objective-c - Objective-C：奇怪的三元行为

Question

我有一个简单的函数，它返回NSNumbers 的原始总和，但如果它们是 -1，则不计算它们：

- (NSInteger)total {
    return [[self obj1] integerValue] == -1 ? 0 : [[self obj1] integerValue] + 
           [[self obj2] integerValue] == -1 ? 0 : [[self obj2] integerValue] +
           [[self obj3] integerValue] == -1 ? 0 : [[self obj3] integerValue];
}

在这种情况下total将始终返回 0。

但如果我这样写：

- (NSInteger)total {
    NSInteger ret = 0;
    ret += [[self obj1] integerValue] == -1 ? 0 : [[self obj1] integerValue];
    ret += [[self obj2] integerValue] == -1 ? 0 : [[self obj2] integerValue];
    ret += [[self obj3] integerValue] == -1 ? 0 : [[self obj3] integerValue];
    return ret;
}

total将返回正确的值。

我不喜欢以一种方式写它而不是另一种方式，但我看不出第一种方式有什么问题。

Answer 1

Operator precedence. The ternary operator has the third lowest precedence in C, so the addition is evaluated before it could get to the conditional part. Use parentheses:

return ([[self obj1] integerValue] == -1 ? 0 : [[self obj1] integerValue]) + 
       ([[self obj2] integerValue] == -1 ? 0 : [[self obj2] integerValue]) +
       ([[self obj3] integerValue] == -1 ? 0 : [[self obj3] integerValue]);