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Home » Tip Bank » C++
Language: C++
Expertise: Intermediate
Sep 25, 2000

Binding a Reference to an Rvalue

In a previous tip, http://www.devx.com/free/tips/tipview.asp?content_id=1585 , I explained the concept of lvalues and rvalues. Binding a reference to an rvalue is allowed as long as the reference is const. The rationale behind this restriction is straightforward: you can't change an rvalue, and only a const reference ensures that the program doesn't try to modify an rvalue through its reference. In the following example, the function f() takes a const reference to int:

 
void f(const int & i);

int main()
{
 f(2); // OK
}

The program passes the rvalue 2 as an argument to f(). C++ creates a temporary object of type int with the value 2 and binds it to the const reference argument passed to f(). The temporary and its reference exist from the moment f() is called until it returns and are destroyed immediately afterwards. Note that had we declared i without the const qualifier, the function f() might have tried to change the value of its argument, thereby causing undefined behavior. The same rules apply to objects: you can bind a temporary object only to a const reference:

 
struct A{};
void f(const A& a);
int main()
{
 f(A()); // OK, binding a temporary A to a const reference
}

Danny Kalev
 
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