C++ allows us to call a function without specifying all its arguments. In such type of cases, the function allots a default value to the parameter which does not have a corresponding argument in the function call. Default values are specifies when the function is declared. The compiler looks at the proto type to see how many arguments a function uses and alerts the programme for possible default values. Here is an example of a proto type (function declarations) with default values:
Float amount (float principal, Int period, float rate = 0.15);
The default value is specified in a manner syntactically similar to a variable initialization. The above proto type declares a default value of 0.15 to the argument rate. A subsequent function call like
Value = amount (5000, 7); // one argument missing
Passes the value of 5000 to principal and 7 to period and then less the function use default value of 0.15 for rate.
The call value = amount (5000, 5, 0.12); // no missing argument
Passes an explicit value of 0.15 to rate.
A default argument is checked for type at the time at the declaration and evaluated at the time of the call. One significant point to note down is that only the trailing arguments can own default values and thus we should add defaults from right to left. We cannot give a default value to an argument in the mid of an argument list. Some examples of function declarations with default values are:
Int mul (i, Int j = 5, Int k = 10); // legal
Int mul (i, Int j = 5, Int k = 10); // legal
Int mul (i, Int = 5, Int j ); // illegal
Int mul (i, Int j = 5, Int k = 10); // illegal
Default arguments are useful in situations where some arguments always have the same value. For example, bank interest may remain similar for all customers for a exact period of deposit. It also gives great flexibility to the programmers. A function can be written with more parameters than are required for its common applications. With the use of default arguments, a programmer can use only those arguments which are meaningful to an exact situation.