C && C++ (1)

Posted on Edited on In Valine:

describe some differences between c and c++,(Function Union and Quote)

Function

  • Function declaration

    if Function declaration's parameter is empty of void,This two declaration in c++ are same :

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    f();				//This function don't carray any parameters
    f(void);			//This function don't carray any parameters

    but they are different in c:

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    f(void);			//This function don't carray any parameters
    f();				//Don't give the information of parameters ,It have chance to carrary 						//paramters

  • Macro definition

    Macro definition may cause some error that we can't expect ,for example:

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    #include <iostream>
    using namespace std;
    #define doub(x) x*2
    int main()
    {
    	for(int i =1 ;i<4;i++)
    		cout<<i<<" doubled is "<<doub(i)<<endl;
    	cout<<"1+2 doubled is "<<doub(1+2)<<endl;
    	return 0;
    }

    in line 8 ,we are expecting the result is :

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    1+2 doubled is 6

    but in fact ,the result is :

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    1+2 double is 5

    because the macro definition is replace the expression in the place where it is called :

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    cout<<"1+2 doubled is "<<doub(1+2)<<endl;
    cout<<"1+2 doubled is "<<1+2*2<<endl;

  • Inline function

    to avoid the unsafe of Marco definition ,We can use inline function to replace the macro function to achieve more safe :

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    #include <iostream>
    using namespace std;
    inline int doub(int x);
    int main()
    {
    	for(int i =1 ;i<4;i++)
    		cout<<i<<" doubled is "<<doub(i)<<endl;
    	cout<<"1+2 doubled is "<<doub(1+2)<<endl;
    	return 0;
    }
    inline int doub(int x){
        return x*2;
    }

  • function with default value of parameters

    the parameters with default value must be in the right of parameters with no initial value:

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    int fun(int i ,int j=5,int k);     //incorrect 
    int fun(int i,int j,int k=5);      //correct

  • Function Overloading

    The same function name can be used to define different tasks

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    #include <iostream>
    using namespace std;
    int square(int i)
    {
        return i*i;
    }
    long square(long i)
    {
        return i*i;
    }
    double square(double i)
    {
        return i*i;
    }
    int main()
    {
        int i =12;
        long j =1234;
        double k =1.23;
        cout<<"int i'quare is "<<square(i)<<endl;
        cout<<"long j's quare is "<<square(j)<<endl;
        cout<<"double k's quare is "<<square(k)<<endl;
        return 0;
    }

    if only the type of return is not the same ,the numbers of parameter and the type of parameters is same,The function couldn't be overloaded:

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    int nihao(int x,int y);
    void nihao(int x,int y);     //That is incorrect ,because only the type of return is not the same

    if the overloading function with default value parameter, it can be incorrect :

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    void nihao(int a,int b=0);
    void nihao(int a);				//This is also incorrect the compiler can't decide which function should be choosed

Scope operator ::

  • Priority

    Local variables have higher priority than global variables

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    #include<iostream>
    using namespace std;
    int avar =10;			//global variable
    int main()
    {
    	int avar=3;			//local variable
    	cout<<"avar is "<<avar<<endl;
    	return 0;
    }

    if global variables wants to be called in function ,we can add :: in the front of variables

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    cout<<"local variable"<<avar<<endl;
    cout<<"global variable"<<::avar<<endl;

Union

  • What is the Union

    Union is a custom type ,we can create union in two way in c:

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    union data{
        int n;
        char ch;
        double f;
    };
    union data a,b,c;

    //second way
    union data{
        int n;
        char ch;
        double f;
    }a,b,c;

    union's space is the largest memory footprint of all member ,int,char,double,the double's memory footprint is largest ,so the union data's memory footprint is 8 bits(the size of double)

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    #include<stdio.h>

    union data{
    	int n;
    	char ch;
    	short m;
    };

    int main()
    {
    	union data a;
    	printf("%d,%d\n",sizeof(a),sizeof(union data));
    	a.n = 0x40;
    	printf("%X,%c,%hX\n",a.n,a.ch,a.m);
    	a.ch ='9';
    	printf("%X,%c,%hX\n",a.n,a.ch,a.m);
    	a.m = 0x2059;
    	printf("%X,%c,%hX\n",a.n,a.ch,a.m);
    	a.n =0x3e25ad54;
    	printf("%X,%c,%hX\n",a.n,a.ch,a.m);
    	
    	return 0;
    }

    you may get the result as follows:

