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Concept
Inline function
Summary
In the C and C++ programming languages, an inline function is one qualified with the keyword inline; this serves two purposes:
It serves as a compiler directive that suggests (but does not require) that the compiler substitute the body of the function inline by performing inline expansion, i.e. by inserting the function code at the address of each function call, thereby saving the overhead of a function call. In this respect it is analogous to the register storage class specifier, which similarly provides an optimization hint.
The second purpose of inline is to change linkage behavior; the details of this are complicated. This is necessary due to the C/C++ separate compilation + linkage model, specifically because the definition (body) of the function must be duplicated in all translation units where it is used, to allow inlining during compiling, which, if the function has external linkage, causes a collision during linking (it violates uniqueness of external symbols). C and C++ (and dialects such as GNU C and Visual C++) resolve this in different ways.
An inline function can be written in C or C++ like this:
inline void swap(int *m, int *n)
{
int tmp = *m;
m = *n;
n = tmp;
}
Then, a statement such as the following:
swap(&x, &y);
may be translated into (if the compiler decides to do the inlining, which typically requires optimization to be enabled):
int tmp = x;
x = y;
y = tmp;
When implementing a sorting algorithm doing lots of swaps, this can increase the execution speed.
C++ and C99, but not its predecessors K&R C and C89, have support for inline functions, though with different semantics. In both cases, inline does not force inlining; the compiler is free to choose not to inline the function at all, or only in some cases. Different compilers vary in how complex a function they can manage to inline. Mainstream C++ compilers like Microsoft Visual C++ and GCC support an option that lets the compilers automatically inline any suitable function, even those not marked as inline functions.
Official source
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