Search

6.6a — An introduction to std::string_view

In the previous lesson, we talked about C-style strings, and the dangers of using them. C-style strings are fast, but they’re not as easy to use and as safe as std::string.

But std::string (which we covered in lesson 4.12 -- An introduction to std::string), has some of its own downsides, particularly when it comes to const strings.

Consider the following example:

As expected, this prints

hello hello hello

Internally, this program creates 3 copies of the string “hello”. The first is the string literal “hello”, which is known at compile-time and stored in the binary. The following two std::string objects create copies of this string. Because std::string is designed to be modifiable, each std::string must contain its own copy of the string, so that a given std::string can be modified without affecting any other std::string objects.

This holds true for const std::string, even though they can’t be modified.

Introducing std::string_view

Consider a window in your house, looking at a car sitting on the street. You can look through the window and see the car, but you can’t touch or move the car. Your window just provides a view to the car, which is a completely separate object.

C++17 introduces another way of using strings, std::string_view, which lives in the <string_view> header.

Unlike std::string, which keeps its own copy of the string, std::string_view provides a view of a string that is defined elsewhere.

We can re-write the above code to use std::string_view by replacing every std::string with std::string_view.

The output is the same, but no more copies of the string “hello” are created. std::string_view is not only fast, but has many of the functions that we know from std::string.

Best practice

Use std::string_view instead of const char*.

Prefer std::string_view over std::string for read-only strings, unless you already have a std::string.

View modification functions

Back to our window analogy, consider a window with curtains. We can close either the left or right curtain to reduce what we can see. We don’t change what’s outside, we just reduce the visible area.

Similarly, std::string_view contains functions that let us manipulate the view of the string. This allows us to change the view without modifying the viewed string.

The functions for this are remove_prefix, which removes characters from the left side of the view, and remove_suffix, which removes characters from the right side of the view.

This program produces the following output:

Peach
each
ea

For advanced readers

Internally, std::string_view uses a const char* to mark the beginning of the string and an integer to keep track of the length.

The remove_prefix functions advances the const char* forward and decreases the length respectively. remove_suffix decreases the internal length.

Unlike real curtains, a std::string_view cannot be opened back up. Once you change the visible area, you can’t go back (There are tricks which we won’t go into).

std::string_view works with non-null-terminated strings

Unlike C-style strings and std::string, std::string_view doesn’t use null terminators to mark the end of the string. Rather, it knows where the string ends because it keeps track of its length.

This program prints:

aeiou

Ownership issues

Being only a view, a std::string_view‘s lifetime is independent of that of the string it is viewing. If the viewed string goes out of scope, std::string_view has nothing to observe and accessing it causes undefined behavior.

What's your name?
nascardriver
Hello nascardriver
Your name is �P@�P@

When we created str and filled it with std::cin, it created its internal string in dynamic memory. When str goes out of scope at the end of askForName, the internal string dies along with str. The std::string_view doesn’t know that the string no longer exists and allows us to access it. Accessing the released string through view in main causes undefined behavior, which on the author’s machine produced weird characters.

Warning

Make sure that the underlying string viewed with a std::string_view does not go out of scope while using the std::string_view.

Converting a std::string_view to a std::string

An std::string_view will not implicitly convert to a std::string, but can be explicitly converted:

This prints:

ball
ball

Converting a std::string_view to a C-style string

Some old functions (such as the old strlen function) still expect C-style strings. To convert a std::string_view to a C-style string, we can do so by first converting to a std::string:

This prints:

ball has 4 letter(s)

However, creating a std::string every time we want to pass a std::string_view as a const char* is expensive, so this should be avoided if possible.

Opening the window (kinda) via the data() function

The string being viewed by a std::string_view can be accessed by using the data() function, which returns a C-style string. This provides fast access to the string being viewed (as a C-string). But it should also only be used if the std::string_view‘s view hasn’t been modified (e.g. by remove_prefix or remove_suffix) and the string being viewed is null-terminated.

In the following example, std::strlen doesn’t know what a std::string_view is, so we need to pass it str.data():

balloon
7

When a std::string_view has been modified, data() doesn’t always do what we’d like it to. The following example demonstrates what happens when we access data() after modifying the view:

all has 6 letter(s)
str.data() is alloon
str is all

Clearly this isn’t what we’d intended, and is a consequence of trying to access the data() of a std::string_view that has been modified. The length information about the string is lost when we access data(). std::strlen and std::cout keep reading characters from the underlying string until they find the null-terminator, which is at the end of “balloon”.

Warning

Only use std::string_view::data() if the std::string_view‘s view hasn’t been modified and the string being viewed is null-terminated. Using std::string_view::data() of a non-null-terminated string can cause undefined behavior.

Incomplete implementation

Being a relatively recent feature, std::string_view isn’t implemented as well as it could be.

There’s no reason why line 5 and 6 shouldn’t work. They will probably be supported in a future C++ version.

8.6 -- Pointers
Index
6.6 -- C-style strings

Leave a Reply

You can use these HTML tags

<a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code class="" title="" data-url=""> <del datetime=""> <em> <i> <q cite=""> <s> <strike> <strong> <pre class="" title="" data-url=""> <span class="" title="" data-url="">