Difference between const method and method with const attributes

Question

What is the difference between const at the beginning:

const int MyClass::showName(string id){
...
}

And :

int MyClass::showName(const string id) {
...
}

And :

int MyClass::showName(string id) const{
...
}

Answer 1

const modifies the thing it's next to, like a word like "little". "A little yard with a shed" is different from "a yard with a little shed". Let's look at your code examples:

  const int MyClass::showName(string id) {

The returned integer is const. This has no effect because the caller receives a copy and had no ability to modify the original anyways.

 int MyClass::showName(const string id) {

The variable id is const. This means that code in showName() like id += "foo"; will be a syntax error because you aren't allowed to modify id.

  int MyClass::showName(string id) const {

The method showName() is const. Suppose you have member variables of your class, for instance

class MyClass {
  int something;
  int showName(string id) const;
}

We call showName() a const member function, meaning modifying other parts of the object isn't allowed inside showName(). Writing code in showName() like something = 1; is a syntax error because it tries to modify a member of MyClass.