Question regarding cout

why does these two statement behaving in the same manner?

Code:

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        (cout<<"hello");        //1st

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Code:

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        cout<<"hello";          //2nd

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and this one throwing an error:

Code:

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        (cout<<"hello";)        //3rd

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Because
(expression;)
in C++ is a syntax error.
It's nothing special about cout.

Try
(a=1;)

1 works because cout doesn't return void.

in 1st shouldn't it throw some error since the operator << sends its right-hand-operands i.e. "hello") into the left hand operand i.e. ostream object cout & it should be asking for the left-parenthesis (the left-parenthesis being placed before cout)?

The parentheses isn't part of operator <<'s operands. Parentheses was used for grouping.

One necessary use:

Code:

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#include <iostream>
#include <cstdlib>
bool coinflip()
{
  return std::rand() % 2 != 0;
}
int main()
{
  std::cout << (coinflip() ? "hello" : "bye") << std::endl;
}

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Like I said, cout returns a value, so you can put it inside parenthesised expressions.

Here's another example.

Code:

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#include <iostream>
using namespace std;
int main()
{
  if ( cout << "hello world" << endl ) {
  }
}

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Similarly for cin

Code:

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if ( cin >> myvar ) {
  // do stuff with valid data
} else {
  // oops
}

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