References: Second Names for Objects
intermediate20 min readLesson 67 of 204
Bind once, borrow forever — when to pass by value, T&, and const& — and the rules that keep references safe.
A reference is a second name for an existing object. Once bound, a reference can never refer to anything else — assignment goes through it to the object. References must be initialized and can never be null.
void double_it(int& out) { out *= 2; } // non-const: writes through
void report(const std::string& name) { // const&: read-only borrow
std::cout << name << '\n';
}
int main_cj_reference() {
int x = 5;
int& alias = x; // binding, not copying
alias = 9; // x is now 9
double_it(x); // x is now 18
return 0;
}
Choosing between by-value, by-reference, and const&
| You want... | Pass by | Why |
|---|---|---|
| a private copy the callee may change | value | cheap for small types |
| to modify the caller's object | T& | the callee writes through |
| read-only access, no copy | const T& | zero copy, cannot write |
| an optional "no object" state | pointer (later) or std::optional | references cannot be null |
Rules that keep references safe
- Never return a reference to a local — the object dies before the caller reads it.
- Range-based loops over containers should take
const auto&(read) orauto&(modify). - A reference member makes a type non-assignable; usually prefer a pointer or value member.
std::vector<int> data{3, 1, 2};
for (const auto& value : data) std::cout << value; // read: no copy
for (auto& value : data) value *= 10; // write in place