several kinds; you rarely need all of them, but you must recognize each.

#include <string>
#include <vector>

class Inventory {
public:
    Inventory() = default;                          // default ctor
    explicit Inventory(int cap) : cap_{cap} {}      // converting ctor → explicit!
    Inventory(std::string owner, int cap)           // multi-parameter
        : owner_{std::move(owner)}, cap_{cap} {}

private:
    std::string owner_{"warehouse"};
    int cap_{0};
    std::vector<int> items_{};
};

Member initializer lists

Prefer : member_{value} over assigning in the body — members are initialized once, directly, and const/reference members have no other option. The order of the list does not matter; members initialize in declaration order.

explicit, always for one-argument constructors

Without explicit, Inventory inv = 5; compiles — a silent conversion from int. explicit makes that a compile error while Inventory inv{5}; still works. Default rule: single-argument constructors are explicit unless you mean implicit conversion.

When you write none

If you declare no constructors, the compiler generates a default one that value-initializes members with {} defaults. Often that is all you need — struct Point { double x{}; double y{}; }; needs nothing else.