Why mutable c

The flag function here is a constant function. The mutable function can be changed as per necessity. It gets changed dynamically whenever the mutable variable is called. This value is returned and sent it as a Boolean variable. PlacedOrder "Veg Lasagne" ; c1. BillChanged ; c1. Now let us try to display by uncommenting the code as below. We are trying to display the variable which was not declared as mutable and is constant.

Make below changes in the main. The error states that num is not a mutable value and it is only read-only object as it is declared a constant. Does the 'mutable' keyword have any purpose other than allowing the variable to be modified by a const function? Ask Question. Asked 13 years, 1 month ago. Active 6 years, 7 months ago. Viewed k times. Improve this question. M4N Rob Rob A question though, if you are not modifying anything, why do u need to use a mutex in first place?

I just want to understand this. A really naive example, imagine if I only give non-const handles to friends, enemies get a const handle. Friends can modify, enemies cannot. Note: here's a great example of using the keyword mutable : stackoverflow. Add a comment. Active Oldest Votes. Improve this answer. KeithB KeithB Richard: you're missing the point. There is no "logical const" keyword, true, rather, it's a conceptual differentiation the programmer makes to decide which members should be excluded by being made mutable, based on an understanding of what constitutes the logical observable state of the object.

Why does one need mutable on lambdas? Wouldn't it suffice to capture variable by reference? Giorgio: The difference is that the modified x within the lambda remains within the lambda, i. The change is not visible outside, the original x is still unchanged. Consider that lambdas are implemented as functor classes; captured variables correspond to member variables.

Show 10 more comments. With your const reference or pointer you are constrained to: only read access for any visible data members permission to call only methods that are marked as const. Community Bot 1 1 1 silver badge. Dan L Dan L 1, 1 1 gold badge 10 10 silver badges 7 7 bronze badges.

I don't agree with because it forces the API client to destroy the const protection of the objects. Basically it lets you bypass constness on arbitrary members at a specific call site , while mutable let's you remove const on a specific member at all call sites. The method is logically const since the final state is the same as the initial, but you want to make that transient change.

Frank Szczerba. Examples the author gives include caching and temporary debugging variables. John Millikin John Millikin k 38 38 gold badges silver badges bronze badges. I think this link gives the best example of a scenario where mutable is helpful. It almost appears they are exclusively used for debugging. The usage of mutable can make the code more readable and cleaner. In the following example, the read can be const as expected.

There's one extremely popular use case: ref counts. It's useful in situations where you have hidden internal state such as a cache. Adam Rosenfield Adam Rosenfield k 94 94 gold badges silver badges bronze badges. Here are some valid reasons to declare and use mutable data: Thread safety. Declaring a mutable boost::mutex is perfectly reasonable.

Counting the number of calls to a function, given some or all of its arguments. Computing some expensive answer, and then storing it for future reference rather than recomputing it again. Lloyd Lloyd Good answer, except for comment regarding mutable being a "hint". This makes it seem as if the mutable member sometimes won't be mutable if the compiler placed the object into ROM.

The behaviour of mutable is well defined. Apart from placing a const object in read-only memory, the compiler may also decide to optimize const fucntion calls out of a loop, for example. A mutable statistics counter in an otherwise const function will still allow such optimization and count only one call instead of preventing optimization just for the sake of counting more calls.

HagenvonEitzen - I'm pretty sure that is incorrect. A compiler cannot hoist functions out of a loop unless it can prove there are no side effects. Lambda function declaration. Fundamental types. Function types. Compound types. Storage duration specifiers. Default initialization. Value initialization. Zero initialization. Copy initialization. Direct initialization.

Aggregate initialization. Constant initialization. Reference initialization. Value categories. Order of evaluation. Operator precedence. Alternative representations.