C++ – Understanding the Implementation of ‘const’

c++constants

How does a compiler, C or C++, (for example, gcc) honors the const declaration?

For example, in the following code, how does the compiler keeps track that the variable ci is const and cannot be modified?

int
get_foo() {
    return 42;
}

void
test()
{
    int i = get_foo();
    i += 5;

    const int ci = get_foo();
    // ci += 7;  // compile error: assignment of read-only variable ?ci?
}

Best Answer

Much like any other bit of symbol information for variables (address, type etc.). Usually there is a symbol table which stores information about declared identifiers, and this is consulted whenever it encounters another reference to the identifier.

(A more interesting question is whether this knowledge is only present during compilation or even during runtime. Most languages discard the symbol table after compiling, so you can manipulate the compiled code and force a new value even if the identifier was declared 'const'. It takes a sophisticated runtime system (and code signing) to prevent that.)

Related Solutions

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.

C vs C++ – How Does ‘const’ Differ?

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.

Best Answer

Related Solutions

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

C vs C++ – How Does ‘const’ Differ?

Related Question