Const correctness for value parameters

Question

I know there are few question about const correctness where it is stated that the declaration of a function and its definition do not need to agree for value parameters. This is because the constness of a value parameter only matters inside the function. This is fine:

// header int func(int i);  // cpp int func(const int i) {     return i; } 

Is doing this really a best practice? Because I've never seen anyone do it. I've seen this quotation (not sure of the source) in other places this has been discussed:

"In fact, to the compiler, the function signature is the same whether you include this const in front of a value parameter or not."

"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."

The second paragraph says to not put the const in the declaration. I assume this is because the constness of a value parameter is meaningless as part of a interface definition. It is an implementation detail.

Based on this recommendation, is it also recommended for the pointer values of pointer parameters? (It is meaningless on a reference parameter since you can't reassign a reference.)

// header int func1(int* i); int func2(int* i);  // cpp int func1(int* i) {     int x = 0;      *i = 3; // compiles without error     i = &x; // compiles without error      return *i; } int func2(int* const i) {     int x = 0;      *i = 3; // compiles without error     i = &x; // compile error      return *i; } 

Summary: Making value parameters is useful to catch some logic errors. Is it a best practice? Do you go to the extreme of leaving the const out of the header file? Is it just as useful to const pointer values? Why or why not?

Some references:

C++ const keyword - use liberally? Use of 'const' for function parameters

An example of when const value parameters are useful:

bool are_ints_equal(const int i, const int j) {     if (i = j) { // without the consts this would compile without error         return true;     } else {         return false;     }     // return i = j; // I know it can be shortened } 

Answer 1

I've read many times that making value parameters in a function const is a bad thing to do because it's unnecessary.

However, I find it occasionally helpful to me as a check that my implementation doesn't do something I don't intend (as in the example at the end of your question).

So, while it may not add value to the caller, it does sometimes add a small bit of value to me as an implementer, and it doesn't take anything away from the caller. So I see no harm using it.

For example, I may be implementing a C function that takes a couple pointers to a buffer - a pointer to the start, and a pointer to the end. I'm going to put data in the buffer, but want to ensure that I don't overrun the end. So inside the function there's code that will increment a pointer as I'm adding data to it. Making the pointer to the end of the buffer a const parameter will ensure that I don't code up a bug that accidentally increments the end boundary pointer instead of the pointer I really should be incrementing.

So a fillArray function with a signature like this:

size_t fillArray( data_t* pStart, data_t* const pEnd); 

will prevent me from accidentally incrementing pEnd when I really mean to increment pStart. It's not a huge thing, but I'm pretty sure everyone who has programmed for any period of time in C has run across such a bug.