5.7 — Introduction to std::string

In lesson 5.2 -- Literals, we introduced C-style string literals:

#include <iostream>

int main()
{
    std::cout << "Hello, world!"; // "Hello world!" is a C-style string literal.
    return 0;
}

While C-style string literals are fine to use, C-style string variables behave oddly, are hard to work with (e.g. you can’t use assignment to assign a C-style string variable a new value), and are dangerous (e.g. if you copy a larger C-style string into the space allocated for a shorter C-style string, undefined behavior will result). In modern C++, C-style string variables are best avoided.

Fortunately, C++ has introduced two additional string types into the language that are much easier and safer to work with: std::string and std::string_view (C++17). Unlike the types we’ve introduced previously, std::string and std::string_view aren’t fundamental types (they’re class types, which we’ll cover in the future). However, basic usage of each is straightforward and useful enough that we’ll introduce them here.

Introducing std::string

The easiest way to work with strings and string objects in C++ is via the std::string type, which lives in the <string> header.

We can create objects of type std::string just like other objects:

#include <string> // allows use of std::string

int main()
{
    std::string name {}; // empty string

    return 0;
}

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

#include <string>

int main()
{
    std::string name { "Alex" }; // initialize name with string literal "Alex"
    name = "John";               // change name to "John"

    return 0;
}

Note that strings can be composed of numeric characters as well:

std::string myID{ "45" }; // "45" is not the same as integer 45!

In string form, numbers are treated as text, not as numbers, and thus they can not be manipulated as numbers (e.g. you can’t multiply them). C++ will not automatically convert strings to integer or floating point values or vice-versa (though there are ways to do so that we’ll cover in a future lesson).

String output with std::cout

std::string objects can be output as expected using std::cout:

#include <iostream>
#include <string>

int main()
{
    std::string name { "Alex" };
    std::cout << "My name is: " << name << '\n';

    return 0;
}

This prints:

My name is: Alex

Empty strings will print nothing:

#include <iostream>
#include <string>

int main()
{
    std::string empty{ };
    std::cout << '[' << empty << ']';

    return 0;
}

Which prints:

[]

std::string can handle strings of different lengths

One of the neatest things that std::string can do is store strings of different lengths:

#include <iostream>
#include <string>

int main()
{
    std::string name { "Alex" }; // initialize name with string literal "Alex"
    std::cout << name << '\n';

    name = "Jason";              // change name to a longer string
    std::cout << name << '\n';

    name = "Jay";                // change name to a shorter string
    std::cout << name << '\n';

    return 0;
}

This prints:

Alex
Jason
Jay

In the above example, name is initialized with the string "Alex", which contains five characters (four explicit characters and a null-terminator). We then set name to a larger string, and then a smaller string. std::string has no problem handling this! You can even store really long strings in a std::string.

This is one of the reasons that std::string is so powerful.

String input with std::cin

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

#include <iostream>
#include <string>

int main()
{
    std::cout << "Enter your full name: ";
    std::string name{};
    std::cin >> name; // this won't work as expected since std::cin breaks on whitespace

    std::cout << "Enter your favorite color: ";
    std::string color{};
    std::cin >> color;

    std::cout << "Your name is " << name << " and your favorite color is " << color << '\n';

    return 0;
}

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

Enter your full name: John Doe
Enter your favorite color: Your name is John and your favorite color is Doe

Hmmm, that isn’t right! What happened? It turns out that when using operator>> to extract a string from std::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 input into variable name, only "John" was extracted, leaving " Doe" inside std::cin. When we then used operator>> to get extract input into variable color, it extracted "Doe" instead of waiting for us to input an color. 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() requires two arguments: the first is std::cin, and the second is your string variable.

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

#include <iostream>
#include <string> // For std::string and std::getline

int main()
{
    std::cout << "Enter your full name: ";
    std::string name{};
    std::getline(std::cin >> std::ws, name); // read a full line of text into name

    std::cout << "Enter your favorite color: ";
    std::string color{};
    std::getline(std::cin >> std::ws, color); // read a full line of text into color

    std::cout << "Your name is " << name << " and your favorite color is " << color << '\n';

    return 0;
}

Now our program works as expected:

Enter your full name: John Doe
Enter your favorite color: blue
Your name is John Doe and your favorite color is blue

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, which alter the way that input is accepted. The std::ws input manipulator tells std::cin to ignore any leading whitespace before extraction. Leading whitespace is any whitespace character (spaces, tabs, newlines) that occur at the start of the string.

