CODEWITHSUNDEEP
c++gcc-warning
user11554405
user11554405

Reputation:

Left-hand operand of comma has no effect vs Right-hand operand of comma has no effect

I'm very beginner in C++ and I'm currently reading a book to get better in it. While reading a section about C++'s only ternary operator, I played around some code and got a question:

So, when I do this: 

std::cout << "Hello ", "Mars";

I get a warning:

warning: right operand of comma operator has no effect [-Wunused-value]

And when I do this: 

std::cout << (5 > 4 ? "Hello ", "Mars" : "Bye ", "World");
  1. for the second operand of ?: I get:

    warning: left operand of comma operator has no effect [-Wunused-value]

  2. for the third operand of ?: I get:

    warning: left operand of comma operator has no effect [-Wunused-value]

So my question is, why do I get the first warning for the second operand of ?:, while I get the second warning for the third operand of ?:?

Upvotes: 0

Views: 3212

Answers (2)

463035818_is_not_an_ai
463035818_is_not_an_ai

Reputation: 122238

From cppreference:

In a comma expression E1, E2, the expression E1 is evaluated, its result is discarded (although if it has class type, it won't be destroyed until the end of the containing full expression), and its side effects are completed before evaluation of the expression E2 begins (note that a user-defined operator, cannot guarantee sequencing) (until C++17).

It is the result of left operand that is discarded. However, if you consider operator precedence, where the comma operator is always last, then 

std::cout << "Hello ", "Mars";

is the same as 

(std::cout << "Hello ") , ( "Mars" ) ;

and the compiler is trying to be clever. This 

std::cout << "Hello "

does have an effect even if you ignore the returned value. Note that operator<< returns a reference to the stream and you typically ignore that value. Hence, discarding it via the comma operator does not prevent it to print something on the screen.

On the other hand 

"Mars";

as a statement has no effect and that is what the compiler is trying to tell in that case.

Upvotes: 4

1201ProgramAlarm
1201ProgramAlarm

Reputation: 32732

It's because of how you're using them. If you'd written

"Mars", std::cout<<"Hello ";

you'd get the "left operand has no effect" warning. If you'd have assigned your expression to a variable

const char *msg = std::cout << "Hello ", "Mars";

you wouldn't get any warnings. When this code is executed, "Hello " would be sent to cout, and msg would be pointing to "Mars".

For the ternary operator, the right operand of the comma operator is the result of the operator, which is the expression value for the ternary operator. Your example is the same as if you'd have written

std::cout << (5 > 4 ? "Mars" : "World");

Upvotes: 3

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