The extension of function parameters in the 7th lesson

1. default values for function parameters

(1) C + + can provide a default value for a parameter when a function is declared (note that the declaration is not available in the definition )

(2) When a function call does not provide a value for the parameter, the default value is used

Preliminary study on default parameter value of "instance analysis"

#include <stdio.h>//The default value can only be provided when a function declarationintMulintx =0);//the default value for parameter x is 0intMain () {printf ("%d\n", Mul ());//incoming default value 0printf"%d\n", Mul (-1));//Incoming-1printf"%d\n", Mul (2));//Incoming 2    return 0; }intMulintX//definition, cannot provide default value, compiler will error{    returnX *x;}

(3) Rules for default values of function parameters

① declaration , the default value must be supplied from right to left

When the ② function is called , if a default value is used, subsequent arguments must use the default value .

Error example of the "programming experiment" default parameter

#include <stdio.h>//default parameters must be supplied from right to left, such as//int Add (int x = 0,int y = 1,int z) is wrongintAddintXinty =1,intz =2);intMain () {//The 2nd parameter y uses the default value, then the subsequent z must also use the default value//such as Add (1,, 3), the call is wrong. printf"%d\n", Add (0));//x = 0, y = 1, z = 2printf ("%d\n", Add (2,3));//x = 2, y = 3, z = 2printf"%d\n", Add (3,2,1));//x = 3, y = 2, z = 1    return 0; }intAddintXintYintz) {    returnX + y +Z;}

2. Function-placeholder parameters in C + +

(1) The placeholder parameter has only the parameter type declaration, and no parameter name declaration, such as: int func (int x,int)

(2) Under normal circumstances, the placeholder parameter cannot be used inside the function body

(3) Significance of placeholder parameters

The ① placeholder parameter is used in conjunction with the default parameter .

② compatible C language programs may appear in the non-standard wording

"Programming Experiment" placeholder parameters and default parameter values

#include <stdio.h>//The int func () in C can accept arbitrary arguments, so in subsequent calls it may//There are different invocations such as func (1), Func (2, 3), and such code is in C + +//wrong, so in order to be compatible with the C language of this nonstandard notation, you can provide Func with two//The two preceding calls are legal if the take-up parameter such as func (int = 0,int = 0)//at a very low cost, you can make C code available in C + +. Makes you feel like a fake.//Buddha C + + can also be like C language, accept any parameter! //placeholder parameter, and the default value is 0intFuncintx =0,int=0);intMain () {printf ("%d\n", func ());//0printf"%d\n", Func (1));//1printf"%d\n", Func (2,3));//2    return 0; }//The 2nd parameter is a placeholder parameter (no function name), so it cannot be used inside a function//This parameter only plays a role in the placeholderintFuncintXint){    returnx;}

3. Summary

(1) Default value for function parameters supported in C + +

(2) If a function call does not provide a parameter value, the default value is used

(3) The default value of the parameter must be supplied from right to left

(4) If a default parameter is used when the function is called, the following parameter must use the default value

(5) Support for placeholder parameters in C + + for compatibility with non-canonical wording

The extension of function parameters in the 7th lesson