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4.12 — An introduction to std::string

The very first C++ program you wrote probably looked something like this:

So what is “Hello, world!” exactly? “Hello, world!” is a collection of sequential characters called a string. In C++, we use strings to represent text such as names, addresses, words, and sentences. String literals (such as “Hello, world!\n”) are placed between double quotes to identify them as strings.

Because strings are commonly used in programs, most modern programming languages include a fundamental string data type. In C++, strings aren’t a fundamental type (they’re actually a compound type, and defined in the C++ standard library rather than as part of the core language). But strings are straightforward and useful enough that we’ll introduce them here rather than wait until the chapter on compound types (chapter 8).

std::string

To use strings in C++, we first need to #include the <string> header to bring in the declarations for std::string. Once that is done, we can define variables of type std::string.

Just like normal variables, you can initialize or assign values to strings as you would expect:

Note that strings can hold numbers as well:

In string form, numbers are treated as text, not numbers, and thus they can not be manipulated as numbers (e.g. you can’t multiply them). C++ will not automatically convert string numbers to integer or floating point values.

String output

Strings can be output as expected using std::cout:

This prints:

My name is: Alex

Empty strings will print nothing:

Which prints:

[]

String input with std::cin

Using strings with std::cin may yield some surprises! Consider the following example:

Here’s the results from a sample run of this program:

Enter your full name: John Doe
Enter your age: Your name is John and your age is Doe

Hmmm, that isn’t right! What happened? It turns out that when using operator>> to extract a string from cin, operator>> only returns characters up to the first whitespace it encounters. Any other characters are left inside std::cin, waiting for the next extraction.

So when we used operator>> to extract a string into variable name, only "John" was extracted, leaving " Doe" inside std::cin. When we then used operator>> to get variable age, it extracted "Doe" instead of waiting for us to input an age. Then the program ends.

Use std::getline() to input text

To read a full line of input into a string, you’re better off using the std::getline() function instead. std::getline() takes two parameters: the first is std::cin, and the second is your string variable.

Here’s the same program as above using std::getline():

Now our program works as expected:

Enter your full name: John Doe
Enter your age: 23
Your name is John Doe and your age is 23

What the heck is std::ws?

In lesson 4.8 -- Floating point numbers, we discussed output manipulators, which allow us to alter the way output is displayed. In that lesson, we used the output manipulator function std::setprecision() to change the number of digits of precision that std::cout displayed.

C++ also supports input manipulators (defined in the iomanip header), which alter the way that input is accepted. The std::ws input manipulator tells std::cin to ignore any leading whitespace. Note that std::ws is not a function.

Let’s explore why this is useful. Consider the following program:

Here’s some output from this program:

Pick 1 or 2: 2
Now enter your name: Hello, , you picked 2

This program first asks you to enter 1 or 2, and waits for you to do so. All good so far. Then it will ask you to enter your name. However, it won’t actually wait for you to enter your name! Instead, it prints the “Hello” string, and then exits. What happened?

It turns out, when you enter a value using operator>>, std::cin not only captures the value, it also captures the newline character ('\n') that occurs when you hit the enter key. So when we type 2 and then hit enter, std::cin receives gets the string "2\n". It then extracts the 2 to variable choice, leaving the newline character behind for later. Then, when std::getline() goes to read the name, it sees "\n" is already in the stream, and figures we must have previously entered an empty string! Definitely not what was intended.

We can amend the above program to use the std::ws input manipulator, to tell std::getline() to ignore any leading whitespace characters:

Now this program will function as intended.

Pick 1 or 2: 2
Now enter your name: Alex
Hello, Alex, you picked 2

Best practice

If using std::getline to read strings, use the std::ws input manipulator to ignore leading whitespace.

Key insight

Using the extraction operator (>>) with std::cin ignores leading whitespace.
std::getline does not ignore leading whitespace unless you use input manipulator std::ws.

String length

If we want to know how many characters are in a std::string, we can ask the std::string for its length. The syntax for doing this is different than you’ve seen before, but is pretty straightforward:

This prints:

Alex has 4 characters

Note that instead of asking for the string length as length(myName), we say myName.length(). The length() function isn’t a normal standalone function -- it’s a special type of function that belongs to std::string called a member function. We’ll cover member functions, including how to write your own, in more detail later.

