Bartłomiej Filipek, 20th October 2022, cppstories.com
20 Smaller yet Handy
C++20 Features
Part 2 - library
About Me
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Author of cppstories.com
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~15y professional coding experience
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4x Microsoft MVP, since 2018
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C++ ISO Member
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@Xara.com since 2014
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Mostly text related features for advanced document
editors
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Somehow addicted to C++
The plan
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About C++20
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10 Language Features
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10 Library Features
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More in the future
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About C++20
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80 Library features and 70 language changes
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https://en.cppreference.com/w/cpp/compiler_support#cpp20
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Do you use C++20?
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Have you tried
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modules
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std::format
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concepts
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coroutines
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extended std::chrono?
11. Math Constants
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template<class T> inline constexpr T e_v = /* unspecified */;
template<class T> inline constexpr T log2e_v = /* unspecified */;
template<class T> inline constexpr T log10e_v = /* unspecified */;
...
...
template<class T> inline constexpr T inv_sqrt3_v = /* unspecified */;
template<class T> inline constexpr T egamma_v = /* unspecified */;
template<class T> inline constexpr T phi_v = /* unspecified */;
namespace std::numbers
12. More constexpr in the Library
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constexpr std::complex
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constexpr algorithms P0202
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Making std::vector constexpr - P1004
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Making std::string constexpr - P0980
constexpr new: https://godbolt.org/z/becbas5Mz
example for constexpr algorithm
https://godbolt.org/z/cds48cxPK
https://godbolt.org/z/P59r888Gd - GCC
parsing params: https://godbolt.org/z/xrPj4TKac
13. .starts_with() and .ends_with()
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#include <string>
#include <iostream>
#include <string_view>
int main(){
const std::string url = "https://isocpp.org";
// string literals
if (url.starts_with("https") && url.ends_with(".org"))
std::cout << "you're using the correct site!\n";
if (url.starts_with('h') && url.ends_with('g'))
std::cout << "letters matched!\n";
}
14. contains() member function of associative containers
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for (auto& key: {"hello", "something"}) {
if (strToInt.contains(key))
std::cout << key << ": Found\n";
else
std::cout << key << ": Not found\n";
}
15. Consistent Container Erasure
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In C++20, we get a bunch of free functions that have overloads for many containers and can remove elements:
erase(container, value); erase_if(container, predicate);
#include <iostream>
#include <vector>
int main() {
std::vector vec { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
std::erase_if(vec, [](auto& v) { return v % 2 == 0; });
for (int i = 0; auto &v : vec)
std::cout << i++ << ": " << v << '\n';
}
void erase(basic_string<charT, traits, Allocator>& c, const U& value);
c.erase(remove(c.begin(), c.end(), value), c.end());
16. Source Location
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void log(const string_view& message, const source_location& location =
source_location::current()) {
std::cout << “info:”
<< location.file_name() << “:”
<< location.line() << “ “
<< location.function_name() << “ “
<< message << ‘\n’;
}
17. std::bind_front - for partial function application
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void func(int a, int b, int c, int d) { }
using namespace std::placeholders;
auto f1 = std::bind(func, 42, 128, _1,_2);
// vs
auto f2 = std::bind_front(func, 42, 128);
f1(100, 200);
f2(100, 200);
18. Heterogeneous lookup for unordered containers
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struct string_hash {
using is_transparent = void;
[[nodiscard]] size_t operator()(const char *txt) const {
return std::hash<std::string_view>{}(txt);
}
[[nodiscard]] size_t operator()(std::string_view txt) const {
return std::hash<std::string_view>{}(txt);
}
[[nodiscard]] size_t operator()(const std::string &txt) const {
return std::hash<std::string>{}(txt);
}
};
std::unordered_map<std::string, int, string_hash, std::equal_to<>>
19. Smart pointer creation with default initialization
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new T[]()
// vs
new T[]
auto ptr = std::make_unique_for_overwrite<int[]>(COUNT);
20. Safe integral comparisons and ssize
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const long longVal = -100;
const size_t sizeVal = 100;
std::cout << std::boolalpha;
std::cout << std::cmp_less(longVal, sizeVal);
void printReverseSigned(const std::vector<int>& v) {
for (auto i = std::ssize(v)-1; i >= 0; --i)
std::cout << i << “: “ << v[i] << ‘\n’;
}
And more!
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List of supported features: https://en.cppreference.com/w/cpp/compiler_support#cpp20
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C++20 - The Complete Guide, by N Josuttis - https://leanpub.com/cpp20
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Google Chrome: C++20, How Hard Could It Be - presentation and discussion on Reddit:
https://www.reddit.com/r/cpp/comments/xnk3fm/google_chrome_c20_how_hard_could_it_be/
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My articles on C++20: https://www.cppstories.com/tags/cpp20/
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Summary
Abbreviated Function Templates and Constrained Auto
Math constants
Template Syntax For Generic Lambdas
More constexpr in the Library
Constexpr Improvements
.starts_with() and .ends_with()
using enum
contains() member function of associative containers
Class-types in non-type template parameters
Consistent Container Erasure
New keyword constinit
Source Location
Designated Initializers
std::bind_front - for partial function application
Nodiscard Attribute Improvements
Heterogeneous lookup for unordered containers
Range-based for loop with Initializer
Smart pointer creation with default initialization
New keyword consteval - immediate functions
Safe integral comparisons
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Bonus
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C++23 almost ready! - Feature freeze, sent for voting
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some features:
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deducing this
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static operator()
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stacktrace
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more ranges, views and algorithms
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std::format improvements and std::print
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std::expected
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std::flat_map and std::flat_set
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module std
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std::generator
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...
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Carbon, CppFront?