Difference between revisions of "cpp/ranges/cdata"
m (→Example: + ::ranges) |
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std::string src {"hello world!\n"}; | std::string src {"hello world!\n"}; | ||
| − | // std::cdata(src)[0] = 'H'; // error, src.data() is treated as | + | // std::ranges::cdata(src)[0] = 'H'; // error, src.data() is treated as read-only |
| − | std::data(src)[0] = 'H'; // OK, src.data() is a non-const storage | + | std::ranges::data(src)[0] = 'H'; // OK, src.data() is a non-const storage |
char dst[20]; // storage for a C-style string | char dst[20]; // storage for a C-style string | ||
Revision as of 10:11, 18 December 2021
| Defined in header <ranges>
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| inline namespace /*unspecified*/ { inline constexpr /*unspecified*/ cdata = /*unspecified*/; |
(since C++20) (customization point object) |
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| Call signature |
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| template< class T > requires /* see below */ |
(since C++20) | |
Returns a pointer to the first element of a contiguous range denoted by a const-qualified argument.
Let CT be
- const std::remove_reference_t<T>& if the argument is an lvalue (i.e.
Tis an lvalue reference type), - const T otherwise,
a call to ranges::cdata is expression-equivalent to ranges::data(static_cast<CT&&>(t)).
If ranges::cdata(t) is valid, then it returns a pointer to a object.
Contents |
Expression-equivalent
Expression e is expression-equivalent to expression f, if
- e and f have the same effects, and
- either both are constant subexpressions or else neither is a constant subexpression, and
- either both are potentially-throwing or else neither is potentially-throwing (i.e. noexcept(e) == noexcept(f)).
Customization point objects
The name ranges::cdata denotes a customization point object, which is a const function object of a literal semiregular class type. For exposition purposes, the cv-unqualified version of its type is denoted as __cdata_fn.
All instances of __cdata_fn are equal. The effects of invoking different instances of type __cdata_fn on the same arguments are equivalent, regardless of whether the expression denoting the instance is an lvalue or rvalue, and is const-qualified or not (however, a volatile-qualified instance is not required to be invocable). Thus, ranges::cdata can be copied freely and its copies can be used interchangeably.
Given a set of types Args..., if std::declval<Args>()... meet the requirements for arguments to ranges::cdata above, __cdata_fn models
- std::invocable<__cdata_fn, Args...>,
- std::invocable<const __cdata_fn, Args...>,
- std::invocable<__cdata_fn&, Args...>, and
- std::invocable<const __cdata_fn&, Args...>.
Otherwise, no function call operator of __cdata_fn participates in overload resolution.
Example
#include <cstring> #include <iostream> #include <ranges> #include <string> int main() { std::string src {"hello world!\n"}; // std::ranges::cdata(src)[0] = 'H'; // error, src.data() is treated as read-only std::ranges::data(src)[0] = 'H'; // OK, src.data() is a non-const storage char dst[20]; // storage for a C-style string std::strcpy(dst, std::ranges::cdata(src)); // [data(src), data(src) + size(src)] is guaranteed to be an NTBS std::cout << dst; }
Output:
Hello world!
See also
| (C++20) |
obtains a pointer to the beginning of a contiguous range (customization point object) |
| (C++17) |
obtains the pointer to the underlying array (function template) |
