问题
I have :
#include<stdio.h>
int main()
{
int a=5,b=6;
(a>b)?b=a:b=b; // Here is the error
return 0;
}
But if I replace :
(a>b)?b=a:b=b; // Error
with
(a>b)?(b=a):(b=b); // No-Error
I understand the lvalue is a value to which something can be assigned and how is it different from rvalue, but why is the extra parenthesis making the difference.
回答1:
Assignment has a lower precedence than the ternary operator so the line evaluates like:
((a>b)?b=a:b)=b;
use:
b=(a>b)?a:b;
回答2:
Actually, in C, this code
(a>b)?b=a:b=b;
is parsed by many compilers as
((a>b)?b=a:b)=b;
which is an error, as the expression ((a>b)?b=a:b) evaluates to an rvalue which you try to assign with b which results in an error. Trying to assign an rvalue is an error. If it is not parsed that way, then its simply a syntax error. But a C compiler is NOT allowed to parse it as:
((a>b)?b=a:(b=b)); //not allowed to parse by C language
Because the grammar of C does not allow a compiler to parse the code as above.
But what you've written (the original code) is correct as C++.
Here the grammars of C and C++ differ a lot. And because of that difference you see both languages treat the expression differently. That is, the conditional expression in C++ is different from the conditional expression in C .
Wikipedia has very good and correct explanation for this:
The binding of operators in C and C++ is specified (in the corresponding Standards) by a factored language grammar, rather than a precedence table. This creates some subtle conflicts. For example, in C, the syntax for a conditional expression is:
logical-OR-expression ? expression : conditional-expression
while in C++ it is:
logical-OR-expression ? expression : assignment-expression
Hence, the expression:
e = a < d ? a++ : a = d
is parsed differently in the two languages. In C, this expression is a syntax error, but many compilers parse it as:
e = ((a < d ? a++ : a) = d)
which is a semantic error, since the result of the conditional-expression (which might be a++) is not an lvalue. In C++, it is parsed as:
e = (a < d ? a++ : (a = d))
which is a valid expression.
回答3:
It is really:
((a>b)?b=a:b)=b;
Note: you should simply
b = (a>b)?a:b;
回答4:
When we put an equation in parenthesis it is treated as an expression. And it returns some value which provide solution to the error.
来源：https://stackoverflow.com/questions/6966299/error-lvalue-required-in-this-simple-c-code-ternary-with-assignment