C++ Changing the values of members of a const struct [duplicate] - c++

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C/C++ changing the value of a const
(18 answers)
Closed 8 years ago.
I have a struct which is defined in types.h with the following code:
struct data_Variant {
FlightPlanSteeringDataRecord steeringData;
FlightPlanType flightPlan : 8;
MinitoteLegDataType legDataType : 8; // discriminent, either current or amplified
unsigned spare : 16;
union {
// currentLeg =>
CurrentLegDataRecord currentLegData;
// amplifiedLeg =>
AmplifiedLegDataRecord amplifiedLegData;
} u;
};
I am then trying to pass an instance of that struct as a parameter to a function in a C++ source file called dialogue.cpp:
void dialogue::update( const types::data_Variant& perfData){
...
}
I now want to change the value of some of the members of that struct inside this update() function. However, if I try doing this as I usually would, i.e.
perfData.etaValid = true;
I get a compile error which says: "C2166: l-value specifies const object". As I understand, this is because perfData has been declared as a constant variable. Am I correct in thinking this?
Since I didn't write this part of the code, but only want to use it to update the value displayed on the GUI, I don't really want to change the perfData variable by removing the const keyword, in case I break something else. Is there any way to change the value of a variable that has been declared as const?
I have tried declaring the same struct variable in another part of the code, without using the const keyword, to see if I can change the values of some of its members there... i.e. in Interface.cpp, I have added the following code to a function called sendData():
types::data_Variant& perfData;
perfData.steering.etaValid = true;
perfData.steering.ttgValid = true;
However, I now get the following compile errors on these lines:
error C2653: 'types' is not a class or namespace name
error C2065: data_Variant: undeclared identifier
error C2065: 'perfData': undeclared identifier
error C2228: left of '.steering' must have class/ struct/ union
Is there a way of updating the values of this struct? If so, how should I do it, and what am I doing wrong here?
I have added the following function to the dialogue.cpp source file, as suggested in the answer:
void dialogue::setFPTTGandETAValidityTrue(
FlightPlanMinitoteTypes::FlightPlanMinitoteData_Variant& perfData)
{
SESL_FUNCTION_BEGIN(setFPTTGandETAValidityTrue)
perfData.steeringData.fpETAValid = true;
perfData.steeringData.fpTTGValid = true;
SESL_FUNCTION_END()
}

Answer 1

You could add a wrapper for yourself.
void myupdate(dialogue& dia, types::data_Variant& perfData)
{
perfData.etaValid = true;
dia.update(perfData);
}
Then call myupdate() instead of dialogue::update().

Answer 2

You declare
void dialogue::update( const types::data_Variant& perfData){
...
}
that const is a declaration of you saying: "I won't modify the referenced object in this function". If you want to modify it in dialogue::update you have to remove the const keyword. Wrapping is not a solution, in my opinion, makes the code harder to maintain. Also I vote against remove const with const_cast.
The correct solution is to remove const from method declaration if you want to modify the referenced object inside that function.