If a function is very small and time-critical, declare it `inline`

Reason

Some optimizers are good at inlining without hints from the programmer, but don't rely on it. Measure! Over the last 40 years or so, we have been promised compilers that can inline better than humans without hints from humans. We are still waiting. Specifying inline (explicitly, or implicitly when writing member functions inside a class definition) encourages the compiler to do a better job.

Example

inline string cat(const string& s, const string& s2) { return s + s2; }

Exception

Do not put an inline function in what is meant to be a stable interface unless you are certain that it will not change. An inline function is part of the ABI.

Note

constexpr implies inline.

Note

Member functions defined in-class are inline by default.

Exception

Function templates (including member functions of class templates A<T>::function() and member function templates A::function<T>()) are normally defined in headers and therefore inline.

Note

Consider making functions out of line if they are more than three statements and can be declared out of line (such as class member functions).