peek 成员函数与 get 类似,但有一个重要的区别,当 get 函数被调用时,它将返回输入流中可用的下一个字符,并从流中移除该字符;但是,peek 函数返回下一个可用字符的副本,而不从流中移除它。
因此,get() 是从文件中读取一个字符,但 peek() 只是"看"了下一个字符而没有真正读取它。为了更好地理解这种差异,假设新打开的文件包含字符串 “abc”,则以下语句序列将在屏幕上打印两个字符 “ab”:
char ch = inFile.get () ; // 读取一个字符
cout << ch; //输出字符
ch = inFile.get () ; // 读取另一个字符
cout << ch; //输出字符
但是,以下语句则将在屏幕上打印两个字符 “aa”:
char ch = inFile.peek () ; //返回下一个字符但是不读取它
cout << ch; //输出字符
ch = inFile.get () ; //现在读取下一个字符
cout << ch; //输出字符
当需要在实际阅读之前知道要读取的数据类型时,peek 函数非常有用,因为这样就可以决定使用最佳的输入方法。如果数据是数字的,最好用流提取操作符 >> 读取,但如果数据是非数字字符序列,则应该用 get 或 getline 读取。
// This program demonstrates the peek member function.、
#include <iostream>
#include <string>
#include <fstream>
using namespace std;
int main()
{
// Variables needed to read characters and numbers
char ch;
int number;
// Variables for file handling
string fileName;
fstream inFile, outFile;
// Open the file to be modified
cout << "Enter a file name: ";
cin >> fileName;
inFile.open(fileName.c_str(), ios::in);
if (!inFile)
{
cout << "Cannot open file " << fileName;
return 0;
}
// Open the file to receive the modified copy
outFile.open("modified.txt", ios::out);
if (!outFile)
{
cout << "Cannot open the outpur file.";
return 0;
}
// Copy the input file one character at a time except numbers in the input file must have 1 added to them
// Peek at the first character
ch = inFile.peek();
while (ch != EOF)
{
//Examine current character
if (isdigit(ch))
{
// numbers should be read with >>
inFile >> number;
outFile << number + 1;
}
else
{
// just a simple character, read it and copy it
ch = inFile.get();
outFile << ch;
}
// Peek at the next character from input file
ch = inFile.peek();
}
// Close the files
inFile.close();
outFile.close ();
return 0;
}
程序测试文件内容:
Amy is 23 years old. Robert is 50 years old. The difference between their ages is 27 years. Amy was born in 1986.
程序输出结果:
Amy is 24 years old. Robert is 51 years old. The difference between their ages is 28 years. Amy was born in 1987.