All 12 Const Correctness questions
More in Const Correctness
What does const mean in C++? What is a const member function? Why should you pass objects by const reference? What is the difference between const pointer and pointer to const? What is the mutable keyword? What is const_cast and when is it (rarely) appropriate? Why is const correctness important in API design? What is the difference between physical and logical constness? How do you overload a member function on const? How does const propagate with member objects and pointers (const propagation)?
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C++ › Const Correctness

What is a constant reference and why is it useful?

Beginner

Answer

A const reference (const T&) is a reference through which the referent can't be modified. It's useful for read-only aliasing without copying, for function parameters, and because a const reference can bind to a temporary (extending its lifetime to the reference's scope), which a non-const lvalue reference cannot.
const std::string& r = std::string("temp"); // binds temporary
Real-world example A function takes const std::string& so it can be called with both variables and string literals without copying.

Common follow-ups: Can a const reference bind to a temporary? | Why can't a non-const lvalue reference?

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All 12 Const Correctness questions