User literals(C++11)

[Wunkolo]

Channel

C++Weekly

Topics

What topics are important to cover for this episode?

User-literals and use-cases for user-literals. Such as memory-size user-literals.

// namespace My::Cool::Library::Literals {

constexpr std::size_t operator""_KiB(unsigned long long int x) {
  return 1024ULL * x;
}

constexpr std::size_t operator""_MiB(unsigned long long int x) {
  return 1024_KiB * x;
}

constexpr std::size_t operator""_GiB(unsigned long long int x) {
  return 1024_MiB * x;
}

constexpr std::size_t operator""_TiB(unsigned long long int x) {
  return 1024_GiB * x;
}

constexpr std::size_t operator""_PiB(unsigned long long int x) {
  return 1024_TiB * x;
}

// } // namespace My::Cool::Library::Literals
...
	using namespace My::Cool::Library::Literals;
...
	const std::size_t BufferSize = 8_MiB;
	...
	std::byte LogBuffer[64_KiB];
	...
	Config.StackSize = 16_MiB;
	...
	class Foo {
	public:
		Foo() : memory_size_(64_MiB) {
	...
	std::size_t new_size = round_up(old_size, 64_KiB);
	...
	if( Size >= 1_GiB )
	...
	BufferThing(std::size_t InitialSize = 1_MiB);
	...
	const uint8_t BufferSize = 4_GiB;
	// warning: implicit conversion from 'size_t' (aka 'unsigned long') to 'const uint8_t' (aka 'const unsigned char') changes value from 4294967296 to 0 [-Wconstant-conversion]
	// const uint8_t BufferSize = 4_GiB;
	//               ~~~~~~~~~~   ^~~~~

Length

5-10 minutes

[lefticus]

I've actually planned and recorded this episode at least once, but have never been happy with the results.

[stefanos82]

One thing that irritates me with user literals is the use of underscore, whereas with chrono_literals we do not!

#include <iostream>
#include <chrono>
 
int main()
{
    using namespace std::chrono_literals;
    auto d1 = 250ms;
    std::chrono::milliseconds d2 = 1s;
    std::cout << "250ms = " << d1.count() << " milliseconds\n"
              << "1s = " << d2.count() << " milliseconds\n";
}

@lefticus do you have any idea why is that?

[stefanos82]

ud-suffix | - | an identifier, introduced by a literal operator or a literal operator template declaration (see below). All ud-suffixes introduced by a program must begin with the underscore character _. The standard library ud-suffixes do not begin with underscores.

Aaaah, I didn't see this line clearly, now it makes sense why chrono library does not use underscores!

I learned something new today :smile: