C运算符与评估

Question

#include<stdio.h>
int main()
{
int i=3,val;
val=sizeof (f(i))+ +f(i=1)+ +f(i-1);
printf("%d%d",val,i);
getch();
return 0;

}
int f(int num)
{
return num*5;
}

The compiler compiles the program and gives the output 7 1 ..what do the " + +" mean???

Answer 1

First note that sizeof is compile-time evaluable so the first term in val will be sizeof(int) : int being the return type of f .

The value of the entire expression that you want to assign to val is undefined since + as a binary and unary operator is not sequenced. In essence you don't know if i = 1 will happen before or after the evaluation of i - 1 .

As for your specifics, a + + b is evaluated as a + (+b) . +b is simply a unary plus ( almost a no-op, but does do some subtle type coercion), the other + is the addition operator taking two arguments.