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c++constantsinlineconstexpr
Heston Wilson
Heston Wilson

Reputation: 61

Using inline and constexpr in C++

I am working on a robot program for FRC robotics and was curious about if inline or constexpr or both are appropriate for declaring constants.

inline constexpr double PI = wpi::math::pi 
inline constexpr double TWO_PI = 2.0 * wpi::math::pi;
inline constexpr units::radian_t PI_RAD = units::radian_t(PI);
inline constexpr units::radian_t TWO_PI_RAD = units::radian_t(TWO_PI);

Upvotes: 6

Views: 2075

Answers (1)

Davis Herring
Davis Herring

Reputation: 40023

Omit inline for variables declared in source files; it merely invites name collisions. C++17 added support for inline variables for header files: by all means use it there to avoid stupid things like this:

#ifndef HEADER_HH
#define HEADER_HH
#include<algorithm>
/* inline */ constexpr int bound=30;
inline int clip(int x) {return std::min(x,bound);}
#endif

Any program that #includes this header in two different source files is ill-formed, no diagnostic required(!), because std::min takes its arguments by reference and thus odr-uses the internal-linkage bound which is a different variable in every translation unit, denying a single definition for clip.

In C++20, though, you can use export constexpr (or, for module-private variables, extern constexpr) in a module interface unit with no need for inline. (With sufficiently modern compilers, you can omit the extern.)

All this applies for normal const (as you must use if the initializer is not a constant expression), with the added bonus that inline lets you define a non-constexpr static class member inside the class (which is useful even in a source file; put the class in an unnamed namespace to prevent collisions in that case).

Upvotes: 5

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