Why does std::getline() skip input after a formatted extraction?

While using the std::getline() in C++ codes, you have faced a situation where it skips the input after a formatted extraction. This problem occurs in C++ coding because of how the input is processed, which leads to errors in the code. In this article, we will discuss formatting extraction in C++, the problem that the std::getline() skips after a formatted input extraction, methods to prevent this problem, and the best practices.

Table of Contents:

• Understanding Formatted Extraction (>>) in C++
• The Problem of std::getline() Skipping Input After a Formatted Extraction in C++
• Method to Prevent the std::getline() from Skipping Input in C++
  • Use the std::cin.ignore() to Remove Leftover Newline
  • Use std::ws to Consume Whitespace
• Best Practices
• Conclusion

Understanding Formatted Extraction (>>) in C++

The extraction operator in C++ is used to read the formatted input from the std:: cin. It also processes input based on the expected data type and stops reading when it encounters the whitespaces.

Example:

Output:

The code shows how an integer input is read from the user using the std::cin>>num, and then it is printed by using the std::cout. Here, the extraction operator (>>) stops reading when the first whitespace occurs in the input buffer.

The Problem of std::getline() Skipping Input After a Formatted Extraction in C++

The common problem in C++ occurs when the std::getline() is used immediately after a formatted extraction, for example, std::cin >> var. This problem occurs because >> leaves a newline (‘n’) in the input buffer, which is then read by the std::getline() as an empty line.

Example:

Input:

100

Hello! Intellipaat

Expected Output:

Number: 100

Text: “Hello! Intellipaat”

Actual Output:

The code shows how an integer using the std::cin >> number is read at first, and it leaves a newline in the input buffer, and when the std::getline(std::cin, text) is called, then it starts reading the leftover newline and results in an empty string.

Method to Prevent the std::getline() from Skipping Input in C++

Below, there are two methods that you can use to prevent the std::getline() from skipping input in C++:

1. Use the std::cin.ignore() to Remove Leftover Newline

The std::cin.ignore() in C++ helps to remove the characters from the input buffer that are no longer needed and stops std::getline() from reading an empty line in the given input. It is a reliable method for clearing the buffer before reading a full line of text.

Example:

Output:

The code shows how using the std::cin.ignore() helps to clear the newline, which is left in the input buffer after reading an integer, and making sure that the std::getline() reads the full line of text in a correct manner.

2. Use std::ws to Consume Whitespace

The std::ws in C++ is a stream manipulator that removes the occurring whitespaces, such as spaces, tabs, and newlines, before the input is read. It makes sure that the std::getline() does not encounter a leftover newline from a previous std::cin >> var; operation. It is a concise method that works well when only whitespace needs to be skipped, and it does not handle any unexpected extra characters in the buffer.

Example:

Output:

The code shows how the std::ws is used to remove the occurring whitespaces before calling the std::getline() to read the input. It also makes sure that the full text is read correctly after extracting an integer.

Best Practices

• You must always clear the input buffer after the formatted extraction.
• You should use the std::ws for the simple whitespace handling in your C++ code.
• Always avoid mixing >> and the std::getline() in the code when it is possible.
• You must always check the input to prevent unexpected behavior in the code.
• Always try to use separate input streams for different data types in the code.

Conclusion

As we have discussed above, the std::getline() in C++ is used after a formatted extraction, which skips the input when a newline occurs in the input buffer. You can avoid this issue by using the std::cin.ignore() to clear the input buffer and std::ws to consume the occurring whitespaces. So, by understanding how the std::cin process input, why the std::getline() skips the input after a formatted extraction, and methods to fix it, you can easily write efficient and without input skipped C++ code using the std::getline().

Why does std::getline() skip input after a formatted extraction – FAQs