Question

The code below compiles well

int a=5,b=4,c;
a>b?30:40;

Also does,

int a=5,b=4,c;
a>b?c=30:40;

But why this does not work?

int a=5,b=4,c;
a>b?c=30:c=40;

Answer 1

You are being bitten by precedence. ?: has very low precedence, but not as low as = or , (see the operator precedence table).

Your code is parsed as:

(a>b ? c=30 : c) = 40;

Rather than:

a>b ? c=30 : (c=40);

You don’t need parenthesis around c=30 because ? and : act like parentheses to the expression within.

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Believe it or not, (a>b ? c=30 : c) = 40 is valid C++ (but not valid C). The expression (a>b ? c=30 : c) is an lvalue referencing the variable c, to which 40 is assigned.