The class __boost_string_view__ and other classes derived from __basic_string_view__ represent references to strings or substrings. When you are parsing/processing strings from some external source, frequently you want to pass a piece of text to a procedure for specialized processing. Before __std_string_view__, the canonical way to do this used to be a __std_string__, but that has certain drawbacks:
1) If you are processing a buffer of text (say a HTTP response or the contents of a file), then you have to create the string from the text you want to pass, which involves memory allocation and copying of data.
2) If a routine receives a constant __std_string__ and wants to pass a portion of that string to another routine, then it must create a new string of that substring.
3) If a routine receives a constant __std_string__ and wants to return a portion of the string, then it must create a new string to return.
__boost_string_view__ is designed to solve these efficiency problems. A __boost_string_view__ is a read-only reference to a contiguous sequence of characters, and provides much of the functionality of __std_string__. A __boost_string_view__ is cheap to create, copy and pass by value, because it does not actually own the storage that it points to.
A __boost_string_view__ is implemented as a small struct that contains a pointer to the start of the character `data` and a `count`. A __boost_string_view__ is cheap to create and cheap to copy.
__boost_string_view__ acts as a container; it includes all the methods that you would expect in a container, including iteration support, `operator[]`, `at` and `size`. It can be used with any of the iterator-based algorithms in the STL - as long as you do not need to change the underlying data. For example, __std_sort__ and __std_remove__ will not work.
Besides generic container functionality, __boost_string_view__ provides a subset of the interface of __std_string__. This makes it easy to replace parameters of type `const __std_string__ &` with __boost_string_view__. Like __std_string__, __boost_string_view__ has a static member variable named `npos` to denote the result of failed searches, and to mean "the end".
[caution Because a __boost_string_view__ does not own the data that it refers to, it introduces lifetime issues into code that uses it. The programmer must ensure that the data that a __string_view__ refers to exists as long as the __string_view__ does.]
[note
Boost.Utility also includes the class __string_ref__:
- __string_ref__ is the initial implementation of Jeffrey Yaskin's [@http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2012/n3442.html N3442:
string_ref: a non-owning reference to a string].
- __string_view__ is an updated implementation to reflect the Library Fundamentals TS [@http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4480.html N4480: \[string.view\]]
Please prefer __string_view__ / __basic_string_view__ over __string_ref__ / __basic_string_ref__:
- The __basic_string_view__ class better matches __std_basic_string_view__
- __basic_string_view__ has WAY more constexpr support.
- Code that uses __basic_string_ref__ should continue to work.
- No much code depends on __basic_string_ref__ anymore
Integrating __string_view__ into your code is fairly simple. Wherever you pass a `const __std_string__ &` or __std_string__ as a parameter, that's a candidate for passing a __boost_string_view__.
if ( extract_part ( "ABCDEFG" ).front() == 'C' ) { /* do something */ }
```
Let's figure out what happens in this contrived example.
* First, a temporary string is created from the string literal `"ABCDEFG"`, and it is passed (by reference) to the routine `extract_part`.
* Then a second string is created in the call `__std_string__::substr` and returned to `extract_part` (this copy may be elided by RVO).
* Then `extract_part` returns that string back to the caller (again this copy may be elided).
* The first temporary string is deallocated, and `front` is called on the second string, and then it is deallocated as well.
Two __std_string__ s are created, and two copy operations. That is potentially four memory allocations and deallocations, and the associated copying of data.
Now let's look at the same code with __string_view__:
```
__boost_string_view__ extract_part ( __boost_string_view__ bar ) {
return bar.substr ( 2, 3 );
}
if ( extract_part ( "ABCDEFG" ).front() == "C" ) { /* do something */ }
```
No memory allocations. No copying of character data. No changes to the code other than the types. There are two __string_view__ s created, and two __string_view__ s copied, but those are cheap operations.
The header file [@../../../include/boost/utility/string_view.hpp `<boost/utility/string_view.hpp>`] defines a template __boost_basic_string_view__, and four specializations __string_view__, __wstring_view__, __u16string_view__, __u32string_view__ - for `char` / `wchar_t` / `char16_t` / `char32_t`.
`#include <boost/utility/string_view.hpp>`
Construction and copying:
```
constexpr basic_string_view (); // Constructs an empty string_view
constexpr basic_string_view(const charT* str); // Constructs from a NULL-terminated string
constexpr basic_string_view(const charT* str, size_type len); // Constructs from a pointer, length pair
template<typename Allocator>
basic_string_view(const __std_basic_string__<charT, traits, Allocator>& str); // Constructs from a std::string
