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#include <boost/phoenix/core/reference.hpp>
Values are immutable constants. Attempting to modify a value will result
in a compile time error. When we want the function to modify the parameter,
we use a reference instead. For instance, imagine a lazy function add_assign
:
void add_assign(T& x, T y) { x += y; } // pseudo code
Here, we want the first function argument, x, to be mutable. Obviously, we cannot write:
add_assign(1, 2) // error first argument is immutable
In C++, we can pass in a reference to a variable as the first argument in our example above. Yet, by default, the library forces arguments passed to partially applied functions to be immutable values (see Values). To achieve our intent, we use:
expression::reference<T>::type
This is similar to expression::value<T>::type
before but instead holds a reference to a variable.
We normally don't instantiate expression::reference<T>::type
objects directly. Instead we use:
ref(v)
For example (where i
is
an int
variable):
add_assign(ref(i), 2)
References are actors. Hence, references can be evaluated. Such invocation gives the reference's identity. Example:
int i = 3; char const* s = "Hello World"; cout << ref(i)() << ref(s)();
prints out "3 Hello World"
Another free function
cref(cv)
may also be used. cref(cv)
creates an expression::reference<T const>::type
object. This is similar to expression::value<T>::type
but when the data to be passed as argument to a function is heavy and expensive
to copy by value, the cref(cv)
offers a lighter alternative.