为什么要编译c++代码?它做什么?(复制)

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Uses of C comma operator [duplicate] 20 answers
C逗号运算符的使用[重复] 20个答案

I had this problem when I accidentally deleted the method name. The code went from

我不小心删除了方法名称时遇到了这个问题。代码来自

bool bRet = MethodName(pData, pOutFilename);

至

bool bRet = (pData, pOutFilename);

but still compiled? What does this code do? What does it mean? It seems to return true, is this always the case (even if pData is null)?

但仍然编译？这段代码有什么作用？这是什么意思？它似乎返回true，总是这样（即使pData为null）？

Any ideas are welcome!

欢迎任何想法！

3 个解决方案

#1

it is the "comma operator" which

它是“逗号运算符”

evaluates its first operand and discards the result, and then evaluates the second operand and returns this value (and type).

计算其第一个操作数并丢弃结果，然后计算第二个操作数并返回该值（和类型）。

#2

Your expression bool bRet = (pData, pOutFilename); is a valid expression, and it's equivalent to the expression bool bRet = pOutFilename;

你的表达式bool bRet =（pData，pOutFilename）;是一个有效的表达式，它等同于表达式bool bRet = pOutFilename;

In bool bRet = (pData, pOutFilename);, first expression pData is evaluated, then the second expression pOutFilename is evaluated, then value of second expression is assigned to bRet (this is how , operator works from left-to-right).

在bool bRet =（pData，pOutFilename）;中，计算第一个表达式pData，然后计算第二个表达式pOutFilename，然后将第二个表达式的值赋给bRet（这是操作符从左到右的方式）。

Read: Comma Operator: ,

阅读：逗号运营商：，

The comma operator , has left-to-right associativity. Two expressions separated by a comma are evaluated left to right. The left operand is always evaluated, and all side effects are completed before the right operand is evaluated.

逗号运算符具有从左到右的关联性。用逗号分隔的两个表达式从左到右进行计算。始终评估左操作数，并在评估右操作数之前完成所有副作用。

To understand the importance of parenthesis ( ) in your expression, consider my example below. Observe the output in this example (I have C example):

要了解括号（）在表达式中的重要性，请考虑下面的示例。观察这个例子中的输出（我有C例子）：

int main () {
   int i = 10, b = 20, c= 30;
   i = b, c;   // i = b
   printf("%i\n", i);

   i = (b, c); // i = c
   printf("%i\n", i);
}

output:

输出：

20
30

To understand the output: look at precedence table , have lower precedence than =. In you expression you have overwrite the precedence using parenthesis.

要理解输出：查看优先级表，优先级低于=。在表达式中，您使用括号覆盖优先级。

#3

Its a , comma operator. If you have an expression like this:

它是一个逗号运算符。如果你有这样的表达式：

i = (a, b);

b will be stored into i .

b将存储到i中。

So in your case:

所以在你的情况下：

bRet = pOutFilename;

pOutFilename will be stored into bRet.

pOutFilename将存储到bRet中。

#1