Does the `inline` Keyword Still Matter When Using C Templates?

Impact of the 'Inline' Keyword on Template Inlining

While templates reside in headers, compilers possess the ability to determine the benefits of function inlining. However, this raises the question of whether the 'inline' keyword serves a purpose in conjunction with templates.

Changing Role of the 'Inline' Keyword

Contrary to its original interpretation as a directive for inlining functions, the 'inline' keyword has evolved into a means of preventing ODR (One Definition Rule) violations. This semantic shift has led to ambiguity regarding the actual meaning of 'inline'.

Function Template Inlining Behavior

The 'inline' keyword is not irrelevant for function templates. C explicitly defines that function templates are not inherently inline. This is demonstrated in the following example:

// tpl.h
template<class T> void f(T) {}
template<class T> inline T g(T) {}

When compiling this code with explicit instantiations, a multiple definition error occurs for g because it is not declared as inline.

Best Practices

Vandevoorde and Josuttis recommend a rule of thumb: Use 'inline' explicitly to indicate your intention of inlining. This consistent approach simplifies code maintenance and ensures the compiler makes optimal inlining decisions.

Conclusion

In conclusion, the 'inline' keyword still plays a role with templates, providing clarity and ensuring proper inlining behavior. By using 'inline' when necessary and following consistent guidelines, developers can improve code efficiency and readability.

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