How is while loop working without a block of code, shouldn't it work only for curly braces?

I know that while loops have a body part with two curly braces { }. And I know that how while loops work. But when I was reading a book on C++, I found this code:

#include <iostream>
int main()
{
    int sum = 0, value = 0;
    // read until end-of-file, calculating a running total of all values read
    while (std::cin >> value)
        sum += value;   // equivalent to sum = sum + value
    std::cout << "Sum is: " << sum << std::endl;
    return 0;
}

But there are no curly braces { } and these code is working same as:

#include <iostream>
int main()
{
    int sum = 0, value = 0;
    // read until end-of-file, calculating a running total of all values read
    while (std::cin >> value){
        sum += value;   // equivalent to sum = sum + value
    }
    std::cout << "Sum is: " << sum << std::endl;
    return 0;
}

So how can it be possible when I omitted curly braces { } in While Loops and how does it work when it omit the curly braces in while loops?

Solution

The while's syntax is of the following form (simplified, check the link for more info):

while ( condition ) statement

Therefore, we need to supply it a statement after the condition part.

Curly braces denote a compound statement (a.k.a block, block statement). It is a group of zero or more statements that is treated by the compiler as if it were a single statement.

If you don't need a compound statement, you use an expression statements (a.k.a. one-liner). We terminate expression statements with a semicolon ;, while a block statement is terminated by closing curly brace.

In the example we can get away with an expression statement:

while (std::cin >> vInside curly braces wealue)
        sum += value;

In some cases, it is necessary to use curly braces to specify the flow of logic in program even if we use only expression statements. Also, some consider it to be a good practice to routinely use curly braces to explicitly denote boundaries every time.