std::ranges:: data

Returns a pointer to the first element of a contiguous range.

If T is an array type and std:: remove_all_extents_t < std:: remove_reference_t < T >> is incomplete, then the call to ranges::data is ill-formed, no diagnostic required.

If the argument is an lvalue or ranges:: enable_borrowed_range < std:: remove_cv_t < T >> is true , a call to ranges::data is expression-equivalent to:

1. decay-copy ( t. data ( ) ) (until C++23) auto ( t. data ( ) ) (since C++23) , if that expression is valid and its type is a pointer to an object type.
2. Otherwise, std:: to_address ( ranges:: begin ( t ) ) , if the expression ranges:: begin ( t ) is valid and its type models std::contiguous_iterator .

In all other cases, a call to ranges::data is ill-formed, which can result in substitution failure when ranges :: data ( e ) appears in the immediate context of a template instantiation.

Customization point objects

The name ranges::data 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 __data_fn .

All instances of __data_fn are equal. The effects of invoking different instances of type __data_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::data 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::data above, __data_fn models

• std:: invocable < __data_fn, Args... > ,
• std:: invocable < const __data_fn, Args... > ,
• std:: invocable < __data_fn & , Args... > , and
• std:: invocable < const __data_fn & , Args... > .
  

Otherwise, no function call operator of __data_fn participates in overload resolution.

Notes

If the argument is an rvalue (i.e. T is an object type) and ranges:: enable_borrowed_range < std:: remove_cv_t < T >> is false , the call to ranges::data is ill-formed, which also results in substitution failure.

If ranges :: data ( e ) is valid for an expression e , then it returns a pointer to an object.

The C++20 standard requires that if the underlying data function call returns a prvalue, the return value is move-constructed from the materialized temporary object. All implementations directly return the prvalue instead. The requirement is corrected by the post-C++20 proposal P0849R8 to match the implementations.

Example

#include <cstring>
#include <iostream>
#include <ranges>
#include <string>
 
int main()
{
    std::string s{"Hello world!\n"};
 
    char a[20]; // storage for a C-style string
    std::strcpy(a, std::ranges::data(s));
    // [data(s), data(s) + size(s)] is guaranteed to be an NTBS
 
    std::cout << a;
}

Hello world!

See also