Motivation
Const variables
Exercise
Homework
13. Const correctness
Ján Dugáček
February 10, 2019
Ján Dugáček 13. Const correctness
Motivation
Const variables
Exercise
Homework
Table of Contents
1 Motivation
2 Const variables
Const variables
Const objects
Violating constness
Remarks
3 Exercise
4 Homework
Ján Dugáček 13. Const correctness
Motivation
Const variables
Exercise
Homework
Motivation
If a variable isn’t intended to be changed, it’s better to mark it
so in order to prevent someone else who edits the code from
changing it
i n t doStuff ( const i n t x ) {
// . . .
x = x % 12; // Error , x i s const
// . . .
r e t u r n x + r e s u l t ;
// We can be c e r t a i n t h i s i s the o r i g i n a l value
}
Large objects can be passed as reference (std::string&) to
prevent unnecessary copies and a const reference (const
std::string&) ensures it’s not edited inside the function
Ján Dugáček 13. Const correctness
Motivation
Const variables
Exercise
Homework
Const variables
Const objects
Violating constness
Remarks
Const variables
const f l o a t a = 42;
const unsigned i n t b = 9;
std : : pair <const int , f l o a t > c{ 3 , 5};
c . f i r s t = 2; // Error , the f i r s t i s const
c . second = 2 . 0 ; // The second i s not const
const std : : pair <int , f l o a t > d{ 3 , 5};
d . second = 1 5 . 1 7 ; // Error : The whole o b j e c t i s const
std : : pair <int , f l o a t > e = d ;
// Const can be copied i n t o non−const
std : : pair <int , f l o a t >& d = e ;
// Error : Cannot make non−const r e f e r e n c e to const
There is no way to accidentally change something that is const
Anything can be converted into const
Ján Dugáček 13. Const correctness
Motivation
Const variables
Exercise
Homework
Const variables
Const objects
Violating constness
Remarks
Const objects
const std : : s t r i n g t e x t ( " Free beer " ) ;
i n t found = t e x t . f i n d ( " " ) ; // Okay , s t r i n g not modified
std : : s t r i n g beer = t e x t . s u b s t r ( found + 1 ) ;
t e x t [ found ] = ’_’ ; // Error , s t r i n g i s modified
How does the compiler know which methods modify the object
and which ones don’t?
Ján Dugáček 13. Const correctness
Motivation
Const variables
Exercise
Homework
Const variables
Const objects
Violating constness
Remarks
Const objects #2
s t r u c t importantData {
i n t p r e c i o u s ;
i n t& g e t P r e c i o u s () const {
p r e c i o u s = 2;
// Error , we are i n a const o b j e c t
std : : cout << p r e c i o u s << std : : endl ; // Okay
r e t u r n p r e c i o u s ;
// Error , t u r n i n g i n t o a nonconst r e f e r e n c e
}
};
Methods can be declared const, which allows them to be
called if the object is const, but all member variables are const
inside them
Ján Dugáček 13. Const correctness
Motivation
Const variables
Exercise
Homework
Const variables
Const objects
Violating constness
Remarks
Const objects #3
c l a s s importantData {
i n t p r e c i o u s ;
p u b l i c :
i n t g e t P r e c i o u s () const {
r e t u r n p r e c i o u s ;
} // Returned value i s a copy
i n t& g e t P r e c i o u s () {
r e t u r n p r e c i o u s ;
} // E d i t i n g the r e t u r n value e d i t s the member
void s e t P r e c i o u s ( i n t s e t ) {
p r e c i o u s = s e t ;
}
};
There can be const and nonconst versions of functions chosen
accordingly to the constness of the object
Ján Dugáček 13. Const correctness
Motivation
Const variables
Exercise
Homework
Const variables
Const objects
Violating constness
Remarks
Violating constness
c l a s s importantData {
i n t p r e c i o u s ;
mutable i n t imprecious ;
p u b l i c :
i n t g e t P r e c i o u s () const {
imprecious = p r e c i o u s − 1; // Okay , mutable
r e t u r n p r e c i o u s ;
}
};
Some objects hide some internal attributes that don’t affect
their external behaviour but serve for some other purposes, like
caching, synchronising and other stuff, these attributes have to
be marked with keyword mutable (use it only if this is the
case)
Ján Dugáček 13. Const correctness
Motivation
Const variables
Exercise
Homework
Const variables
Const objects
Violating constness
Remarks
Utterly violating constness
c l a s s importantData {
i n t p r e c i o u s ;
mutable i n t imprecious ;
p u b l i c :
i n t g e t P r e c i o u s () const {
imprecious = p r e c i o u s − 1;
r e t u r n p r e c i o u s ;
}
void specialDebugFunction ( i n t s e t ) const {
const_cast<int >( p r e c i o u s ) = s e t ;
}
};
You can modify a const variable by removing its constness
using const_cast, but this is needed very rarely
Ján Dugáček 13. Const correctness
Motivation
Const variables
Exercise
Homework
Const variables
Const objects
Violating constness
Remarks
Remarks
Constness doesn’t serve to limit you, its purpose is to protect
yourself from making mistakes
You should use it to make sure you don’t break your code
when you return to it a year later and protect others from
using your code incorrectly
In very rare cases, compiler uses the knowledge that something
is const to make some optimisations
It’s rather important when methods accepts references to
objects, as it’s the only way to ensure they don’t modify the
variables in the function that called it
It’s crucial when using more threads (usually to leverage more
processor cores), because more cores can read a variable
simultaneously, but cannot write into it simultaneously
Ján Dugáček 13. Const correctness
Motivation
Const variables
Exercise
Homework
Exercise
1 Create a class that gives access to lines in a file, but reads the
(possibly gigabytes long) file only up to the line that’s
requested
2 Create a SortedVector class that encapsulates a vector of
some numbers and has a method to sort it
3 Create a Collection class that allows reading numbers,
adding them and changing them only when non-const and
doesn’t alow removing any
4 Create a Factoriser class that can factor integers and
remember all integers it has already factorised to make it
factor faster
Use const wherever there’s any reason to use it
Ján Dugáček 13. Const correctness
Motivation
Const variables
Exercise
Homework
Advanced exercise
1 Create a class that reads a file by chunks of 16 bytes and holds
them in a vector, allowing a const access and a non-const
access that saves the changes to disk when done (returns some
kind of wrapper whose destructor makes the class update the
object)
2 Create a Decimator class that holds numbers representing
some data, each with time and value, and has a method to
remove the one that will produce data with the smallest
possible mean square difference
Use const wherever there’s any reason to use it
Ján Dugáček 13. Const correctness
Motivation
Const variables
Exercise
Homework
Homework
Write a class that represents some measured data as a vector
of pairs of time and value, has a method to return an
interpolated value at some time between known values and
caches recently interpolated values so that it would not have
to compute them again if requested again shortly later
You have two weeks to do it
You must keep const correctness (and use mutable where it
makes sense)
Ján Dugáček 13. Const correctness