Why should I ever use inline code?

#define unsafe(i) ( (i) >= 0 ? (i) : -(i) )

[...]
unsafe(x++); // x is incremented twice!
unsafe(f()); // f() is called twice!
[...]

// header.hpp
#include <iosfwd> // for std::ostream&

#ifndef HEADER_HPP
#define HEADER_HPP

template <typename IteratorT>
/*inline*/ void print_all(IteratorT begin, IteratorT end, std::ostream& out, char sep = ' ') {
    while (begin != end)
        out << *begin++ << sep;
}

#endif // !defined(HEADER_HPP)

// test.cpp
#include <iostream>
#include <vector>
#include "header.hpp"

void test_function() {
    using namespace std;
    vector<int> vec;
    int x;
    while (cin >> x) vec.push_back(x);
    cout << "Results: ";
    print_all(vec.begin(), vec.end());
    cout << endl;
}

// main.cpp
#include <iostream>
#include <vector>
#include "header.hpp"

void test_function(); // Forward declaration.

int main() {
    using namespace std;
    vector<int> xyz(23, 42);
    print_all(xyz);
    cout << endl << "Test call:" << endl;
    test_function();
}

$ g++ -Wall -pedantic main.cpp test.cpp

CPoint
{
  public:

    inline int x() const { return m_x ; }
    inline int y() const { return m_y ; }

  private:
    int m_x ;
    int m_y ;

};