64,637
社区成员
发帖
与我相关
我的任务
分享逻辑与和逻辑或的优先级问题
我在网上找到的说法几乎都是说与优先级高于或，我自己写了个测试代码
bool p1()
{
cout<<"p1"<<endl;
return false;
}
bool p2()
{
cout<<"p2"<<endl;
return true;
}
bool p3()
{
cout<<"p3"<<endl;
return true;
}
int main()
{
if(p1()||p2()&&p3());
return 0;
}
打印的的顺序是p1 p2 p3 ,也就是按顺序执行的啊，与并没有高于或，而且另外一个问题就是它们的结合性都是左到右，如果与的优先级高于或的话，那么在这个例子里与的先执行，将导致或的执行顺序是先右后左了，不符合理论啊？？
希望大家指教，问题出在哪啊？网上的资料和我的代码测试和推论不符合。。。
bool p1()
{
cout<<"p1"<<endl;
return false;
}
bool p2()
{
cout<<"p2"<<endl;
return true;
}
bool p3()
{
cout<<"p3"<<endl;
return true;
}
int main()
{
if(p1()||p2()&&p3());
return 0;
}
打印的的顺序是p1 p2 p3 ,也就是按顺序执行的啊，与并没有高于或，而且另外一个问题就是它们的结合性都是左到右，如果与的优先级高于或的话，那么在这个例子里与的先执行，将导致或的执行顺序是先右后左了，不符合理论啊？？
希望大家指教，问题出在哪啊？网上的资料和我的代码测试和推论不符合。。。
...全文
请发表友善的回复…
发表回复
幻夢之葉 2014-06-24
- 打赏
- 举报
你计算表达式前后顺序，跟结合顺序没什么关联吧？！
你这个结构只能说明该编译器
求
//输出的效果，相当于表达式的效果
f1() = true;
f2() = true;
f3() = true;
然后
true || （true && true) //完全没有违反结合性
lin5161678 2014-06-24
- 打赏
- 举报
楼主错误的理解的优先级的意义了
优先级没有确定求值顺序的作用
所以 不管 && 优先级高 还是 || 优先级高
求值顺序在这里都是从左向右[&& || 是有限的几个确定求值顺序的运算符]
优先级真正作用仅用于确定运算符和哪些子表达式结合 而已
buyong 2014-06-24
- 打赏
- 举报
分析汇编也能看出来，就是费劲：
0x40055b push rbp
0x40055c mov rbp,rsp
0x40055f sub rsp,0x10
0x400563 mov eax,0x0
0x400568 call 0x40051c <p1>
0x40056d test eax,eax
0x40056f jne 0x40058d <main+50>
0x400571 mov eax,0x0
0x400576 call 0x400531 <p2>
0x40057b test eax,eax
0x40057d je 0x400594 <main+57>
0x40057f mov eax,0x0
0x400584 call 0x400546 <p3>
0x400589 test eax,eax
0x40058b je 0x400594 <main+57>
0x40058d mov eax,0x1
0x400592 jmp 0x400599 <main+62>
0x400594 mov eax,0x0
0x400599 mov DWORD PTR [rbp-0x4],eax
0x40059c mov eax,0x0
0x4005a1 leave
0x4005a2 ret
sx666777888 2014-06-24
- 打赏
- 举报
汇编看不懂啊
sx666777888 2014-06-24
- 打赏
- 举报
觉得你的回答最精辟！！
赵4老师 2014-06-24
- 打赏
- 举报
//C++ Operators
// Operators specify an evaluation to be performed on one of the following:
// One operand (unary operator)
// Two operands (binary operator)
// Three operands (ternary operator)
// The C++ language includes all C operators and adds several new operators.
// Table 1.1 lists the operators available in Microsoft C++.
// Operators follow a strict precedence which defines the evaluation order of
//expressions containing these operators. Operators associate with either the
//expression on their left or the expression on their right; this is called
//“associativity.” Operators in the same group have equal precedence and are
//evaluated left to right in an expression unless explicitly forced by a pair of
//parentheses, ( ).
// Table 1.1 shows the precedence and associativity of C++ operators
// (from highest to lowest precedence).
//
//Table 1.1 C++ Operator Precedence and Associativity
// The highest precedence level is at the top of the table.
