When to pass by value, reference or pointer in C++?

Case	Use
Small type (int, double, pointer)	T by value
Big object, only read	const T&
Function must change the argument	T&
Argument may be missing	T* (or std::optional)
Function keeps its own copy	T by value, then std::move
Read only text	std::string_view

#include <iostream>
#include <string>
#include <string_view>
#include <vector>

void a(int n) { std::cout << "value " << n << '\n'; }
void b(const std::vector<int>& v) { std::cout << "const ref " << v.size() << '\n'; }
void c(int& n) { n++; }
void d(const int* p) { std::cout << (p ? "pointer set" : "no value") << '\n'; }
void e(std::string_view s) { std::cout << "view " << s << '\n'; }

int main() {
    int n = 1;
    a(n);
    b({1, 2, 3});
    c(n);
    d(&n);
    d(nullptr);
    e("text");
    std::cout << n << '\n';
}

See References and Pointers.

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