CODEWITHSUNDEEP
c++c++11
Hello world
Hello world

Reputation: 3

Is putting const before and after a function header equivalent?

While reading some code, I didn't understand the meaning of the const modifier. For example, are the following two lines equivalent?

void output() const;
const void output();

Upvotes: 0

Views: 191

Answers (2)

arsdever
arsdever

Reputation: 1262

For any case const T function(ARGS ...) means that the function returns const T object.

For T function(ARGS ...) const things are different. First, you can't have this kind of function wherever you want. This function must be only as a non-static member function of user-defined type (AKA struct or class).

class A
{
  T foo() const; // const function
  const T foo1() const; // wanted to show that you can use both together.
  static const T foo2(); // this is another option
  static T foo3() const; // error: static function cant be const
  static const T foo4() const; // error: the same as above

}

Non-static const member functions promise, that the object you call the function on, can't be changed.

From the above example

An a;
// here you can call any function on the object a that compiles
const A b;
// here you can call only the functions (for non-static ones) that have the `const` modifier

Also, note, that if you try a member variable in the const function, you will get a compiler error:

class B
{
  int var;
  void foo() const {var = 1;} // error: can't modify value
  void foo1() {var = 1;} // alright
}

To make it possible to modify a variable in the const function, you should declare the variable as a mutable one.

class C
{
  mutable int var1;
  void foo() {var1 = 1;} // OK
  void bar() const {var1 = 1;} // Also OK
}

This might seem to be useless, but the use case is some kind of counters, etc.

Upvotes: 4

Adrian Costin
Adrian Costin

Reputation: 438

In void output() const, const means that you "promise" the compiler not to modify the member data of the class using that output() function. Which means output() will only be used as a read-only function.

const void output() means that the function return type will be constant.

Upvotes: 4

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