Inline Functions in C++
Inline Functions
Consider a function that returns a reference to the shorter of its two string parameters:
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// finder shorter of two strings
const string &shorterString(const string &s1, const string &s2) {
return s1.size() < s2.size() ? s1 : s2;
}
The benefits of using a function call include:
- It is easier to read and understand.
- It is easier to make changes than to find and change every occurrence of the equivalent expression.
- It is easier to reuse this piece of code.
- It is easier to test the code.
However, there is one potential drawback to making such a small piece of code a function. Calling a function is slower than evaluating the equivalent expression. This is because a function call does a lot of work: registers are saved before the call and restored after the return; stack space are allocated; the arguments are copied; and the program branches to a new location. This can result in a substantial performance penalty.
Inline Functions Avoid Function Call Overhead
A function specified as inline is expanded everywhere it is invoked. If we make the shorterString an inline function, then this call:
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cout << shorterString(s1, s2) << endl;
will be expanded during compilation into:
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cout << (s1.size() < s2.size() ? s1 : s2) << endl;
The run-time overhead of making shorterString a function is thus removed.
To define the shorterString function as an inline function, we just put the keyword inline before the function’s return type.
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// inline version: find shorter of two strings
inline const string &shorterString(const string &s1, const string &s2) {
return s1.size() < s2.size() ? s1 : s2;
}
Note that the inline specification is only a request to the compiler. The compiler may choose to ignore the request if it is a recursive function or a super long function.
Unlike other function definitions, inlines should be defined in header files. Whenever an inline function is added to or changed in a header file, every source file that uses that header must be recompiled.