12.4 — Lvalue references to const

In the previous lesson (12.3 -- Lvalue references), we discussed how an lvalue reference can only bind to a modifiable lvalue. This means the following is illegal:

int main()
{
    const int x { 5 }; // x is a non-modifiable (const) lvalue
    int& ref { x }; // error: ref can not bind to non-modifiable lvalue

    return 0;
}

This is disallowed because it would allow us to modify a const variable (x) through the non-const reference (ref).

But what if we want to have a const variable we want to create a reference to? A normal lvalue reference (to a non-const value) won’t do.

Lvalue reference to const

By using the const keyword when declaring an lvalue reference, we tell an lvalue reference to treat the object it is referencing as const. Such a reference is called an lvalue reference to a const value (sometimes called a reference to const or a const reference).

Lvalue references to const can bind to non-modifiable lvalues:

int main()
{
    const int x { 5 };    // x is a non-modifiable lvalue
    const int& ref { x }; // okay: ref is a an lvalue reference to a const value

    return 0;
}

Because lvalue references to const treat the object they are referencing as const, they can be used to access but not modify the value being referenced:

#include <iostream>

int main()
{
    const int x { 5 };    // x is a non-modifiable lvalue
    const int& ref { x }; // okay: ref is a an lvalue reference to a const value

    std::cout << ref << '\n'; // okay: we can access the const object
    ref = 6;                  // error: we can not modify an object through a const reference
    
    return 0;
}

Initializing an lvalue reference to const with a modifiable lvalue

Lvalue references to const can also bind to modifiable lvalues. In such a case, the object being referenced is treated as const when accessed through the reference (even though the underlying object is non-const):

#include <iostream>

int main()
{
    int x { 5 };          // x is a modifiable lvalue
    const int& ref { x }; // okay: we can bind a const reference to a modifiable lvalue

    std::cout << ref << '\n'; // okay: we can access the object through our const reference
    ref = 7;                  // error: we can not modify an object through a const reference

    x = 6;                // okay: x is a modifiable lvalue, we can still modify it through the original identifier

    return 0;
}

In the above program, we bind const reference ref to modifiable lvalue x. We can then use ref to access x, but because ref is const, we can not modify the value of x through ref. However, we still can modify the value of x directly (using the identifier x).

Initializing an lvalue reference to const with an rvalue

Perhaps surprisingly, lvalues references to const can also bind to rvalues:

#include <iostream>

int main()
{
    const int& ref { 5 }; // okay: 5 is an rvalue

    std::cout << ref << '\n'; // prints 5

    return 0;
}

When this happens, a temporary object is created and initialized with the rvalue, and the reference to const is bound to that temporary object.

A temporary object (also sometimes called an anonymous object) is an object that is created for temporary use (and then destroyed) within a single expression. Temporary objects have no scope at all (this makes sense, since scope is a property of an identifier, and temporary objects have no identifier). This means a temporary object can only be used directly at the point where it is created, since there is no way to refer to it beyond that point.

Const references bound to temporary objects extend the lifetime of the temporary object

Temporary objects are normally destroyed at the end of the expression in which they are created.

However, consider what would happen in the above example if the temporary object created to hold rvalue 5 was destroyed at the end of the expression that initializes ref. Reference ref would be left dangling (referencing an object that had been destroyed), and we’d get undefined behavior when we tried to access ref.

To avoid dangling references in such cases, C++ has a special rule: When a const lvalue reference is directly bound to a temporary object, the lifetime of the temporary object is extended to match the lifetime of the reference.

#include <iostream>

int main()
{
    const int& ref { 5 }; // The temporary object holding value 5 has its lifetime extended to match ref

    std::cout << ref << '\n'; // Therefore, we can safely use it here

    return 0;
} // Both ref and the temporary object die here

In the above example, when ref is initialized with rvalue 5, a temporary object is created and ref is bound to that temporary object. The lifetime of the temporary object matches the lifetime of ref. Thus, we can safely print the value of ref in the next statement. Then both ref and the temporary object go out of scope and are destroyed at the end of the block.

So why does C++ allow a const reference to bind to an rvalue anyway? We’ll answer that question in the next lesson!

Constexpr lvalue references Optional

When applied to a reference, constexpr allows the reference to be used in a constant expression. Constexpr references have a particular limitation: they can only be bound to objects with static duration (either globals or static locals). This is because the compiler knows where static objects will be instantiated in memory, so it can treat that address as a compile-time constant.

A constexpr reference cannot bind to a (non-static) local variable. This is because the address of local variables is not known until the function they are defined within is actually called.

int g_x { 5 };

int main()
{
    [[maybe_unused]] constexpr int& ref1 { g_x }; // ok, can bind to global

    static int s_x { 6 };
    [[maybe_unused]] constexpr int& ref2 { s_x }; // ok, can bind to static local

    int x { 6 };
    [[maybe_unused]] constexpr int& ref3 { x }; // compile error: can't bind to non-static object

    return 0;
}

When defining a constexpr reference to a const variable, we need to apply both constexpr (which applies to the reference) and const (which applies to the type being referenced).

int main()
{
    static const int s_x { 6 }; // a const int
    [[maybe_unused]] constexpr const int& ref2 { s_x }; // needs both constexpr and const

    return 0;
}

Given these limitations, constexpr references typically don’t see much use.