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

Does Inlining Templates Meant Anything?

Despite templates being defined within headers, the use of the inline keyword with them has significance due to potential compiler optimizations.

Compiler's Inline Handling

Modern compilers can certainly determine when inlining a function is beneficial. However, the inline keyword serves a different purpose specific to templates.

Template Inlining Semantics

Function templates exist in two forms: non-fully specialized and fully specialized.

• Non-fully specialized: Contain unknown types and may not be inlined by default.
• Fully specialized: Use only known types and require the inline keyword to be explicitly specified.

Example

Consider the following code:

#include "tpl.h"

int main() {
  f(5);  // Non-fully specialized
  g(5);  // Fully specialized
  return 0;
}

tpl.h:

template<class T> void f(T);
template<class T> inline T g(T);

Compiling this code results in an error for g since it's a fully specialized template and inline was not specified.

Rule of Thumb

To avoid confusion and ensure consistent behavior, follow the rule of thumb:

Write inline if you mean it.

By explicitly indicating the inline nature of templates, you make it clear to both the compiler and other developers that these functions should be inlined if possible.

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