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4.4b — An introduction to std::string

What is a 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!”) are placed between double quotes to identify them as a string.

Because strings are commonly used in programs, most modern languages include a built-in string data type. C++ includes one, not as part of the core language, but as part of the standard library.

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 input and output

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

This prints:

My name is: Alex

However, 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 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, waiting for the next extraction. When we used operator>> again to extract a string into variable age, we got “Doe” instead of “23”. If we had done a third extraction, we would have gotten “23”.

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

Mixing std::cin and std::getline()

Reading inputs with both std::cin and std::getline may cause some unexpected behavior. Consider the following:

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” line, and then exits. What happened?

It turns out, when you enter a numeric value using cin, cin not only captures the numeric value, it also captures the newline. So when we enter 2, cin actually gets the string “2\n”. It then extracts the 2 to variable choice, leaving the newline stuck in the input stream. Then, when std::getline goes to read the name, it sees “\n” is already in the stream, and figures we must have entered an empty string! Definitely not what was intended.

A good rule of thumb is that after reading a numeric value with std::cin, remove the newline from the stream. This can be done using the following:

If we insert this line directly after reading variable choice, the extraneous newline will be removed from the stream, and the program will work as expected!

Rule: If reading numeric values with std::cin, it’s a good idea to remove the extraneous newline using std::cin.ignore().

What’s that 32767 magic number in your code?

That tells std::cin.ignore() how many characters to ignore up to. We picked that number because it’s the largest signed value guaranteed to fit in a (2-byte) integer on all platforms.

Technically, the correct way to ignore an unlimited amount of input is as follows:

But this requires remembering (or looking up) that horrendous line of code, as well as remembering what header to include. Most of the time you won’t need to ignore more than a line or two of buffered input, so for practical purposes, 32767 works about as well, and has the benefit of being something you can actually remember in your head.

Throughout these tutorials, we use 32767 for this reason. However, it’s your choice of whether you want to do it the “obscure, complex, and correct” way or the “easy, practical, but not ideal” way.

Appending strings

You can use operator+ to concatenate two strings together, or operator+= to append one string to another.

Here’s an example of both, also showing what happens if you try to use operator+ to add two numeric strings together:

This prints:

4511
45 volts

Note that operator+ concatenated the strings “45” and “11” into “4511”. It did not add them as numbers.

String length

If we want to know how long a string is, we can ask the 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 like we’ve used up to this point -- 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. It also has a lot of other capabilities that we haven’t touched on here. 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

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.

Quiz solutions

1) Show Solution

4.5 -- Enumerated types
Index
4.4a -- Explicit type conversion (casting)

248 comments to 4.4b — An introduction to std::string

  • Jon

    Okay, haven't posted a test since the gravity ball one back in 2.10! But use of strings is something I really want to get solid!

    Here's my test:

    Learn CPP 4.4b.cpp

    I'm not sure the best way to get it not to error if I input a string for the age, however. std::stoi() doesn't really like getting letters it turns out.

    • nascardriver

      Hi Jon!

      * Line 6, 22, 24: Initialize your variables with uniform initialization.
      * Reading the age as an @std::string and converting it to an int is slow. Read the age as an int.

      Other than that your code looks fine, good job!

      • Jon

        Hey nascardriver!

        Yeah, I'm caught in a pickle about the initialization! My work, for whom I'm learning C++, seems to prefer the '=' form of initialization so I've been getting in that habit, even if it's not as good as uniform.

        As for reading age as int, would you suggest another more specific function to read an int - of is there some way that I could keep my function as generic, but do a check on the input to ensure it won't have to convert? Or would that check be just as slow as converting it?

        • nascardriver

          You could use templates, but for now I'd stick to writing a separate function or use std::cin::operator>> directly without a wrapper.

          References
          * Chapter 13 - Templates

  • RelearningCGuy

    Is there a reason you declare strings with direct initialization instead of uniform initialization?

    Why not:

    • nascardriver

      Hi there!

      Some of the lessons are older than uniform initialization (Uniform initialization was add to C++ in 2011).
      Uniform initialization is preferred.

      • RelearningCGuy

        Ah gotcha, thanks. I've been testing everything as I go along, and it seems like the new uniform initialization works basically everywhere (even in for loops, etc).  Are there any cases in 11+ in which it is beneficial to use direct instead?

        • nascardriver

          Uniform initialization can cause unexpected behavior in combination with the 'auto' keyword.

          References
          * Lesson 4.8 - The auto keyword

          • RelearningCGuy

            Makes sense since it would not know to what type it should be initialized, especially if you tried to init to default values {}. Thanks!

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