const references
- C++ course
- Beginner
- Lesson 74/116
- 5 min
Learn safe and efficient passing with const references.
const references
If you do not want a parameter to be modified, use a const reference: void printValue(const int& n). This means n is read-only and is not copied.
A const reference is both safe and efficient. Safe, because the function cannot accidentally change the caller's variable. Efficient, because large objects are not copied. So when passing types like std::string or std::vector to a function, const T& is preferred.
Why const reference?
Passing a large object to a function by value is expensive, because the whole object is copied. Pass by reference avoids the copy but allows the function to modify the object.
To get both benefits, use a const reference: no copy, no modification. This is the standard way to pass large objects to a function in C++. For example void print(const std::string& s).
Practice The printValue function
Define a void printValue(const int& n) function; in the body print n with std::cout. In main declare int x = 7; and call printValue(x).
Starter code
#include <iostream>
// Define the printValue function
int main() {
// Declare x
// Add a printValue call
return 0;
}Sample solution
#include <iostream>
void printValue(const int& n) {
std::cout << n << "\n";
}
int main() {
int x = 7;
printValue(x);
return 0;
}Expected output
7
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 const reference parameter
void printValue(____ int& n) { std::cout << n; }void printValue(const int& n) { std::cout << n; }Explanation: const int& n is a read-only reference parameter.
2 Which is a const reference declaration?
- const int p;
- int const p = x;
- const int& r = x; Correct answer
- int& const r = x;
Explanation: const int& r = x; declares a read-only reference.
3 Match each expression to its meaning
| Term | Meaning |
|---|---|
const int& n |
Const reference |
printValue(x) |
Call |
void |
Returns no value |
n |
Read-only |