A Summary of Interface Principles

ByeongGil Jeon

Interfacesclass Csv {

public:Csv(istream& fin = cin, string sep = ",") :

fin(fin), fieldsep(sep) {}

int getline(string& str);string getfield(int n);int getnfield() const { return nfield; }

private:...

};

A Set of Principles from the book

• Hide implementation details

• Choose a small orthogonal set of primitives

• Don’t reach behind the user’s back

• Do the same thing the same way everywhere

Hide Implementation Details(1)

• Terms: Information hidingEncapsulationAbstractionModularization

Hide Implementation Details(2)

• Avoid global variables• Against publicly visible data

However,Extern FILE __job[_NFILE];#define stdin (&__job[0])#define stdout (&__job[1])#define stderr (&__job[2])

Two underscores for visible private names

Choose a Small Orthogonal Set of Primitives(1)

• A large interface is harder to write and maintain

• Example (functions that will write a single character to an output stream)char c;putc(c, fp);fputc(c, fp);fprintf(fp, “%c”, c);fwrite(&c, sizeof(char), 1, fp);

Multiple ways of doing the same thingNot all are necessary

Choose a Small Orthogonal Set of Primitives(2)

• Do one thing, and do it well• Don’t add to an interface just

because it’s possible to do so• Don’t fix the interface when it’s the

implementation that’s broken

• For instance, rather than having many functions, it would be better to have one function that was best

Don’t Reach behind the user’s back

• Should not write secret files and variables or change global data• The use of one interface should not demand another one just for the

convenience• Should be circumspect about modifying data in its caller

Example char str[] = "now # is the time for all # good men to come to the # aid of their

country"; char delims[] = "#"; char *result = NULL; result = strtok( str, delims ); while( result != NULL ) { printf( "result is \"%s\"\n", result ); result = strtok( NULL, delims ); }

Reference: http://www.cppreference.com/wiki/string/c/strtokEach subsequent call, with a NULL as the value of str, starts searching from the

saved pointer

Do the same thing the same way everywhere

• Should be easy to predict how to use an unfamiliar function– Example:• the algorithms for STL containers present a

very uniform interface• The standard I/O function fread and fwrite

would be easier to remember if they looked like read and write functions they are based on

A Set of Principles from the book

• Hide implementation details

• Choose a small orthogonal set of primitives

• Don’t reach behind the user’s back

• Do the same thing the same way everywhere

Question?