Function Definition

CorX · 05-12-2009, 09:16 AM

I was wondering what the "const" after the function definition means. Not the first one i.e the type but the second one (outside the brackets).

This code was inside a class if that means anything.

Code:

 const string& getSomething() const { return something; }

what is this -------------------------------------------------^^^^

Thanks,
CorX

Brafil · 05-12-2009, 09:29 AM

It means this function won't modify any attribute of the instance that is not declared as mutual. Only such functions can be called within constant instances. The same with volatile.

CorX · 05-12-2009, 09:36 AM

Thanks, I understand now.

bithub · 05-12-2009, 09:40 AM

Quote:

It means this function won't modify any attribute of the instance that is not declared as mutual

I think you meant mutable.

Quote:

Only such functions can be called within constant instances

No, they can be called from constant and non-constant class instances. A const instance can only call const functions, whereas a non-const instance can call either.

Quote:

The same with volatile

Volatile has nothing to do with const.

Brafil · 05-12-2009, 09:54 AM

Ooops... Was thinking about mutually-recursive acronyms...

1. This means that "only" functions declared const can be called from constant instances. It doesn't mean they can't be called from other ones.

2. Volatile means volatile instances can be passed to this method as this.

brewbuck · 05-12-2009, 10:28 AM

Quote:

Originally Posted by bithub

Volatile has nothing to do with const.

Volatile and const are intimately related. Anywhere that const may syntactically appear, volatile may appear. And like const, the only way to expunge volatile from a type is by using const_cast<>.

There is a reason why const and volatile are referred to as "CV qualifiers." C being Const, V being Volatile.

linuxdude · 05-14-2009, 09:39 AM

What does this const actually mean again? I don't know what it does.

Quote:

any attribute of the instance

is a vague statement to me.

**laserlight** · 05-14-2009, 09:58 AM

"Any attribute of the instance" can be interpreted to mean "any member variable of the object".

linuxdude · 05-14-2009, 11:56 AM

Ok, so it's a guarantee that member variables not defined as mutable aren't changed.

Code:

class s {
  public:
   void func (void) const {
      x = 1;
   }
  private:
   mutable int x;
};

Works fine, but

Code:

class s {
  public:
   void func (void) const {
      x = 1;
   }
  private:
   int x;
};

produces a

Code:

error: assignment of data-member ‘s::x’ in read-only structure

Thanks

05-12-2009, 09:16 AM	#1
CorX Registered User Join Date: Mar 2009 Posts: 8	Function Definition I was wondering what the "const" after the function definition means. Not the first one i.e the type but the second one (outside the brackets). This code was inside a class if that means anything. Code: const string& getSomething() const { return something; } what is this -------------------------------------------------^^^^ Thanks, CorX
CorX is offline

05-12-2009, 09:29 AM	#2
Brafil Making mistakes Join Date: Dec 2008 Posts: 347	It means this function won't modify any attribute of the instance that is not declared as mutual. Only such functions can be called within constant instances. The same with volatile.
Brafil is offline

05-12-2009, 09:36 AM	#3
CorX Registered User Join Date: Mar 2009 Posts: 8	Thanks, I understand now.
CorX is offline

05-12-2009, 09:54 AM	#5
Brafil Making mistakes Join Date: Dec 2008 Posts: 347	Ooops... Was thinking about mutually-recursive acronyms... 1. This means that "only" functions declared const can be called from constant instances. It doesn't mean they can't be called from other ones. 2. Volatile means volatile instances can be passed to this method as this.
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05-14-2009, 09:58 AM	#8
laserlight C++ Witch Join Date: Oct 2003 Location: Singapore Posts: 10,354	"Any attribute of the instance" can be interpreted to mean "any member variable of the object". __________________ C + C++ Compiler: MinGW port of GCC Build + Version Control System: SCons + Bazaar Look up a C/C++ Reference and learn How To Ask Questions The Smart Way
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05-14-2009, 11:56 AM	#9
linuxdude Registered User Join Date: Mar 2003 Location: Louisiana Posts: 926	Ok, so it's a guarantee that member variables not defined as mutable aren't changed. Code: class s { public: void func (void) const { x = 1; } private: mutable int x; }; Works fine, but Code: class s { public: void func (void) const { x = 1; } private: int x; }; produces a Code: error: assignment of data-member ‘s::x’ in read-only structure Thanks
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