overhead（额外开销） for a function call
- the processing time required by a device prior to the execution of a command
- push a parameters
- push return address
- prepare return values
- pop all pushed
inline functions
-
an inline function is expanded in place,like a preprocessor macro,so the overhead of the function call is eliminated
-
repeat inline keyword at declaration and definition
-
an inline function definition may not generate any code in .obj file
inline function in header file
- so you can put inline functions' bodies in header file.then #include it where the function is needed
- never be afraid of multi-definition of inline functions,since they no body at all
- definitions of inline functions are just declarations
tradeoff of inline functions
- body of the called function is to be inserted into the caller
- this may expand the code size
- but deduces the overhead of calling time
- so it gains speed at the expenses of space
- in most time case,it is worth
- it is most much better than macro in C.it checks the types of parameters
inline may not in-line
- the compile does not have to honor your request to make a function inline.it might decide the function is too large or notice that it calls itself(recursion is not allowed or indeed possible for inline functions),or the feature might not be implemented for your particular compiler
inline inside classes
- any function you define inside a class declaration is automatically an inline
class Point{
int i,j,k;
public:
Point(){i=j=k=0;}
Point(int ii,int jj, int kk){i=ii,j=jj,k=kk;}
void print(string&msg = ""){
if(msg.size!=0) cout<<msg<<endl;
cout<<i<<j<<k<<endl;
}
｝;
int main(){
Point p,q(1,2,3);
p.print();
q.print();
} access functions
- they are small functions that allow you to read or change part of the state of an object---that is ,an internal variable or variables
reducing clutter
- member functions defined within classes use the Latin in situ(in place) and maintains that all definitions should be placed outside the class to keep the interface clean
class Point{
int i,j,k;
public:
Point();
Point(int ii,int jj, int kk);
void print(string&msg = "");
｝;
inline Point(){i=j=k=0;}
inline Point(int ii,int jj, int kk){i=ii,j=jj,k=kk;}
inline void print(string&msg = ""){
if(msg.size!=0) cout<<msg<<endl;
cout<<i<<j<<k<<endl;
}inline or not?
- inline:
- small function,2 or 3 lines
- Frequently called functions,e.g. inside loops
- not inline?
- very large functions,more than 20 lines
- recursive functions
