const to Non-const Conversion in C

Question

I m really annoyed by const keyword these days  as I m not quite familiar with it  I had a vector that stores all const pointers like vector lt const BoxT lt T gt    gt   Q exclude  and in the constructor of another class  I need an element in this queue to be passed in as a parameter and assign it to a non-const member  My question is    How do I assign a const variable to a non-const variable  I know this doesn t make sense because after all  a const is a const  and should not be changed by any mean  But that annoying member variable REALLY has to be changed during the process  I might also change the data type in the vector to be non-const  but that would be too much work  Or does anyone know how to avoid such situation

User · Answer

Changing a constant type will lead to an Undefined Behavior.

However, if you have an originally non-const object which is pointed to by a pointer-to-const or referenced by a reference-to-const then you can use const_cast to get rid of that const-ness.

Casting away constness is considered evil and should ~~not~~ be avoided. You should consider changing the type of the pointers you use in vector to non-const if you want to modify the data through it.

User · Answer

void SomeClass  changeASettingAndCallAFunction   const       someSetting   0    Can t do this     someFunctionThatUsesTheSetting        Another solution is to call said function in-between making edits to variables that the const function uses  This idea was what solved my problem being as I was not inclined to change the signature of the function and had to use the  changeASettingAndCallAFunction  method as a mediator   When you call the function you can first make edits to the setting before the call  or  if you aren t inclined to mess with the invoking place  perhaps call the function where you need the change to the variable to be propagated  like in my case    void SomeClass  someFunctionThatUsesTheSetting   const          We really don t want to touch this functions implementation      ClassUsesSetting  classUsesSetting   ClassUsesSetting  PropagateAcrossClass someSetting                    Do important stuff            void SomeClass  changeASettingAndCallAFunction   const        someFunctionThatUsesTheSetting                     Have to do this            void SomeClass  nonConstInvoker        someSetting   0      changeASettingAndCallAFunction        Now  when some reference to  someFunctionThatUsesTheSetting  is invoked  it will invoke with the change to someSetting

User · Answer

Leaving this here for myself  If I get this error  I probably used const char  when I should be using char  const  This makes the pointer constant  and not the contents of the string  const char  const makes it so the value and the pointer is constant also

User · Answer

You can assign a const object to a non-const object just fine  Because you re copying and thus creating a new object  constness is not violated   Like so   int main        const int a   3     int b   a      It s different if you want to obtain a pointer or reference to the original  const object   int main        const int a   3     int amp  b   a           or int  b    amp a         error  invalid initialization of reference of type  int amp   from            expression of type  const int    You can use const cast to hack around the type safety if you really must  but recall that you re doing exactly that  getting rid of the type safety  It s still undefined to modify a through b in the below example   int main        const int a   3     int amp  b   const cast lt int amp  gt  a       b   3      Although it compiles without errors  anything can happen including opening a black hole or transferring all your hard-earned savings into my bank account   If you have arrived at what you think is a requirement to do this  I d urgently revisit your design because something is very wrong with it

User · Answer

The actual code to cast away the const-ness of your pointer would be   BoxT lt T gt    nonConstObj   const cast lt BoxT lt T gt    gt  constObj     But note that this really is cheating   A better solution would either be to figure out why you want to modify a const object  and redesign your code so you don t have to     or remove the const declaration from your vector  if it turns out you don t really want those items to be read-only after all

[c++] const to Non-const Conversion in C++

Examples related to c++

Examples related to constants