Conclusion

std::string is complex, leveraging many language features that we haven’t covered yet. Fortunately, you don’t need to understand these complexities to use std::string for simple tasks, like basic string input and output. We encourage you to start experimenting with strings now, and we’ll cover additional string capabilities later.

Quiz time

Question #1


Write a program that asks the user to enter their full name and their age. As output, tell the user how many years they’ve lived for each letter in their name (for simplicity, count spaces as a letter).

Sample output:

Enter your full name: John Doe
Enter your age: 46
You've lived 5.75 years for each letter in your name.

Show Solution


4.13 -- Literals
Index
4.11 -- Chars

611 comments to 4.12 — An introduction to std::string

  • Mal

    Using std:ws only prevents leading whitespace chars from causing issues.  Using your example (below), if a user enters "2 abc" it will reproduce the original error because 'abc' has been left in the input buffer.  I understand that you could check at the time of input that you are only getting  a '1' or '2' returned. However, when you use cout you can flush the buffer with 'std::endl;' is there a way of flushing the cin buffer before asking for input?
      

  • vilala

    hi ad
    in linux, i dont use std::ws but cout print full string?

    • curiosCoder

      I am also having the same doubt, on my linux machine i am getting the same result with and without using std::ws. I don't know for sure but Linux auto ignores the whitespaces when using getline() function. But it would be helpfull if someone can explain the reason behind it.

  • Dasu Pradyumna

    In the Key Insight, the first line is -
    "Using the extraction operator (>>) with std::cin ignores leading whitespace."
    But shouldn't it be "does not ignore leading whitespace"? We use std::ws to ignore the whitespace right?

    • Alex

      Formatted input (e.g. std::cin >> value) skips leading whitespace by default.
      Unformatted input (e.g. std::getline(std::cin, value)) does not skip leading whitespace. In this case, using std::ws can be used to skip leading whitespace (e.g. std::getline(std::cin >> std::ws, value))

  • Jacob

    I don't get line 15 in Q1.
    Why are you casting at all if name.length() is already an integer?

    • Alex

      Because name.length() returns an object of type size_t, which is an unsigned type. Converting an unsigned type to a signed type (int) is a conversion, and should be done using a static_cast to tell the compiler that this conversion is legit.

  • yeokaiwei

    Hi Alex,
    I think there's something interesting to add about strings.

    There Ain’t No Such Thing As Plain Text.

    It depends on what encoding you use.

    https://www.joelonsoftware.com/2003/10/08/the-absolute-minimum-every-software-developer-absolutely-positively-must-know-about-unicode-and-character-sets-no-excuses/

    • Alex

      I'm well aware. It's not just about strings, it's about chars, character sets, fonts, and entire systems for translating and displaying text, including substitution substrings. And probably a whole host of other related topics.

      Localization / Internationalization (i18n) of programs is a topic in-and-of itself that I'd love to cover... someday -- but it will likely require an entire chapter to do justice to... and it's a little outside the C++ core language, which is what I'm more focused on right now.

  • dumdum

    Just checked the solution to the problem and realised I could've used static cast. Does this work or is it bad practice?

  • Rascal

    mm, I managed to run the following without the need for the iomanip lib?

  • Ahmed

    i think the quiz answer is kinda hard to read and in this example we're not using those variables again so for the sake of simplicity i reduced the code here's my answer:

  • Гомункул

    I'm sure the developer himself will be proud of you.

  • askingboy

    std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');

    do we need that for getline too ?

    • Alex

      You can use std::cin.ignore whenever you want to clear any stored but unextracted input from std::cin.

      I updated this lesson today in such a way that use of std::cin.ignore is no longer required for the examples in this lesson.

