5.10 — Introduction to std::string_view

Consider the following program:

#include <iostream>

int main()
{
    int x { 5 }; // x makes a copy of its initializer
    std::cout << x << '\n';

    return 0;
}

When the definition for x is executed, the initialization value 5 is copied into the memory allocated for variable int x. For fundamental types, initializing and copying a variable is fast.

Now consider this similar program:

#include <iostream>
#include <string>

int main()
{
    std::string s{ "Hello, world!" }; // s makes a copy of its initializer
    std::cout << s << '\n';

    return 0;
}

When s is initialized, the C-style string literal "Hello, world!" is copied into memory allocated for std::string s. Unlike fundamental types, initializing and copying a std::string is slow.

In the above program, all we do with s is print the value to the console, and then s is destroyed. We’ve essentially made a copy of “Hello, world!” just to print and then destroy that copy. That’s inefficient.

We see something similar in this example:

#include <iostream>
#include <string>

void printString(std::string str) // str makes a copy of its initializer
{
    std::cout << str << '\n';
}

int main()
{
    std::string s{ "Hello, world!" }; // s makes a copy of its initializer
    printString(s);

    return 0;
}

This example makes two copies of the C-style string “Hello, world!”: one when we initialize s in main(), and another when we initialize parameter str in printString(). That’s a lot of needless copying just to print a string!

std::string_view C++17

To address the issue with std::string being expensive to initialize (or copy), C++17 introduced std::string_view (which lives in the <string_view> header). std::string_view provides read-only access to an existing string (a C-style string, a std::string, or another std::string_view) without making a copy. Read-only means that we can access and use the value being viewed, but we can not modify it.

The following example is identical to the prior one, except we’ve replaced std::string with std::string_view.

#include <iostream>
#include <string_view> // C++17

// str provides read-only access to whatever argument is passed in
void printSV(std::string_view str) // now a std::string_view
{
    std::cout << str << '\n';
}

int main()
{
    std::string_view s{ "Hello, world!" }; // now a std::string_view
    printSV(s);

    return 0;
}

This program produces the same output as the prior one, but no copies of the string “Hello, world!” are made.

When we initialize std::string_view s with C-style string literal "Hello, world!", s provides read-only access to “Hello, world!” without making a copy of the string. When we pass s to printSV(), parameter str is initialized from s. This allows us to access “Hello, world!” through str, again without making a copy of the string.

Best practice

Prefer std::string_view over std::string when you need a read-only string, especially for function parameters.

std::string_view can be initialized with many different types of strings

One of the neat things about a std::string_view is how flexible it is. A std::string_view object can be initialized with a C-style string, a std::string, or another std::string_view:

#include <iostream>
#include <string>
#include <string_view>

int main()
{
    std::string_view s1 { "Hello, world!" }; // initialize with C-style string literal
    std::cout << s1 << '\n';

    std::string s{ "Hello, world!" };
    std::string_view s2 { s };  // initialize with std::string
    std::cout << s2 << '\n';

    std::string_view s3 { s2 }; // initialize with std::string_view
    std::cout << s3 << '\n';
       
    return 0;
}

std::string_view parameters will accept many different types of string arguments

Both a C-style string and a std::string will implicitly convert to a std::string_view. Therefore, a std::string_view parameter will accept arguments of type C-style string, a std::string, or std::string_view:

#include <iostream>
#include <string>
#include <string_view>

void printSV(std::string_view str)
{
    std::cout << str << '\n';
}

int main()
{
    printSV("Hello, world!"); // call with C-style string literal

    std::string s2{ "Hello, world!" };
    printSV(s2); // call with std::string

    std::string_view s3 { s2 };
    printSV(s3); // call with std::string_view
       
    return 0;
}

std::string_view will not implicitly convert to std::string

Because std::string makes a copy of its initializer (which is expensive), C++ won’t allow implicit conversion of a std::string_view to a std::string. This is to prevent accidentally passing a std::string_view argument to a std::string parameter, and inadvertently making an expensive copy where such a copy may not be required.

However, if this is desired, we have two options:

  1. Explicitly create a std::string with a std::string_view initializer (which is allowed, since this will rarely be done unintentionally)
  2. Convert an existing std::string_view to a std::string using static_cast

The following example shows both options:

#include <iostream>
#include <string>
#include <string_view>

void printString(std::string str)
{
	std::cout << str << '\n';
}

int main()
{
	std::string_view sv{ "Hello, world!" };

	// printString(sv);   // compile error: won't implicitly convert std::string_view to a std::string

	std::string s{ sv }; // okay: we can create std::string using std::string_view initializer
	printString(s);      // and call the function with the std::string

	printString(static_cast<std::string>(sv)); // okay: we can explicitly cast a std::string_view to a std::string

	return 0;
}

Assignment changes what the std::string_view is viewing

Assigning a new string to a std::string_view causes the std::string_view to view the new string. It does not modify the prior string being viewed in any way.

The following example illustrates this:

#include <iostream>
#include <string>
#include <string_view>

int main()
{
    std::string name { "Alex" };
    std::string_view sv { name }; // sv is now viewing name
    std::cout << sv << '\n'; // prints Alex

    sv = "John"; // sv is now viewing "John" (does not change name)
    std::cout << sv << '\n'; // prints John

    std::cout << name << '\n'; // prints Alex

    return 0;
}

In the above example, sv = "John" causes sv to now view the string "John". It does not change the value held by name (which is still "Alex").

Literals for std::string_view

Double-quoted string literals are C-style string literals by default. We can create string literals with type std::string_view by using a sv suffix after the double-quoted string literal. The sv must be lower case.

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

int main()
{
    using namespace std::string_literals;      // access the s suffix
    using namespace std::string_view_literals; // access the sv 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
    std::cout << "moo\n"sv; // sv suffix is a std::string_view literal

    return 0;
}

Related content

We discuss this use of using namespace in lesson 5.9 -- Introduction to std::string. The same advice applies here.

It’s fine to initialize a std::string_view object with a C-style string literal (you don’t need to initialize it with a std::string_view literal).

That said, initializing a std::string_view using a std::string_view literal won’t cause problems (as such literals are actually C-style string literals in disguise).

constexpr std::string_view

Unlike std::string, std::string_view has full support for constexpr:

#include <iostream>
#include <string_view>

int main()
{
    constexpr std::string_view s{ "Hello, world!" }; // s is a string symbolic constant
    std::cout << s << '\n'; // s will be replaced with "Hello, world!" at compile-time

    return 0;
}

This makes constexpr std::string_view the preferred choice when string symbolic constants are needed.

We will continue discussing std::string_view in the next lesson.

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