C++ const keyword

Posted on 2014-01-14
Last Modified: 2014-01-16
What's the difference between:
MyFunction(int i);
MyFunction(const int i);

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When would I want to throw in the const keyword? Can I also throw it in the return value? e.g.
int MyFunction();
const int MyFunction();

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Question by:deleyd
LVL 86

Accepted Solution

jkr earned 60 total points
ID: 39781152
Yes, both is possible. The key difference is that both the argument and the return value are considered to be unchangeable by the compiler. IOW, 'i' will be a constant value when be passed in a call and (if applied) the return value will be treated as a const value. See also
LVL 86

Assisted Solution

jkr earned 60 total points
ID: 39781158

>> When would I want to throw in the const keyword?

Always when you are sure that the value isn't to be changed. The Wikipedia article will provide some more explanatory examples ;o)

Assisted Solution

philrosenberg earned 20 total points
ID: 39781935
It is more relevant when passing references or pointers. For example if I have std::vector<double>, which is basically a C++ array of doubles if you are not used to using the std template library. It could have many elements and I don't want to copy them all when I pass it to a function, so I pass it by reference

int myFunc( std::vector<double> &myVector );

However this gives two related side affects, first if someone else wrote myFunc, I wouldn't know that my vector would be guaranteed to be unchanged, so if I needed it to stay unchanged it would make me think I needed to copy it myself then pass the copy. The second is that if I have a const std::vector<double> I cannot pass it in. For example

const std::vector<double> unityVector(5, 1.0); //create a const vector of 5 elements each with a value of 1.0, that cannot be changed
int result=myFunc(unityVector); //won't compile

To fix this, we instead use

int myFunc( const std::vector<double> &myVector );

In your case you are passing by value, so it actually makes very little difference, basically by making i const you are ensuring that you don't accidentally change i within your function. You may consider this a good thing, but I think some people find it a bit misleading as they expect consts to be references.

When it comes to the return type, if you return a const then it cannot be modified until after it has been assigned so if you had

const int myFunc( int i );
int increment( int &i )
    return i;

The following wouldn't compile
int myInt=++myFunc(5); //cannot increment the const return
iny myOtherInt=increment(myFunc(5)); //still doesn't work as I can't pass the const returned value into the increment function

but the following would be fine
int myInt=myFunc(5);

Note also that the following fails
increment(5); //5 is a const so cannot be passed into the function which takes a non const
LVL 16

Assisted Solution

HooKooDooKu earned 20 total points
ID: 39782307
As I see it, the 'const' key word has two purposes:
1. It's a way to check yourself to make sure you don't try to accidentally change a value you didn't intent to.
2. It gives the compiler a hint on how to optimize code.  

Simple Example:
for( int i = 0; i < x; i++ )
{ do stuff }

If 'x' above is declared as const, the compiler can utilize that information to optimize this loop by doing things like loading 'x' into a register, and simply referencing that register each time it needs to decide if the for...loop should continue.

But if 'x' is NOT declared as const, the compiler must fetch the value of 'x' each time it needs to decide if the for...lop should continue because something inside the for...loop could be changing the value of 'x'.

Author Closing Comment

ID: 39785937
Thank you for the explanations. (I apologize I should have made the question worth more points as the answer was a bit complicated.)

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