2 simple variable initialization question

Question

Possible Duplicate:
Is there a difference in C++ between copy initialization and direct initialization?

Is there any difference between,

int x(5);
int x = 5;

& What's the difference between,

int const x(5);
const int x(5);

Answer 1

Of the 4 lines you show, only 1 is valid C.

int x(5);        /* INVALID C */
int x = 5;
int const x(5);  /* INVALID C */
const int x(5);  /* INVALID C */

I don't know the meaning of the invalid lines in other languages. In C, the meaning of the valid line is to create an object with name x, type int, and value 5.

---

To define a "const" (better: a "read-only") object, the C syntax is

const int x = 5;
int const x = 5;

but x cannot be used in places where only constants are allowed. x is not a constant: it is an object of type const int

#define CONSTANT 42
enum /*unnamed*/ { First=1, Second, Third };
switch (var) {
    case x: break; /* invalid use of x; only constants allowed in case labels */
    case 15: break; /* ok; 15 is a constant */
    case CONSTANT: break; /* ok; CONSTANT is a constant with value 42 */
    case Second: break; /* ok; Second is a constant with value 2 */
}

Answer 2

For the first, there's no difference for built-in types. For class types, the T x = y notation requires a copy constructor, the T x(y) notation doesn't. EDIT: Also, as one of the commenters pointed out, T x = y will not work if the T(y) constructor is declared explicit.

For the second, there's no difference as far as the compiler is concerned, but the second (const int) can lead to confusion in many cases, especially when typedefs are involved, e.g.:

typedef int* IntPtr;
IntPtr const cpi1;  //  Const pointer to a non-const int.
const IntPtr cpi2;  //  Also a const pointer to a non-const int.

In both definitions, the const applies to the pointer.

Because of this confusion, it's generally considered better practice to put the const after what it modifies. Now—for a long time, putting the const at the start was usual, with the result that the practice is widespread.

Answer 3

A really good rule of thumb regarding const:

Read Declarations Right-to-Left.

(see Vandevoorde/Josutiss "C++ Templates: The Complete Guide")

E.g.:

int const x; // x is a constant int
const int x; // x is an int which is const

// easy. the rule becomes really useful in the following:
int const * const p; // p is const-pointer to const-int
int const &p;        // p is a reference to const-int
int * const * p;     // p is a pointer to const-pointer to int.

Ever since I follow this rule-of-thumb, I never misinterpreted such declarations again.

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Answer 4

No both are exactly same until you are using an Argument. It's just a matter of coding style.

[Note: But Following are not same:

int x;  // x is variable
int x();  // x is function declaration

]

Answer 5

About the const keyword:

• It makes const what is immediately to its left
• If there is nothing to the left, then it applies to the right.

So, int const i and const int i are the same.

But, int *const i and const int *i are not the same: in the first expression, the pointer is constant, in the second expression, the integer pointed to is constant. Als int const* i is equivalent to the second one.

Answer 6

No difference. Alternate ways to initialize the variable to a value.

But did you try this on your system? What did you get?

Answer 7

There's no difference in c++, in c, however - int x(5); is not allowed

Answer 8

int x(5);
int x = 5;

No difference.

int const x(5);
const int x(5);

No difference.

int x = 5;
const int *p = &x;
int *const q = &x;

Here is a difference in p and q. Using p you can't change the value of x, but you can assign a new address to p. Using q, you cannot point to any other memory location, but you can modify x using q.