Inline when the function is small and you''d like the body to replace its usage in every instance. For example:

int min(int a, int b){  return (a < b) ? a : b;} 

This kind of function used to be coded as a C macro, but macros don''t do typechecking (and you have to be careful to parenthesize correctly, or unexpected behaviors could occur. Since it''s such a small function, the overhead of allocating memory on the stack and so forth is so much more than the function body that it''s better to actually place that code everywhere you call min(). In such cases, make the function inline.

Also inline where there may be a lot of compund usage, like arithmetic operators:

class vector3d{public:...inline float &operator[] (int j);inline vector3d &operator*= (float s);// note that the next function is _not_ inlinefriend float &operator* (vector3d &v1, vector3d &v2);...private:  float coords[4];};// inline functionfloat &vector3d::operator[] (int j){  return coords[j % 4];}// inline functionvector3d &vector3d::operator*= (float s){  for(int j = 0; j < 4; ++j) coords[j] *= s;  return *this;}// friend function - dot productfloat &operator* (vector3d &v1, vector3d &v2){  // ignore w-coord; it''s for homogeniety  return (v1[0]*v2[0] + v1[1]*v2[1] + v1[2]*v2[2]);} 

The [] operator is better inlined because it is used so heavily in other operators that in-place expansion speeds up execution remarkably.