Portability of User-Defined Literals

Ensuring the portability of user-defined literals across different compilers is crucial for maintaining cross-platform compatibility in C++ projects. Here are some best practices to follow:

Stick to the Standard

The C++ standard specifies that user-defined literals must start with an underscore (_). Adhering to this rule ensures that your literals are compliant with the standard and are less likely to cause conflicts with future standard library literals.

Use Well-Defined Types

When defining user-defined literals, use the standard types that the C++ standard supports for literals:

• unsigned long long for integer literals
• long double for floating-point literals
• const char* for string literals
• char for character literals

Here’s an example of portable user-defined literals:

1#include <iostream>
2
3class Distance {
4 public:
5  Distance(float value) : value{value} {}
6  float value;
7};
8
9std::ostream& operator<<(std::ostream& os, Distance d) {
10  os << d.value << " meters";
11  return os;
12}
13
14Distance operator""_meters(long double val) {
15  return Distance{static_cast<float>(val)};
16}
17
18Distance operator""_kilometers(long double val) {
19  return Distance{static_cast<float>(val * 1000)};
20}
21
22int main() {
23  Distance d = 1.5_kilometers;
24  std::cout << d;
25}

11500 meters

Avoid Compiler-Specific Extensions

Some compilers might allow non-standard features or extensions. Avoid using these extensions to maintain portability. Stick to the features defined in the C++ standard.

Test on Multiple Compilers

Regularly test your code on multiple compilers, such as GCC, Clang, and MSVC. This practice helps identify and resolve any portability issues early in the development process.

Use Namespaces

Wrap your user-defined literals in namespaces to prevent naming conflicts. This practice helps ensure that your literals do not clash with literals from other libraries or future standard library additions.

1#include <iostream>
2
Distance Class|Show
3class Distance {/*...*/};
14
15namespace distance_literals {
16  Distance operator""_meters(long double val) {
17    return Distance{static_cast<float>(val)};
18  }
19}
20
21int main() {
22  using namespace distance_literals;
23  Distance d = 2.0_meters;
24  std::cout << d;
25}

12 meters

By following these practices, you can ensure that your user-defined literals remain portable and maintainable across different compilers and platforms.