The 'const' system is one of the really messy features of C++.

• It is simple in concept: Variables declared with const added become constants and cannot be altered by the program.

However it is also used to bodge in a substitute for one of the missing features of C++ and there it gets horridly complicated and sometimes frustratingly restrictive.

The following attempts to explain how const is used and why it exists.

Declare a constant as if it was a variable but add const before it.

const int Constant1=96;

Const also works with pointers but one has to be careful where const is put as that determines whether the pointer, or what it points to is constant.

const int * Constant2

declares that Constant2 is a variable pointer to a constant integer and:

int const * Constant2

is an alternative syntax which does the same, whereas:

int * const Constant3

declares that Constant3 is constant pointer to a variable integer and:

int const * const Constant4

declares that Constant4 is constant pointer to a constant integer.

A pointer (constant or otherwise) to a ‘const’ value can be very useful.

• This is useful for returning constant strings and arrays from functions which, because they are implemented as pointers, the program could otherwise try to alter and crash.
• Instead of a difficult to track down crash, the attempt to alter unalterable values will be detected during compilation.

If a function which returns a fixed string is written like:

char *Function1()
{ 
  return "Some text";
}

…then the program could crash if it accidentally tried to alter the value doing:

Function1()[1]=’a’;

The solution:

const char *Function1()
{ 
  return "Some text";
}

void Subroutine1(int Parameter1)
{ 
  printf("%d",Parameter1);
}

void Subroutine1()
{ 
  original_value = 5;
  Subroutine2(original_value);
}
 
void Subroutine2(int Parameter1)
{ 
  Parameter1=96;
}

The addition of an & to the parameter name in C++ (which was a very confusing choice of symbol because an ‘&’ in front of variables elsewhere in C generates pointers!) causes the actual variable itself, rather than a copy, to be used as the parameter in the subroutine and therefore can be written to thereby passing data back out the subroutine.

void Subroutine3(int &Parameter1)
{ 
  Parameter1=96;
}

void Subroutine4(int *Parameter1)
{ 
  *Parameter1=96;
}

There is a second common use for passing data by reference or pointer instead of as a copy.

• That is when copying the variable would waste too much memory or take too long.
• This is particularly likely with large & compound user-defined variable types (‘structures’ in C & ‘classes’ in C++).

So a subroutine declared as:

void Subroutine4(big_structure_type &Parameter1); 

might be using & because it is going to alter the variable passed to it; or it might just be to save copying time.

• There is no way to tell which it is if the function provided by an unknown library.
• This could be a risk if one needs to trust the subroutine not to alter the variable.

To solve this, ‘const’ can be used in the parameter list. E.g.

void Subroutine4(big_structure_type const &Parameter1);

which will cause the variable to be passed without copying but stop it from then being altered.

• This is messy because it is essentially making an in-only variable passing method from a both-ways variable passing method which was itself made from an in-only variable passing method just to trick the compiler into doing some optimization.
• Ideally, the programmer should not need to control this detail of specifying exactly how variables are passed, just say which direction the information goes and leave the compiler to optimize it automatically, but C was designed for raw low-level programming on far less powerful computers than are standard these days so the programmer has to do it explicitly.

In Object Oriented Programming, calling a ‘method’ (the Object Oriented name for a function) of an object gives an extra complication.

• As well as the variables in the parameter list, the method has access to the member variables of the object itself which are always passed directly not as copies.

For example a trivial class, ‘Class1’, defined as:

class Class1
{ 
  void Method1();
  int MemberVariable1;
}

has no explicit parameters at all to ‘Method1’ but calling it in an object in this class might alter ‘MemberVariable1’ of that object if ‘Method1’ happened to be, for example,

void Class1::Method1()
{ 
  MemberVariable1=MemberVariable1+1;
}

The solution to that is to put ‘const’ after the parameter list like:

class Class2
{ 
  void Method1() const;
  int MemberVariable1;
}

• which will ban Method1 in Class2 from doing anything which can attempt to alter any member variables in the object.

Of course one sometimes needs to combine some of these different uses of ‘const’ which can get confusing as in:

const int*const Method3(const int*const&)const; 

where the 5 uses of ‘const’ respectively mean that the variable pointed to by the returned pointer & the returned pointer itself will not be alterable and that the method does not alter the variable pointed to by the given pointer, the given pointer itself & the object of which it is a method!.

Besides the confusingness of the const syntax, there are some useful things which the system prevents programs doing.

One in particular annoys me because my programs often needed to be optimized for speed.

This is that a method which is declared ‘const’ cannot even make changes to the hidden parts of its object that would not make any changes that would be apparent from the outside.

This includes storing intermediary results of long calculations which would save processing time in subsequent calls to the class’s methods. Instead it either has to pass such intermediary results back to the calling routine to store and pass back next time (messy) or recalculate from scratch next time (inefficient).

In later versions of C++, the mutable keyword was added which enables const to be overridden for this purpose but it totally relies on trusting the programmer to only use it for that purpose so, if you have to write a program using someone else's class which uses ‘mutable’ then you cannot guarantee that ‘mutable’ things will really be constant which renders ‘const’ virtually useless.

One cannot simply avoid using ‘const’ on class methods because ‘const’ is infectious.

• An object which has been made ‘const’, for example by being passed as a parameter in the ‘const &’ way, can only have those of its methods that are explicitly declared ‘const’ called (because C++’s calling system is too simple to work out which methods not explicitly declared ‘const’ do not actually change anything).
• Therefore class methods that do not change the object are best declared const so that they are not prevented from being called when an object of the class has somehow acquired ‘const’ status.
• In later versions of C++, an object or variable which has been declared ‘const’ can be converted to changeable by use of const_cast which is a similar bodge to ‘mutable’ and using it likewise renders ‘const’ virtually useless.

http://duramecho.com/ComputerInformation/WhyHowCppConst.html

https://www.cprogramming.com/tutorial/const_correctness.html