C++ “const” keyword explanation

Question

When reading tutorials and code written in C++, I often stumble over the const keyword.

I see that it is used like the following:

const          


        

Answer 1

The difference between the two is that the first has type void(char) and the second has type int()const.

A function that has such a type with const at the end can only be a member function of a class, and it means that the member function does not change the class value (which this refers to) as seen from outside the class. The compiler will check that to a degree, and any straight write to a class member in a const member function results in a compile time error, and the function can straightly only call const member functions on itself (special directives exist so you can tell the compiler that a member write won't change the class' value as seen from outside. This is done by the mutable keyword).

In the functions you presented, one had a parameter of type char const. Such a parameter cannot be changed inside its function. It has no effect on the function's type though, and no effect to the callers of the function.

Answer 2

The first function example is more-or-less meaningless. More interesting one would be:

void myfunc( const char *x );

This tells the compiler that the contents of *x won't be modified. That is, within myfunc() you can't do something like:

strcpy(x, "foo");

The second example, on a C++ member function, means that the contents of the object won't be changed by the call.

So given:

class {
  int x;
  void myfunc() const;
}

someobj.myfunc() is not allowed to modify anything like:

x = 3;