//+------------------+-----------------------------------------+---------------+
//| Operator | Name or Meaning | Associativity |
//+------------------+-----------------------------------------+---------------+
//| :: | Scope resolution | None |
//| :: | Global | None |
//| [ ] | Array subscript | Left to right |
//| ( ) | Function call | Left to right |
//| ( ) | Conversion | None |
//| . | Member selection (object) | Left to right |
//| -> | Member selection (pointer) | Left to right |
//| ++ | Postfix increment | None |
//| -- | Postfix decrement | None |
//| new | Allocate object | None |
//| delete | Deallocate object | None |
//| delete[ ] | Deallocate object | None |
//| ++ | Prefix increment | None |
//| -- | Prefix decrement | None |
//| * | Dereference | None |
//| & | Address-of | None |
//| + | Unary plus | None |
//| - | Arithmetic negation (unary) | None |
//| ! | Logical NOT | None |
//| ~ | Bitwise complement | None |
//| sizeof | Size of object | None |
//| sizeof ( ) | Size of type | None |
//| typeid( ) | type name | None |
//| (type) | Type cast (conversion) | Right to left |
//| const_cast | Type cast (conversion) | None |
//| dynamic_cast | Type cast (conversion) | None |
//| reinterpret_cast | Type cast (conversion) | None |
//| static_cast | Type cast (conversion) | None |
//| .* | Apply pointer to class member (objects) | Left to right |
//| ->* | Dereference pointer to class member | Left to right |
//| * | Multiplication | Left to right |
//| / | Division | Left to right |
//| % | Remainder (modulus) | Left to right |
//| + | Addition | Left to right |
//| - | Subtraction | Left to right |
//| << | Left shift | Left to right |
//| >> | Right shift | Left to right |
//| < | Less than | Left to right |
//| > | Greater than | Left to right |
//| <= | Less than or equal to | Left to right |
//| >= | Greater than or equal to | Left to right |
//| == | Equality | Left to right |
//| != | Inequality | Left to right |
//| & | Bitwise AND | Left to right |
//| ^ | Bitwise exclusive OR | Left to right |
//| | | Bitwise OR | Left to right |
//| && | Logical AND | Left to right |
//| || | Logical OR | Left to right |
//| e1?e2:e3 | Conditional | Right to left |
//| = | Assignment | Right to left |
//| *= | Multiplication assignment | Right to left |
//| /= | Division assignment | Right to left |
//| %= | Modulus assignment | Right to left |
//| += | Addition assignment | Right to left |
//| -= | Subtraction assignment | Right to left |
//| <<= | Left-shift assignment | Right to left |
//| >>= | Right-shift assignment | Right to left |
//| &= | Bitwise AND assignment | Right to left |
//| |= | Bitwise inclusive OR assignment | Right to left |
//| ^= | Bitwise exclusive OR assignment | Right to left |
//| , | Comma | Left to right |
//+------------------+-----------------------------------------+---------------+
带头大哥_ 2014-06-23
- 打赏
- 举报
其实你这个问题说复杂也复杂，讲不咋个情况，说简单也简单，关键看你理解思维
你看这个两个 (a||b)&&c和a||(b&&c) 你觉得这个2个效果一样不？
答案肯定是不！你能从中看出点什么不？a||b&&c这个出来的效果可以看做是这样的a||(b&&c)，&&优先结合b和c，b和c两个发生作用，而没有去和a发生作用，a是和b&&c这个整体发生作用！而（a||b)&&c恰恰相反！但是计算顺序还是从左到友的！而不是从右到左，我也不清楚我到到底说清楚没有
多看几遍，你应该可以明白
你看这个两个 (a||b)&&c和a||(b&&c) 你觉得这个2个效果一样不？
答案肯定是不！你能从中看出点什么不？a||b&&c这个出来的效果可以看做是这样的a||(b&&c)，&&优先结合b和c，b和c两个发生作用，而没有去和a发生作用，a是和b&&c这个整体发生作用！而（a||b)&&c恰恰相反！但是计算顺序还是从左到友的！而不是从右到左，我也不清楚我到到底说清楚没有
多看几遍，你应该可以明白
sx666777888 2014-06-23
- 打赏
- 举报
我调换了&&和||顺序后，输出的是 p1,p3
看不懂了，求指教啊[/quote]
这个我知道了，
sx666777888 2014-06-23
- 打赏
- 举报
我调换了&&和||顺序后，输出的是 p1,p3
看不懂了，求指教啊
taodm 2014-06-23
- 打赏
- 举报
楼主是糊涂在了优先级和执行顺序的关系上了。
你自己反复调整&&和||的位置，就可能发现点有意思的东西了。
