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Thinking in C++
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Inlines inside classes

To define an inline function, you must ordinarily precede the function definition with the inline keyword. However, this is not necessary inside a class definition. Any function you define inside a class definition is automatically an inline. For example:

//: C09:Inline.cpp
// Inlines inside classes
#include <iostream>
#include <string>
using namespace std;

class Point {
  int i, j, k;
  Point(): i(0), j(0), k(0) {}
  Point(int ii, int jj, int kk)
    : i(ii), j(jj), k(kk) {}
  void print(const string& msg = "") const {
    if(msg.size() != 0) cout << msg << endl;
    cout << "i = " << i << ", "
         << "j = " << j << ", "
         << "k = " << k << endl;

int main() {
  Point p, q(1,2,3);
  p.print("value of p");
  q.print("value of q");
} ///:~

Here, the two constructors and the print( ) function are all inlines by default. Notice in main( ) that the fact you are using inline functions is transparent, as it should be. The logical behavior of a function must be identical regardless of whether it’s an inline (otherwise your compiler is broken). The only difference you’ll see is in performance.

Of course, the temptation is to use inlines everywhere inside class declarations because they save you the extra step of making the external member function definition. Keep in mind, however, that the idea of an inline is to provide improved opportunities for optimization by the compiler. But inlining a big function will cause that code to be duplicated everywhere the function is called, producing code bloat that may mitigate the speed benefit (the only reliable course of action is to experiment to discover the effects of inlining on your program with your compiler).

Thinking in C++
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   Reproduced courtesy of Bruce Eckel, MindView, Inc. Design by Interspire