C vs C++ – How Does ‘const’ Differ?

c++constants

How does the const qualification on variables differ in C and C++?

from: Does "const" just mean read-only or something more?

"What prompted this question was this answer: https://stackoverflow.com/questions/4024318#4024417 where he states const "just" means read-only in C. I thought that's all const meant, regardless of whether it was C or C++. What does he mean?"

Best Answer

const in C cannot be used to build constant expressions.

For example :

#include <stdio.h>
int main()
{
   int i = 2;
   const int C = 2;
   switch(i)
   {
      case C  : printf("Hello") ;
      break;

      default : printf("World");
   }
}

doesn't work in C because case label does not reduce to an integer constant.

Related Solutions

C++ Constants – ‘const int’ vs. ‘int const’ as Function Parameters

The trick is to read the declaration backwards (right-to-left):

const int a = 1; // read as "a is an integer which is constant"
int const a = 1; // read as "a is a constant integer"

Both are the same thing. Therefore:

a = 2; // Can't do because a is constant

The reading backwards trick especially comes in handy when you're dealing with more complex declarations such as:

const char *s;      // read as "s is a pointer to a char that is constant"
char c;
char *const t = &c; // read as "t is a constant pointer to a char"

*s = 'A'; // Can't do because the char is constant
s++;      // Can do because the pointer isn't constant
*t = 'A'; // Can do because the char isn't constant
t++;      // Can't do because the pointer is constant

C++ – What Does ‘const’ Mean for Variables and Functions?

void myfunc(const char x);

This means that the parameter x is a char whose value cannot be changed inside the function. For example:

void myfunc(const char x)
{
  char y = x;  // OK
  x = y;       // failure - x is `const`
}

For the last one:

int myfunc() const;

This is illegal unless it's inside a class declaration - const member functions prevent modification of any class member - const nonmember functions cannot be used. in this case the definition would be something like:

int myclass::myfunc() const
{
  // do stuff that leaves members unchanged
}

If you have specific class members that need to be modifiable in const member functions, you can declare them mutable. An example would be a member lock_guard that makes the class's const and non-const member functions threadsafe, but must change during its own internal operation.

Best Answer

Related Solutions

C++ Constants – ‘const int’ vs. ‘int const’ as Function Parameters

C++ – What Does ‘const’ Mean for Variables and Functions?

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