assignment operator

[blast3r]

Hey i just started reading c++ today, and i was wondering if someone could tell me why we would need this:

// assignment operator

#include <iostream>
using namespace std;

int main ()
{
  int a, b;         // a:?,  b:?
  a = 10;           // a:10, b:?
  b = 4;            // a:10, b:4
  a = b;            // a:4,  b:4
  b = 7;            // a:4,  b:7

  cout << "a:";
  cout << a;
  cout << " b:";
  cout << b;

  return 0;
}

Why can't we just write that a = 4 in the start?
sorry if this is a noobish question

[jsmith]

int a = 4;

is completely valid and preferred to

int a;
a = 4;

[felipe21]

Just to make it clearer.

in C++ you declare and define variables.When you use:

int a;

you are just declaring a as an integer, and you have to define it later,
but as jsmith said, it is perfectly valid to declare and define a variable in a single
statement, like:

int a = 4;

and in case you are wondering, you can also do this:

int a = 4, b = 7;

or if they are supposed to be initialized to the same value, even:

int a = b = 7;

[kempofighter]

Or you can make it look like object construction which is how I like to do it. This makes it clearer that the variable a is being constructed and initialized with a value of 4.

int a(4);
int a = 4; // equivalent to previous line but looks more like assignment

int a;  // object constructed but not initialized
a = 4; // value of 4 is assigned to the object. 

[Duthomhas]

int a = b = 7; is invalid -- b is not declared.

The closest you can get to such a construct is the fugly int a = 7, b = a;. Personally, I think it is just better to declare each variable on its own line:

int a = 7;
int b = 7;

or

int a( 7 );
int b( 7 );

Hope this helps.

[felipe21]

int a = b = 7; is invalid -- b is not declared.

My mistake, actually what you can do to assign the same value to
more than one variable is this:

int a, b;
a = b = 5;

[vince510]

you could do int a = int b = 7;

[Return 0]

you could do int a = int b = 7;

That's not valid.

[felipe21]

you could do int a = int b = 7; 

That does not compile with in my machine.

error: expected primary-expression before ‘int’

[Return 0]

It's illegal syntax. The only time you can do something close to that is if all variables are declared prior. For example, in a default constructor call you could do this:

a = b = c = d = 0;

[vince510]

oh my bad. thought it would work.

[wmheric]

Remember = evaluates from right to left.
a = b = 5;
Since b = 5; returns the value of b, it can then be assigned to a a = (b = 5);.

It compiles if the compiler knows what it means.
It doesn't if the compiler have no idea of what it means.

Referring to blast3r's question

Why can't we just write that a = 4 in the start?

.
The answer that immediately comes to my mind is that

It demonstrates what the assignment operator does. For example, how you assign the value of a variable to another.

[twoscoops]

I know you can write

int a(4)

but surely int is a primative type and as such not an object, and therefore not contructed?

why can't we just write a = 4 at the start....


Well, you need to specify what type a is

so

int a = 4; is fine

as is

int a = 4, b = 7;

But this has been said before.

[blast3r]

Thanks for the answers. I think you guys misunderstood my question, but that may be because i asked it in a wrong way. but wmheric got me the answer thanks.