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

#include <iostream>
#include <string>

int main()
{
    std::cout << "Pick 1 or 2: ";
    int choice{};
    std::cin >> choice;

    std::cout << "Now enter your name: ";
    std::string name{};
    std::getline(std::cin, name); // note: no std::ws here

    std::cout << "Hello, " << name << ", you picked " << choice << '\n';

    return 0;
}

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.

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 captures the string "2\n" as input. It then extracts the value 2 to variable choice, leaving the newline character behind for later. Then, when std::getline() goes to extract text to name, it sees "\n" is already waiting in std::cin, 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:

#include <iostream>
#include <string>

int main()
{
    std::cout << "Pick 1 or 2: ";
    int choice{};
    std::cin >> choice;

    std::cout << "Now enter your name: ";
    std::string name{};
    std::getline(std::cin >> std::ws, name); // note: added std::ws here

    std::cout << "Hello, " << name << ", you picked " << choice << '\n';

    return 0;
}

Now this program will function as intended.

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

The length of a std::string

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

#include <iostream>
#include <string>

int main()
{
    std::string name{ "Alex" };
    std::cout << name << " has " << name.length() << " characters\n";

    return 0;
}

This prints:

Alex has 4 characters

Although std::string is required to be null-terminated (as of C++11), the returned length of a std::string does not include the implicit null-terminator character.

Note that instead of asking for the string length as length(name), we say name.length(). The length() function isn’t a normal standalone function -- it’s a special type of function that is nested within std::string called a member function. Because the length() member function is declared inside of std::string, it is sometimes written as std::string::length() in documentation.

We’ll cover member functions, including how to write your own, in more detail later.

Also note that std::string::length() returns an unsigned integral value (most likely of type size_t). If you want to assign the length to an int variable, you should static_cast it to avoid compiler warnings about signed/unsigned conversions:

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

#include <iostream>
#include <string>

int main()
{
    std::string name{ "Alex" };
    std::cout << name << " has " << std::ssize(name) << " characters\n";

    return 0;
}

int len { static_cast<int>(std::ssize(name)) };

Initializing a std::string is expensive

Whenever a std::string is initialized, a copy of the string used to initialize it is made. Making copies of strings is expensive, so care should be taken to minimize the number of copies made.

Do not pass std::string by value

When a std::string is passed to a function by value, the std::string function parameter must be instantiated and initialized with the argument. This results in an expensive copy. We’ll discuss what to do instead (use std::string_view) in lesson 5.8 -- Introduction to std::string_view.

Returning a std::string

When a function returns by value to the caller, the return value is normally copied from the function back to the caller. So you might expect that you should not return std::string by value, as doing so would return an expensive copy of a std::string.

However, as a rule of thumb, it is okay to return a std::string by value when the expression of the return statement resolves to any of the following:

• A local variable of type std::string.
• A std::string that has been returned by value from another function call or operator.
• A std::string temporary that is created as part of the return statement.

In most other cases, prefer to avoid returning a std::string by value, as doing so will make an expensive copy.

Literals for std::string

Double-quoted string literals (like “Hello, world!”) are C-style strings by default (and thus, have a strange type).

We can create string literals with type std::string by using a s suffix after the double-quoted string literal. The s must be lower case.

#include <iostream>
#include <string> // for std::string

int main()
{
    using namespace std::string_literals; // easy access to the s suffix

    std::cout << "foo\n";   // no suffix is a C-style string literal
    std::cout << "goo\n"s;  // s suffix is a std::string literal

    return 0;
}

You probably won’t need to use std::string literals very often (as it’s fine to initialize a std::string object with a C-style string literal), but we’ll see a few cases in future lessons (involving type deduction) where using std::string literals instead of C-style string literals makes things easier (see 10.8 -- Type deduction for objects using the auto keyword for an example).

Constexpr strings

If you try to define a constexpr std::string, your compiler will probably generate an error:

#include <iostream>
#include <string>

int main()
{
    using namespace std::string_literals;

    constexpr std::string name{ "Alex"s }; // compile error

    std::cout << "My name is: " << name;

    return 0;
}

This happens because constexpr std::string isn’t supported at all in C++17 or earlier, and only works in very limited cases in C++20/23. If you need constexpr strings, use std::string_view instead (discussed in lesson 5.8 -- Introduction to std::string_view).

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.