Ch 5 Exception

Compile time

• Syntax error, type errors

Link-time error

Run-time error

• detected by the computer
• detected by a library
• detected by user code

Logical error(?)

Run-time error

Find (possible) errors.

Possible errors

• negative or zero area?
• zero divisor.

who will handle negative or zero area

• Caller(5.5.1): Main function
• Callee(5.5.2): area, framed_area

Caller for area

if(x<=0) error("non-positive x");
if(y<=0) error("non-positive x");

We will see what the error function means later.

Callee for framed_area

int framed_area(int x, int y){
    constexpr int frame_width=2;
    if( x-frame_width<=0 || y-frame_width<=0 ){
        error("error called by framed_area()");
    }// Error handling
    return area(x-frame_width,y-frame_width);
}

Apply previous callee's solution to area()

int area(int length, int width){
    // Error handling
    return length*width;
}

Definition of error

inline void error(const string& s)
{
    throw runtime_error(s);
}

http://www.stroustrup.com/Programming/std_lib_facilities.h

Exception

If a function finds an error that it cannot handle, it does not return normally, instead it throws an exception.

try{

    // Something wrong! Throw an exception.

}
catch(Exception-type1 & e ){
    // Do something
}
catch(Exception-type2 & e ){
    // Do something
}
...

Example

Someone uses cerr instead of cout to report an error.

Check: With an exception of type out_of_range, try and catch the following out_of_range exception.

Scope(8.4)

• global scope
• namespace scope
• class scope
• local scope: { } ,parameters in function
• statement scope : for

Example scope1

void f(int x){ //f:global, x: local
    int z=x+7; //z:local
}
void g(int x){  //g: global, x: local
    int f=x+2; //f: local
    return 2*f;
}

Fun questions

Make a max and abs(absolute value) function.

int max(int a, int b);
int abs(int a);

$$abs(x)=|x|$$

Example scope2

class Hello{
    private:
        vector<int> v; // v: class scope
    public:
        int largest()// As absolute values
        {
            int r=0; //r:local
            for(int i=0;i<v.size();i++){ //i: for-statment scope
                r=max(r, abs(v[i]));
            }
            return r;
        }
        // No r here.
};
//No v here

Example scope3

int x;// global x
int y;// global y
int f(){
    int x; //x: local, hide global x
    x=7;
    {
        int x=y;//local x, hide the previous local x
        ++x;
    }
    ++x;
    return x;
}

A common mistake on functions

void f(){
    void g(){

    }
}

Nested function is not legal in c++ (except lambda).

[8.5] function call and return

declaration

double f(int a, int b);

definition

double f(int a, int b){return a*b;}

void

void g(int a);

void means "does not return a value".
Here name a,b are not important when you implement.

Example: my_find in [273p]

my_find searches for s in vs starting at hint

int my_find( vector<string> vs, string s, int hint){
    if(hint < 0|| hint > vs.size()) hint=0;
    for(int i=hint;i<vs.size();++i)
        if(vs[i]==0) return i;
    if(hint > 0){ //Check before hint.
        for(int i=hint;i vs.size();++i)
            if(vs[i]==0) return i;
    }
    return -1; //Not found.

}

If you just do not want to use a hint, we leave 3rd argument as unnamed.

int my_find( vector<string> vs, string s, int){

    for(int i=-;i<vs.size();++i)
        if(vs[i]==0) return i;
    return -1; //Not found.

}

Pass by value

Check.Make a print function which prints values of 'vector' as the following way.

vector<double> v {1,2,3,4,5};
print(v);
// (1,2,3,4,5)

Return by reference

void print(const vector<double> & v)

What is benefit here?

homework

Make a swap function which swaps two int values.

int a=2, b=3;
swap(a,b);
//a=3, b=2