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 S.4.4b -- 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”. First, there is the string literal “hello”, which is known at compile-time and stored in the binary. One copy is created when we create the char[]. The following two std::string objects create one copy of the string each. 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 object.

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. When we copy a std::string_view, the new std::string_view observes the same string as the copied-from std::string_view is observing.
std::string_view is not only fast, but has many of the functions that we know from std::string.

Because std::string_view doesn’t create a copy of the string, if we change the viewed string, the changes are reflected in the std::string_view.

We modified arr, but str appears to be changing as well. That’s because arr and str share their string. When you use a std::string_view, it’s best to avoid modifications the to underlying string for the remainder of the std::string_view‘s life to prevent confusion and errors.

Best practice

Use std::string_view instead of C-style strings.

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

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.

The same can happen when we create a std::string_view from a std::string and modify the std::string. Modifying a std::string can cause its internal string to die and be replaced with a new one in a different place. The std::string_view will still look at where the old string was, but it’s not there anymore.

Warning

Make sure that the underlying string viewed with a std::string_view does not go out of scope and isn’t modified 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 C-style string 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.

6.7 -- Introduction to pointers
Index
6.6 -- C-style strings

11 comments to 6.6a — An introduction to std::string_view

  • Fan

    OK. I saw that the first example has been modified to use char[] instead of const char *, but then should there also be a copy of the string allocated on the stack frame of main()?

  • Omar Abdelazeem

    when I'm trying to compile this code , the compiler complain that
    'string_view': is not a member of 'std'

    [/

    #include <iostream>
    #include <string_view>

    int main()
    {
        std::string_view text{ "hello" }; // view the text "hello", which is stored in the binary
        std::string_view str{ text }; // view of the same "hello"
        std::string_view more{ str }; // view of the same "hello"

        std::cout << text << ' ' << str << ' ' << more << '\n';

        return 0;
    }

    ]

  • ErwanDL

    Hey nascardriver,

    From the start of this article, you introduce "const char*" to define strings but I don't think we have already seen what it is and what it does earlier in the tutorial. Is it like using "std::string" ?

    Another thing is I have played around a bit with std::string_view and there are some pretty bizarre behaviours, like in the following code :

    This prints :

    Hello Hello
    Hi Hilo

    What is happening here ? Maybe this could be explained in the article as well ?

    Cheers

    • ErwanDL

      Update : I've advanced to lesson 6.8b and the syntax "const char*" is explained there (C-style string symbolic constants). Maybe this lesson on std::string_view should be moved further down the chapter ?

    • nascardriver

      > you introduce "const char*" to define strings but I don't think we have already seen what it is
      It's a `const char[]` with a different syntax, I though it was a part of the previous lesson, but it isn't. I changed every `const char*` to `char[]` or "C-style string". Thanks for letting me know!

      > like in the following code
      I wanted to include an example that shows that `std::string_view` reflects the changes made to its underlying string, but I'd have to move this lesson further back, which I don't want to do, because it would motivate the use of C-style strings.
      Your code invokes undefined behavior (I added a paragraph to the lesson).
      When you assign "Hi" to `str` in line 8, `str`'s old string "Hello" can be invalidated. In your case, since "Hi" is shorter than "Hello", the `std::string` re-used the memory it was using the store "Hello". That memory is now "Hi\0lo" (\0 is a null-terminator).
      If you assigned something longer, the old string could die completely.
      `view` still looks at where the old string was, but `view`'s size isn't changed (The curtains are where they were before). Since `std::string_view` doesn't use null-terminators, the \0 is ignored and "Hilo" is printed.

Leave a Comment

Put all code inside code tags: [code]your code here[/code]