C++ Const Correctness – When to Make a Fundamental Parameter Const

c++const-correctnessconstants

I recently had an exchange with another C++ developer about the following use of const:

void Foo(const int bar);

He felt that using const in this way was good practice.

I argued that it does nothing for the caller of the function (since a copy of the argument was going to be passed, there is no additional guarantee of safety with regard to overwrite). In addition, doing this prevents the implementer of Foo from modifying their private copy of the argument. So, it both mandates and advertises an implementation detail.

Not the end of the world, but certainly not something to be recommended as good practice.

I'm curious as to what others think on this issue.

Edit:

OK, I didn't realize that const-ness of the arguments didn't factor into the signature of the function. So, it is possible to mark the arguments as const in the implementation (.cpp), and not in the header (.h) – and the compiler is fine with that. That being the case, I guess the policy should be the same for making local variables const.

One could make the argument that having different looking signatures in the header and source file would confuse others (as it would have confused me). While I try to follow the Principle of Least Astonishment with whatever I write, I guess it's reasonable to expect developers to recognize this as legal and useful.

Best Answer

Remember the `if(NULL == p)` pattern ?

There are a lot of people who will tell a "you must write code like this":

if(NULL == myPointer) { /* etc. */ }

instead of

if(myPointer == NULL) { /* etc. */ }

The rationale is that the first version will protect the coder from code typos like replacing "==" with "=" (because it is forbidden to assign a value to a constant value).

The following can then be considered an extension of this limited if(NULL == p) pattern:

Why const-ing params can be useful for the coder

No matter the type, "const" is a qualifier that I add to say to the compiler that "I don't expect the value to change, so send me a compiler error message should I lie".

For example, this kind of code will show when the compiler can help me:

void bar_const(const int & param) ;
void bar_non_const(int & param) ;

void foo(const int param)
{
   const int value = getValue() ;

   if(param == 25) { /* Etc. */ } // Ok
   if(value == 25) { /* Etc. */ } // Ok

   if(param = 25) { /* Etc. */ } // COMPILE ERROR
   if(value = 25) { /* Etc. */ } // COMPILE ERROR

   bar_const(param) ;  // Ok
   bar_const(value) ;  // Ok

   bar_non_const(param) ;  // COMPILE ERROR
   bar_non_const(value) ;  // COMPILE ERROR

   // Here, I expect to continue to use "param" and "value" with
   // their original values, so having some random code or error
   // change it would be a runtime error...
}

In those cases, which can happen either by code typo or some mistake in function call, will be caught by the compiler, which is a good thing.

Why it is not important for the user

It happens that:

void foo(const int param) ;

and:

void foo(int param) ;

have the same signature.

This is a good thing, because, if the function implementer decides a parameter is considered const inside the function, the user should not, and does not need to know it.

This explains why my functions declarations to the users omit the const:

void bar(int param, const char * p) ;

to keep the declaration as clear as possible, while my function definition adds it as much as possible:

void bar(const int param, const char * const p)
{
   // etc.
}

to make my code as robust as possible.

Why in the real world, it could break

I was bitten by my pattern, though.

On some broken compiler that will remain anonymous (whose name starts with "Sol" and ends with "aris CC"), the two signatures above can be considered as different (depending on context), and thus, the runtime link will perhaps fail.

As the project was compiled on a Unix platforms too (Linux and Solaris), on those platforms, undefined symbols were left to be resolved at execution, which provoked a runtime error in the middle of the execution of the process.

So, because I had to support the said compiler, I ended polluting even my headers with consted prototypes.

But I still nevertheless consider this pattern of adding const in the function definition a good one.

Note: Sun Microsystems even had the balls to hide their broken mangling with an "it is evil pattern anyway so you should not use it" declaration. see http://docs.oracle.com/cd/E19059-01/stud.9/817-6698/Ch1.Intro.html#71468

One last note

It must be noted that Bjarne Stroustrup seems to be have been opposed to considering void foo(int) the same prototype as void foo(const int):

Not every feature accepted is in my opinion an improvement, though. For example, [...] the rule that void f(T) and void f(const T) denote the same function (proposed by Tom Plum for C compatibility reasons) [have] the dubious distinction of having been voted into C++ “over my dead body”.

Source: Bjarne Stroustrup
Evolving a language in and for the real world: C++ 1991-2006, 5. Language Features: 1991-1998, p21.
http://www.stroustrup.com/hopl-almost-final.pdf

This is amusing to consider Herb Sutter offers the opposite viewpoint:

Guideline: Avoid const pass-by-value parameters in function declarations. Still make the parameter const in the same function's definition if it won't be modified.

Source: Herb Sutter
Exceptional C++, Item 43: Const-Correctness, p177-178.

Related Solutions

C++ – Effect of Using const on Function Parameters

const is 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).

C++ – Is ‘const’ Necessary in Function Parameters When Passing by Value?

First, it's just an implementation detail, and if you put const there, don't put it in the declaration set (header). Only put it in the implementation file:

// header
void MyFunction(int age, House &purchased_house);

// .cpp file
void MyFunction(const int age, House &purchased_house);
{
    ...
}

Whether or not a parameter is const in a definition is purely an implementation detail, and should not be part of the interface.

I've not seen this sort of thing often, and i also don't do this. Having the parameter const would confuse me more often than help, because i would immediately pattern-match-fail it to "const int &age" :) The matter of course is entirely different from having const at another level:

// const is a *good* thing here, and should not be removed,
// and it is part of the interface
void MyFunction(const string &name, House &purchased_house);
{
    ...
}

In this case, the const will affect whether the function can change the caller's argument. Const in this meaning should be used as often as possible, because it can help ensuring program correctness and improve self-documenting the code.