问题
in Does the C/C++ ternary operator actually have the same precedence as assignment operators?
Luchian Grigore's answer says that cases like
a ? b : c = d
will always be inferred as
a ? b : ( c = d )
because both = and ?: associate right to left so
in c++
k = 21 > 3 ? j = 12 : j = 10;
and
k = 1 > 3 ? j = 12 : j = 10;
both are fine.
In C
k = 21 > 3 ? 12 : j = 10
returns error
invalid lvalue in assignment.
Shouldn't above be inferred as (and return no error)
k= 21 > 3 ? 12 : ( j = 10 )
I assume now it is being grouped as
k = ( 21 > 3 ? 12 : j ) = 10
which gives error since in C(not in C++) ternary operator cannot return lvalue. Can anyone tell me exactly how operators are grouped in this case.
回答1:
Your linked question's (Does the C/C++ ternary operator actually have the same precedence as assignment operators?) answer by @hvd shows the answer.
The C++ and C grammars for ?: are different.
In C++, the rightmost operand is allowed to be an assignment expression (so the compiler [greedily] treats the = are part of the ?:) while in C the rightmost operand is a conditional-expression instead. Thus in C as soon as the compiler hits the = the analysis of ?: is complete and it treats it as k = ( 21 > 3 ? 12 : j ) = 10.
回答2:
k=21>3?12:(j=10) gets evaluated as
if ( 21 > 3 )
k = 12;
else
k = ( j = 10 );
Since 21>3 is true, the else condition does not get evaluated and j has undefined value (or whatever value it had prior to this statement).
来源：https://stackoverflow.com/questions/17406503/ternary-operator-and-assignment-operator