Passing Structs to Functions

Passing Structs to Functions

A struct groups several variables into one bigger object. Passing that struct into a function works like passing anything else.

Define and pass by value

Define the struct type, then use it as a parameter type:

struct Person {
  string name;
  int age;
};

void greet(Person p) {
  cout << p.name << " is " << p.age;
}

• Person p is the parameter: a local copy of the struct.
• Inside you use the dot to reach members: p.name, p.age.
• Passing by value copies the entire struct, including all members.

The const reference improvement

Structs can be big, and copying all those fields costs time. For a function that only reads, pass a const reference:

void greet(const Person &p) {
  cout << p.name << " is " << p.age;
}

• const Person & reads "const reference to Person".
• No copy is made (cheap for large structs) and const blocks accidental changes.
• Fast and safe at the same time.

When to choose which

• By value: small structs, or when you want your own copy to modify.
• By const reference: big structs that are read-only - the common default.
• By non-const reference: the function must modify the original struct.

TL;DR

• A struct parameter is declared like any other: void f(Person p).
• Reach members with the dot operator: p.name.
• Pass by value copies the whole struct.
• For read-only access to big structs, pass by const reference.
• Use a non-const reference when the function may modify the original.