When Should You Use the `inline` Keyword with C Function Templates?

Does Using Inline Keyword with Templates Serve a Purpose?

In C , templates offer an efficient way to define functions that can operate on different data types. By declaring a template function as inline, you instruct the compiler to substitute the actual template parameters directly into the function body during compilation. This optimization can improve performance by eliminating the overhead of function calls.

However, there is a misconception that modern compilers automatically inline function templates, making the inline keyword redundant. While modern compilers may often inline functions as appropriate, they rely on the inline keyword to guarantee inlining in specific scenarios.

For example, the inline keyword is crucial in defining explicit specializations of function templates. Explicit specialization allows you to customize the behavior of a template function for specific types. C semantics require the inline keyword to be used when defining explicit specializations of non-inline function templates.

Consider the following example:

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

// a.cc
#include "tpl.h"

// Specialization without inline keyword (error)
int g<>(int);

// b.cc
#include "tpl.h"

// Specialization with inline keyword (OK)
inline void f<>(int);

As seen above, omitting the inline keyword when defining explicit specializations of non-inline function templates leads to compilation errors. This demonstrates the necessity of using the inline keyword to ensure inlining in such cases.

Therefore, while modern compilers may optimize function inlining, the inline keyword still plays a significant role in explicitly instructing the compiler to inline specific functions, including explicit specializations of function templates.

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