Writing to a Stream

The first statement in the body of main executes an expression. In C++ an expression yields a result and is composed of one or more operands and (usually) an operator. The expressions in this statement use the output operator (the « operator) to print a message on the standard output:

std::cout << "Enter two numbers:" << std::endl;

The << operator takes two operands: The left-hand operand must be an ostream object; the right-hand operand is a value to print. The operator writes the given value on the given ostream. The result of the output operator is its left-hand operand. That is, the result is the ostream on which we wrote the given value.

Our output statement uses the << operator twice. Because the operator returns its left-hand operand, the result of the first operator becomes the left-hand operand of the second. As a result, we can chain together output requests. Thus, our expression is equivalent to

(std::cout << "Enter two numbers:") << std::endl;

Each operator in the chain has the same object as its left-hand operand, in this case std::cout. Alternatively, we can generate the same output using two statements:

std::cout << "Enter two numbers:";
std::cout << std::endl;

The first output operator prints a message to the user. That message is a string literal, which is a sequence of characters enclosed in double quotation marks. The text between the quotation marks is printed to the standard output.

The second operator prints endl, which is a special value called a manipulator. Writing endl has the effect of ending the current line and flushing the buffer associated with that device. Flushing the buffer ensures that all the output the program has generated so far is actually written to the output stream, rather than sitting in memory waiting to be written.