Function calls
After a function is declared and defined, it can be called from
anywhere within the program: from within the main
function,
from another function, and even from itself. Calling the function
involves specifying the function name, followed by the function call
operator and any data values the function expects to receive. These
values are the arguments for the parameters defined for the
function. This process is called passing arguments to the function.
You
can pass arguments to the called functions in three ways:
- Pass by value, which copies the value of an argument to the corresponding parameter in the called function;
- Pass by pointer, which passes a pointer argument to the corresponding parameter in the called function;
- Pass by reference (C++ only), which passes the reference of an argument to the corresponding parameter in the called function.
If a class has a destructor or a copy constructor that does more than a bitwise copy, passing a class object by value results in the construction of a temporary object that is actually passed by reference.
The compiler generates an
error when a function argument is a class object and all of the following
conditions are true:
- The class needs a copy constructor.
- The class does not have a user-defined copy constructor.
- A copy constructor cannot be generated for that class.
A function call is always an rvalue.
A function call belongs to one of the following value categories
depending on the result type of the function:
- An lvalue if the result type is an lvalue reference type or an rvalue reference to a function type
- An xvalue if the result type is an rvalue reference to an object type
- A (prvalue) rvalue in other cases