template< class C >
constexpr auto size( const C& c ) -> decltype(c.size());
(1)
(since C++17)
Returns the size of the given range.
1,2) Returns c.size(), converted to the return type if necessary.
3,4) Returns N.
[edit] Parameters
c
-
a container or view with a size
member function
array
-
an array of arbitrary type
[edit] Return value
1) c.size()
3,4) N
[edit] Exceptions
1,2) May throw implementation-defined exceptions.
[edit] Overloads
Custom overloads of size
may be provided for classes and enumerations that do not expose a suitable size()
member function, yet can be detected.
[edit] Possible implementation
[edit] Example
#include <cassert>
#include <cstring>
#include <iostream>
#include <vector>
int main()
{
// Works with containers
std::vector <int> v{3, 1, 4};
assert (std::size(v) == 3);
// And works with built-in arrays too
int a[]{-5, 10, 15};
// Returns the number of elements (not bytes) as opposed to sizeof
assert (std::size(a) == 3);
std::cout << "size of a[]: " << sizeof a << '\n'; // 12, if sizeof(int) == 4
// Provides a safe way (compared to sizeof) of getting string buffer size
const char str[] = "12345";
// These are fine and give the correct result
assert (std::size(str) == 6);
assert (sizeof(str) == 6);
// But use of sizeof here is a common source of bugs
const char* str_decayed = "12345";
// std::cout << std::size(str_decayed) << '\n'; // Usefully fails to compile
std::cout << sizeof(str_decayed) << '\n'; // Prints the size of the pointer!
// Since C++20 the signed size (std::ssize) is available
auto i = std::ssize(v);
for (--i; i != -1; --i)
std::cout << v[i] << (i ? ' ' : '\n');
assert (i == -1);
// Note that the string literal includes the ending null character, which
// will be part of the constructed characters array. This makes std::size
// behave differently from std::strlen and std::string::size:
constexpr char symbols[] = "0123456789";
static_assert(std::size(symbols) == 11);
static_assert(std::string (symbols).size() == 10);
assert (std::strlen (symbols) == 10);
}
Possible output:
[edit] See also
signed integer type returned when subtracting two pointers
(typedef) [edit]
unsigned integer type returned by the
sizeof operator
(typedef) [edit]
returns an integer equal to the size of a range
(customization point object)[edit]
returns a signed integer equal to the size of a range
(customization point object)[edit]