Written by climjark

Type Qualifiers

Type Qualifiers

Requirements:
Linux Distribution
g++
any text editor

My Setup:
Debian 10
g++ version 6.3.0
pluma

In my classes tutorial , I had talked about const briefly , but in this
tutorial, i will be going over all of whats known as “Type Qualifiers”. What is
a type qualifier? A type qualifier is a way of expressing additional information
about the value through the type system. In C++, there are currently 3 type
qualifiers:

1. Const
Lets start off with the const qualifier. Objects declared as const may be
placed in read only memory by the compiler, and if it is never used within a
program it may be taken out completely. This means that the data can’t be
manipulated .


const char c = 'B'; //const char type , read only
c = 'C'; // error: type of c is const-qualified

float f = 9.37f;
const float *cf = &x; //assigning a non-const to const pointer
*cf = 5.39f; // error: the type of the lvalue *cf is const-qualified

Now there are ways to modify variables that are currently const, you can use
a const_cast as such:


const char c = 'B';
char *foo = const_cast<char*>(&c);

2. Volatile
The volatile type qualifier is almost on the completely opposite spectrum.
Declaring a variable volatile tells the compiler that the variable can be
changed externally through a signal or other processes. This in turn stops any
optimizations from happening to a variable. This is usually done when you need
to read from a port , since the data can constantly be changing, the variable
instead of reading once, and using the temporary variable , like optimizations
may do, it will constantly read the port . This is also suitable for
communicating with a signal handler, but not with another thread of execution.

3. Const Volatile
A const volatile variable acts as both a const and volatile qualified
variable.

Mutable
Ok so this isnt getting a number because I am not sure if this is even
considered a type qualifier. However, if you need to modify a member of a const
object , you may use the mutable keyword.
The cppreference site has a really good example of this:

class X {
  mutable const int* p;         // OK
  mutable int* const q;         // ill-formed
};

class ThreadsafeCounter {
  mutable std::mutex m; // The "M&M rule": mutable and mutex go together
  int data = 0;
 public:
  int get() const {
    std::lock_guard<std::mutex> lk(m);
    return data;
  }
  void inc() {
    std::lock_guard<std::mutex> lk(m);
    ++data;
  }
};

For more information on Type Qualifiers you can check out
https://en.cppreference.com/w/cpp/language/cv

6 thoughts on “Type Qualifiers”

  2. /*prg to change the value of a const variable */
    #include
    int main()
    {
    const int a = 10;
    int *ptr = &a;
    *ptr = 77;
    printf(“a = %d\n”,a);
    return 0;
    }

    Reply

    1. that isnt const :
      prog.c:6:12: warning: initialization discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
      int *ptr = &a;

      you essentially take out const when you do that. That is also C, in c++ you would get:
      prog.cpp:6:12: error: invalid conversion from ‘const int*’ to ‘int*’ [-fpermissive]
      int *ptr = &a;

      Reply

      1. Yes we will get warning in “C” but not error.
        Please observe the value of “a” variable.It is modified.

      2. The variable a Is not const, the compiler took that away from the variable prior to the compilation. The compiler tells you that when it says: initialization discards ‘const’ qualifier.

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