Use of 'const' for function parameters
How far do you go with const? Do you just make functions const when necessary or do you go the whole hog and use it everywhere? For example, imagine a simple mutator that takes a single boolean parameter:
void SetValue(const bool b) { my_val_ = b; }
Is that const actually useful? Personally I opt to use it extensively, including parameters, but in this case I wonder if it's worthwhile?
I was also surprised to learn that you can omit const from parameters in a function declaration but can include it in the function definition, e.g.:
.h file
void func(int n, long l);
.cpp file
void func(const int n, const long l)
Is there a reason for this? It seems a little unusual to me.
constis pointless when the argument is passed by value since you will not be modifying the caller's object.
Wrong.
It's about self-documenting your code and your assumptions.
If your code has many people working on it and your functions are non-trivial then you should mark const any and everything that you can. When writing industrial-strength code, you should always assume that your coworkers are psychopaths trying to get you any way they can (especially since it's often yourself in the future).
Besides, as somebody mentioned earlier, it might help the compiler optimize things a bit (though it's a long shot).
The reason is that const for the parameter only applies locally within the function, since it is working on a copy of the data. This means the function signature is really the same anyways. It's probably bad style to do this a lot though.
I personally tend to not use const except for reference and pointer parameters. For copied objects it doesn't really matter, although it can be safer as it signals intent within the function. It's really a judgement call. I do tend to use const_iterator though when looping on something and I don't intend on modifying it, so I guess to each his own, as long as const correctness for reference types is rigorously maintained.
Sometimes (too often!) I have to untangle someone else's C++ code. And we all know that someone else's C++ code is a complete mess almost by definition :) So the first thing I do to decipher local data flow is put const in every variable definition until compiler starts barking. This means const-qualifying value arguments as well, because they are just fancy local variables initialized by caller.
Ah, I wish variables were const by default and mutable was required for non-const variables :)
The following two lines are functionally equivalent:
int foo (int a);
int foo (const int a);
Obviously you won't be able to modify a in the body of foo if it's defined the second way, but there's no difference from the outside.
Where const really comes in handy is with reference or pointer parameters:
int foo (const BigStruct &a);
int foo (const BigStruct *a);
What this says is that foo can take a large parameter, perhaps a data structure that's gigabytes in size, without copying it. Also, it says to the caller, "Foo won't* change the contents of that parameter." Passing a const reference also allows the compiler to make certain performance decisions.
*: Unless it casts away the const-ness, but that's another post.
Extra Superfluous const are bad from an API stand-point:
Putting extra superfluous const's in your code for intrinsic type parameters passed by value clutters your API while making no meaningful promise to the caller or API user (it only hampers the implementation).
Too many 'const' in an API when not needed is like "crying wolf", eventually people will start ignoring 'const' because it's all over the place and means nothing most of the time.
The "reductio ad absurdum" argument to extra consts in API are good for these first two points would be is if more const parameters are good, then every argument that can have a const on it, SHOULD have a const on it. In fact, if it were truly that good, you'd want const to be the default for parameters and have a keyword like "mutable" only when you want to change the parameter.
So lets try putting in const whereever we can:
void mungerum(char * buffer, const char * mask, int count);
void mungerum(char * const buffer, const char * const mask, const int count);
Consider the line of code above. Not only is the declaration more cluttered and longer and harder to read but three of the four 'const' keywords can be safely ignored by the API user. However, the extra use of 'const' has made the second line potentially DANGEROUS!
Why?
A quick misread of the first parameter char * const buffer might make you think that it will not modify the memory in data buffer that is passed in -- however, this is not true! Superfluous 'const' can lead to dangerous and incorrect assumptions about your API when scanned or misread quickly.
Superfluous const are bad from a Code Implementation stand-point as well:
#if FLEXIBLE_IMPLEMENTATION
#define SUPERFLUOUS_CONST
#else
#define SUPERFLUOUS_CONST const
#endif
void bytecopy(char * SUPERFLUOUS_CONST dest,
const char *source, SUPERFLUOUS_CONST int count);
If FLEXIBLE_IMPLEMENTATION is not true, then the API is “promising” not to implement the function the first way below.
void bytecopy(char * SUPERFLUOUS_CONST dest,
const char *source, SUPERFLUOUS_CONST int count)
{
// Will break if !FLEXIBLE_IMPLEMENTATION
while(count--)
{
*dest++=*source++;
}
}
void bytecopy(char * SUPERFLUOUS_CONST dest,
const char *source, SUPERFLUOUS_CONST int count)
{
for(int i=0;i<count;i++)
{
dest[i]=source[i];
}
}
That’s a very silly promise to make. Why should you make a promise that gives no benefit at all to your caller and only limits your implementation?
Both of these are perfectly valid implementations of the same function though so all you’ve done is tied one hand behind your back unnecessarily.
Furthermore, it’s a very shallow promise that is easily (and legally circumvented).
inline void bytecopyWrapped(char * dest,
const char *source, int count)
{
while(count--)
{
*dest++=*source++;
}
}
void bytecopy(char * SUPERFLUOUS_CONST dest,
const char *source,SUPERFLUOUS_CONST int count)
{
bytecopyWrapped(dest, source, count);
}
Look, I implemented it that way anyhow even though I promised not to – just using a wrapper function. It’s like when the bad guy promises not to kill someone in a movie and orders his henchman to kill them instead.
Those superfluous const’s are worth no more than a promise from a movie bad-guy.
But the ability to lie gets even worse:
I have been enlightened that you can mismatch const in header (declaration) and code (definition) by using spurious const. The const-happy advocates claim this is a good thing since it lets you put const only in the definition.
// Example of const only in definition, not declaration
struct foo { void test(int *pi); };
void foo::test(int * const pi) { }
However, the converse is true... you can put a spurious const only in the declaration and ignore it in the definition. This only makes superfluous const in an API more of a terrible thing and a horrible lie - see this example:
struct foo
{
void test(int * const pi);
};
void foo::test(int *pi) // Look, the const in the definition is so superfluous I can ignore it here
{
pi++; // I promised in my definition I wouldn't modify this
}
All the superfluous const actually does is make the implementer's code less readable by forcing him to use another local copy or a wrapper function when he wants to change the variable or pass the variable by non-const reference.
Look at this example. Which is more readable ? Is it obvious that the only reason for the extra variable in the second function is because some API designer threw in a superfluous const ?
struct llist
{
llist * next;
};
void walkllist(llist *plist)
{
llist *pnext;
while(plist)
{
pnext=plist->next;
walk(plist);
plist=pnext; // This line wouldn't compile if plist was const
}
}
void walkllist(llist * SUPERFLUOUS_CONST plist)
{
llist * pnotconst=plist;
llist *pnext;
while(pnotconst)
{
pnext=pnotconst->next;
walk(pnotconst);
pnotconst=pnext;
}
}
Hopefully we've learned something here. Superfluous const is an API-cluttering eyesore, an annoying nag, a shallow and meaningless promise, an unnecessary hindrance, and occasionally leads to very dangerous mistakes.