Use of peek() Method

The peek() method in input streams allows you to look at the next character in the stream without extracting it. This can be useful in several scenarios.

Checking for End of File

One common use of peek() is to check if you've reached the end of the file (EOF). This is especially useful when you need to verify if more input is available before attempting to read:

1#include <iostream>
2#include <sstream>
3
4int main() {
5  std::istringstream input{"Hello\nWorld\n"};
6  std::string extract;
7
8  while (std::getline(input, extract)) {
9    std::cout << "Extracted: " << extract << '\n';
10    if (input.peek() == EOF) {  
11      std::cout << "Reached EOF\n";
12    }
13  }
14}

1Extracted: Hello
2Extracted: World
3Reached EOF

In this example, peek() helps to check if the end of the stream is reached after reading lines.

Lookahead Character

peek() is also useful when you need to decide on an action based on the next character in the stream. For example, you might want to process input differently if the next character is a specific delimiter or a certain value:

1#include <iostream>
2#include <sstream>
3
4int main() {
5  std::istringstream input{"1234A5678"};
6  char nextChar;
7
8  while (input.get(nextChar)) {
9    if (std::isdigit(input.peek())) {  
10      std::cout << "Next is a digit\n";
11    } else {
12      std::cout << "Next is not a digit\n";
13    }
14  }
15}

1Next is a digit
2Next is a digit
3Next is a digit
4Next is not a digit
5Next is a digit
6Next is a digit
7Next is a digit
8Next is a digit
9Next is not a digit

Here, peek() checks the next character without consuming it, allowing the program to branch logic based on the lookahead character.

Repeated Checks

Using peek(), you can repeatedly check the same character without moving the input position. This is useful in parsers or situations where multiple operations might be needed on the same upcoming character:

1#include <iostream>
2#include <sstream>
3
4int main() {
5  std::istringstream input{"Hello World"};
6  char ch;
7
8  while (input.get(ch)) {
9    std::cout << "Char: " << ch << '\n';
10    std::cout << "Peek: " << static_cast<char>(input.peek()) << '\n'; 
11  }
12}

1Char: H
2Peek: e
3Char: e
4Peek: y
5Char: y
6Peek:

This program shows how peek() can be used to preview the next character without extracting it.

Using peek() effectively can help manage input streams more precisely, ensuring your program handles input correctly without prematurely consuming characters.