Warning: second/third operand of conditional has no effect [-Wunused-value]

Question

std::cout << (abs(b - c) < a) && a < b + c ? 1 : 0;

I want to check if given values can create triangle. I got warnings:

second operand of conditional expression has no effect [-Wunused-value]
third operand of conditional expression has no effect [-Wunused-value]

What's wrong?

Answer 1

Your code translates to:

((std::cout << (abs(b - c) < a)) && a < b + c) ? 1 : 0;

Firstly, operator<< has higher operator precedence than operator&&.

Only the value of abs(b - c) < a will be printed and the (a < b + c ? 1 : 0) part will be AND-ed together with the return value of std::ostream::operator bool.

But the warning is about 1 and 0 not being assigned to anything nor having any side effects, because && precedes the ternary conditional (?:).

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The correct code is either:

std::cout << (abs(b - c) < a && a < b + c ? 1 : 0);
// same as std::cout << ((abs(b - c) < a && a < b + c) ? 1 : 0);

or

std::cout << (abs(b - c) < a && (a < b + c ? 1 : 0));

In fact, they're equivalent (apart that one calls operator<< with bool and the other with int), no ternary operator needed:

std::cout << (abs(b - c) < a && a < b + c);