      • askingboy

        do we see the changes and updates ?
        i was looking for answers for ignore. then i refreshed the page . i didnt found that got too confused :D
        but now i get it
        thank you

  • Hi

    std::getline(std::cin, name) also has its faults...(https://stackoverflow.com/questions/62374433/alternatives-of-getline)

    I prefer using std::getline(std::cin >> std::ws, name);

  • TypoMan

    Typo in String Length section:

    "If we want to know how many characters are in a std::string is,"

    -> "If we want to know how many characters are in a std::string,"

  • Hey, what's the best way to convert a non-literal int to a std::string so I can append it to said string? I found a bunch of different ways on the internet, but I don't know which is best:

    - std::ostringstream
    - std::to_string
    - boost::lexical_cast
    - std::stringstream

    • nascardriver

      That depends on how you define "best".
      `boost::lexical_cast` requires an external library, that's not good (It's stream-based, you could just use `std::ostringstream`).
      `std::stringstream` is `std::ostringstream` but supports input, you only want output.
      You only want to convert 1 integer, you don't need a stream, that leaves `std::to_string`.
      `std::to_string` constructs a new `std::string`, that's expensive.
      All gone.

      If you already have a `std::string`, you can use `std::to_chars` to write the result directly into that string, but you have to reserve memory.

      Simplest is `std::to_string`, fastest should be `std::to_chars`, pick your poison.

  • Deva_B.D.M.

    ....It turns out, when you enter a value using operator<<, std::cin not only captures the value........

    operator<< or operator>>

  • Szymon :)

    Why do we  need to do a static_cast<int>name.length() ? Isn't name.length() already an integer?

  • Learner

    int letters{ static_cast<int>(name.length()) };

    1) Why do we need to use static_cast<int> for name.length()? Isnt it integer by default?

    2) How can we check what type does name.length() have? I tried typeid(name.length()).name, but it constantly gives me compiler error. I included <typeinfo>;

  • Ylva

    1. Section 'Mixing std::cin and std::getline()'

    why do we have a newline if we didn't type it?

  • Cuyler

    You say here that myName.length() is the first use of member function syntax, but in fact, we've seen it earlier with std::cin.ignore().

  • Cuyler

    Hey, you asked us to tell you if you passed 32767 to std::cin.ignore(). You use it here and also advocate its use, so there ya go

  • EternalSkid

    Hello alex and nascardriver, i was wondering about the line feed issue by using std::getline after std::cin. Is it true that since std::cin does not extract \n, it's left in the input buffer? Due to this, std::getline thinks it's the end of the string, and mistakes it at the end of the string. Does it discard everything after it? Or does it remain in the input buffer.

    I was thinking about this, what if i had this program. I inputted "\n hi" witht he intention of tricking the interpreter that it's the end of the line, and not extract anything. Surprisingly, it didn't work. Are you able to explain to me why? Once again, thank you for your time!

    • nascardriver

      I don't think I understand your question.
      You input a line feed, `std::getline()` stops extraction (Discarding the line feed), `myString` is empty, " hi" remains in the stream.

      • EternalSkid

        What i mean is that in your above example provided, std::getline sees that \n is already in the stream, and stops extraction. When i try to manipulate a std::getline by inputting a string like "\n hi", why does it not stop extracting? Or does c++ interprete everything i entered as a string? Thanks for clarification.

        Interestingly enough, i tried it with std::cin and integers, and i inputted \n4 4, and somehow both variables got output as 0, did std::cin fail here and why so? I felt it was weird, as shdn't std::cin ignore \n?

  • SuperNoob

    Question 1:

    Why does this code work in my machine? I haven't # included <string>.

    Question 2:
    Who is John Doe? Even I saw this name in a stackoverflow answer.

    • nascardriver

      https://www.learncpp.com/cpp-tutorial/header-files/#missing_include_but_works

      John Doe is a generic name that's used when you don't want to show a real name. Your country probably has such a name too. It's often used when pictures of identity/insurance/credit/etc cards are shown.

  • genovacle

    John Doe = 7 letters.
    Age = 46.

    46 / 7 = 6.571...

    The code needs to be modified to include a counter for white space.

    This gives the correct answer.

  • Aidan Stepniowski

    A more compact version of the Quiz 1 solution:

  • Navigator

    I kept getting the error "Conversion from 'unsigned int' to 'double' requires a narrowing conversion". Changing age from int to double fixed the issue.

  • Tomek

    Thank you for the lessons

  • Waldo Lemmer

    Well explained!

    1. Section "Appending strings", first sentence:
    > You can use operator+ to concatenate two strings together (returning a new string), or operator+= to append a string to the end of an existing string).
    Ends with a stray parenthesis.

    2. Why does `std::ignore()` request input when the input buffer is empty?

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