c++_note_10_io

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11. io

c++的io包括三部分：键盘、文件、内存，分别简称标准i/o、文件i/o、串i/o.

11.1 标准io

11.1.1 标准输入流

#include<iostream>
using namespace std;

int main(int argc, char *argv[])
{
	int a;
	float b;
	char s[10];
	cin>>a;
	cin>>b;
	cin>>s; //不能接收空格
	
	cout<<a<<endl;
	cout<<b<<endl;
	cout<<s<<endl;
	cin.get();
	return 0;
}

/*
1
1.2
a b
1
1.2
a
*/

程序和键盘之间存在输入缓冲区，和屏幕之间存在输出缓冲区。

输入时，若缓冲区没有数据则会阻塞。

#include<iostream>
using namespace std;

int main(int argc, char *argv[])
{
	char c;
	while( (c = cin.get()) != EOF)
	{
		cout<<c<<endl;
	}
	cin.get();
	return 0;
}

/*
aa	//此时已经输入input buf
a
a


^Z
请按任意键继续. . .
*/

cin.get():

• istream& get (char& c);
• istream& get (char* s, streamsize n);
• istream& get (char* s, streamsize n, char delim);
• istream& get (streambuf& sb);
• istream& get (streambuf& sb, char delim);

#include<iostream>
using namespace std;

int main(int argc, char *argv[])
{
	char a,b,c;
	cin.get(a);
	cin.get(b);
	cin.get(c);
	
	cout<<a<<b<<c<<"there is data in buf.\n";
	
	cin.get(a).get(b).get(c);
	cout<<a<<b<<c<<endl;
	
	cin.get();
	return 0;
}
/*
abcdefg
abcthere is data in buf.
def
*/

输入含空格的字符串用getline()：

• istream& getline (char* s, streamsize n );
• istream& getline (char* s, streamsize n, char delim );

#include<iostream>
using namespace std;

int main(int argc, char *argv[])
{
	char s[10];
	cin.getline(s,10);
	cout<<s<<endl;
	
	cin.get();
	return 0;
}

/*
aaaaa sssss dd
aaaaa sss
*/

下面是3个不常用的函数：

• istream& ignore (streamsize n = 1, int delim = EOF);，ignore until delim
• int peek(); 检测缓冲区是否有数据
• istream& putback (char c); 把c退回缓冲区

#include<iostream>
using namespace std;
int main(int argc, char *argv[])
{
	char buf1[10],buf2[10];
	int ret;
	cin>>buf1;
	cin.ignore(1);
	ret = cin.peek();
	cout<<"ret:"<<ret<<endl;
	
	cin>>buf2;
	cout<<"buf1:"<<buf1<<endl;
	cout<<"buf2:"<<buf2<<endl;
	
	cin.get();
	return 0;
}

/*
aa  bb
ret:32
buf1:aa
buf2:bb
*/

下面是官网给出的putback()示例。

// istream::putback example
#include <iostream>     // std::cin, std::cout
#include <string>       // std::string

int main () {
  std::cout << "Please, enter a number or a word: ";
  char c = std::cin.get();

  if ( (c >= '0') && (c <= '9') )
  {
    int n;
    std::cin.putback (c);
    std::cin >> n;
    std::cout << "You entered a number: " << n << '\n';
  }
  else
  {
    std::string str;
    std::cin.putback (c);
    getline (std::cin,str);
    std::cout << "You entered a word: " << str << '\n';
  }
  return 0;
}

11.1.2 标准输出

标准输出流的定义：extern ostream cout;

常用函数：

• ostream& flush(); 让缓冲区的信息立即强制输出。
• ostream& put (char c);，cout.put('a').put('b');
• ostream& write (const char* s, streamsize n);，输出二进制流。
• streamsize width();
• char fill();
• fmtflags setf();

char *p = "hello";
cout.write(p,5)<<endl;

#include<iostream>
using namespace std;
#include <iomanip>
int main(int argc, char *argv[])
{
   cout<<"<start>"
   	<<setw(30)
   	<<setfill('*')
   	<<setiosflags(ios::showbase)
   	<<hex
   	<<100
   	<<"<end>"
   	<<endl;
   cout<<endl;
   cout.width(5);
   cout.fill('*');
   cout.setf(ios::showbase);
   cout.setf(ios::internal);
   cout<<hex<<100<<endl;
   
   cin.get();
   return 0;
}

/*
<start>**************************0x64<end>

0x*64
*/

11.2 文件io

打开文件：void open (const char* filename, ios_base::openmode mode = ios_base::out);

调用bool is_open() const;判断是否打开成功。

读写函数：

• ostream& write (const char* s, streamsize n);
• istream& read (char* s, streamsize n);

输入

std::ifstream ifs;
ifs.open ("test.txt", std::ifstream::in);

char c = ifs.get();
while (ifs.good()) {
    std::cout << c;
    c = ifs.get();
}

ifs.close();

member constant	stands for	access
in *	input	File open for reading: the internal stream buffer supports input operations.
out	output	File open for writing: the internal stream buffer supports output operations.
binary	binary	Operations are performed in binary mode rather than text.
ate	at end	The output position starts at the end of the file.
app	append	All output operations happen at the end of the file, appending to its existing contents.
trunc	truncate	Any contents that existed in the file before it is open are discarded.

输出

std::ofstream ofs;
ofs.open ("test.txt", std::ofstream::out | std::ofstream::app);
ofs << " more lorem ipsum";
ofs.close();
//or
//std::ofstream fout("test.txt");

注意，out方式，若文件存在，则清除内容。

member constant	stands for	access
in	input	File open for reading: the internal stream buffer supports input operations.
out *	output	File open for writing: the internal stream buffer supports output operations.
binary	binary	Operations are performed in binary mode rather than text.
ate	at end	The output position starts at the end of the file. 文件不存在则报错
app	append	All output operations happen at the end of the file, appending to its existing contents.
trunc	truncate	Any contents that existed in the file before it is open are discarded.