Example

class MyClass
{
public:
MyClass(int init){
var = init;
}
void setVar(int input){
var = input;
}
private:
mutable int var;
};

int function(const MyClass &mc){

mc.setVar(10);
return 0;
}

Solution

Use const_cast to remove the constness of the variable.

class MyClass
{
public:
MyClass(int init){
var = init;
}
void setVar(int input){
var = input;
}
private:
int var;
};

int function(const MyClass &mc){

MyClass& tmp = const_cast<MyClass&> (mc);
tmp.setVar(10);

return 0;
}