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    4,4
    40,@,40
    39,9,39
    2059,Y,2059
    3E25AD54,T,AD54

    so the result prove that union members will have Influence each other, if you change one member's value ,the other member's value is changed

  • How to store data

    Union 's data is shared for all member ,so one of they is changed ,others will be effected,but the process is how to go

  • Using union

    student information and teacher information is recorded by the same struct ,but some place are different,for example:

    name Num Sex Profession Score/course
    A 1 f s 89
    B 2 m t math 
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    #include<stdio.h>
    #include<stdlib.h>

    #define NUM 4
    struct{
    	char name[40];
    	int num;
    	char sex;
    	char pro;
    	union d{
    		int score;
    		char course[10];
    	}sc;
    } bodys[NUM];
    int main(){
    	int i;
    	for(i=0;i<NUM;i++)
    	{
    		printf("Please input info:");
    		scanf("%s %d %c %c",&bodys[i].name,&bodys[i].num,&bodys[i].sex,&bodys[i].pro);
    		if (bodys[i].pro == 's'){
    			scanf("%d",&bodys[i].sc.score);
    			printf("\n%s\t%d\t%c\t%c\t%d",
                       bodys[i].name,bodys[i].num,bodys[i].sex,bodys[i].pro,bodys[i].sc.score);
    		}
    		else{
    			scanf("%s",&bodys[i].sc.course);
    			printf("\n%s\t%d\t%c\t%c\t%s",
                       bodys[i].name,bodys[i].num,bodys[i].sex,bodys[i].pro,bodys[i].sc.course);
    		}
    	}
    	return 0;
    }

Quote

  • definition

    give variables a another name after quoted ,the two variables are equivalent when one of them changed ,the other will change

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    int i =5;
    int &j = i;		//delcare j is a integer quote ,and initialize j with i

    but j definited in following way is not allowed:

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    int i;
    int &j;			//incorrect j is not linked some variables
    j = i;

  • quote and pointer

    quote is another name for a variable ,but pointer is a tool pointing to variable

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    #include<iostream>
    using namespace std;
    int main()
    {
    	int i=15;
    	int *iptr=&i;			//define a pointer 
    	int &rptr = i;			//define a quote 
    	cout<<"I is "<<i<<endl;
    	cout<<"*iptr is  "<<*iptr<<endl;
    	cout<<"rptr is "<<rptr<<endl;
    	i=29;
    	cout<<"After changing i to 29:"<<endl;
    	cout<<"i is "<<i<<endl;
    	cout<<"*iptr is  "<<*iptr<<endl;
    	cout<<"rptr is "<<rptr<<endl;
    	return 0;
    }

    quote: No need & ,pointer : Need * .

  • warn

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    int i,k;
    int &j =i;
    j =&k;   		//incorrect , delcare j is the quotation of k is not allowed 
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    void &r = 10; 	//incorrect, void type is not allow 
    int &r = 10 ; 	//incorrect,can't assign quote "r" with value

    //can't quote array
    int a[4] = "amd";
    int &a[4] = a;


    //can't define quotation to quotation
    int n=3;
    int && r =n;
    int &*p = n;

  • used as parameters of function

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    #include<iostream>
    using namespace std;
    void swap(int &a,int &b)
    {
    	int temp =a;
    	a =b;
    	b =temp;
    }
    int main(){
    	int a=5,b=10;
    	cout<<"a="<<a<<" "<<"b="<<b;
    	swap(a,b);
    	cout<<"a="<<a<<" "<<"b="<<b;
    	return 0;
    }

    quotation is used as formal parameter for function variable transfer,but it seem that we have really passed a,b into the function "swap"

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    #include<iostream>
    using namespace std;
    int a[] = {1,3,5,6,7,3};
    int &index(int);
    int main(){
    	index(1)=100;		//in fact ,because the return of funciton is not a number ,but a[i]
    	cout<<"a[1]="<<a[1]<<endl;
    	return 0;
    	
    }
    int &index(int i){
    	return a[i];
    }