This is a fundamental thing that every C++ programmers must know.

const to data

This is the simplest form of pointer definition that we always do.

const int* data3 = arr2; // Const to data
data3[0] = 55;  // ERROR: Data can't be changed
data3 = arr1;    // Success:  Can't change the pointer to somewhere else

Here the data is can’t changed using data3 pointer. but the pointer itself can be modified and can point to some other location.

Usually we don’t care even if pass the pointer to some function and that pointer changes to some other location, as the function parameters points to same location but the pointer is a copy of actual parameter. But things may go different if we wrongly pass reference to function parameters even it’s const. So we’ve to take care this while designing the interfaces.

void Foo(  LPCTSTR& lpszValue )
{
	lpszValue = _T( "codereflect.com" );
}

int _tmain(int argc, _TCHAR* argv[])
{
	LPCTSTR sz = NULL;
	Foo(sz);
	_tprintf( sz );
	return 0;
}

const to pointer

In this form of usage, the pointer can’t be changed to some location but the data can be changed.

int* const data2 = arr2; // const to pointer
data2 = arr1;    // Can't change the pointer to somewhere else
data2[0] = 55;  // SUCCESS: Data can be changed

const to both data and pointer

This is the goodness of both . The most secure pointer.

const int* const data1 = arr1; // Const to both pointer and data
data1 = arr2;    // ERROR: can't change the pointer
data1[0] = 100; // ERROR: can't change the data