如果检查null是否复合，然后C ++中的其他条件总是安全的？ [英] Is compound if checking for null and then other condition in C++ always safe?

问题描述

（这个问题是复合如果检查为空，然后C中的其他条件总是安全吗？但是关于 C ++ ，而不是C.有人指出问题应该更具体。）

(this question is an exact copy of Is compound if checking for null and then other condition in C always safe? but about C++, not C. It was pointed out that the question should be more specific).

我一直在使用以下类型的 if 条件很长时间。

I have been using the following type of if condition for a lot of time.

char* ptr = ...;
if (ptr != NULL && ptr[0] != '\0') // <=== is this always safe?
{ /* ... */ }

它依赖于 ptr！= NULL 在 ptr [0]之前检查！='\ 0'。

在所有标准，编译器和架构下是否安全？或者是否有可能在 ptr！= NULL ptr [0]！='\ 0' >？

Is it safe under all standards, compilers, architectures? Or is there a possibility that ptr[0] != '\0' will be checked before ptr != NULL?

推荐答案

在这种情况下是安全的。 短路评估表示&&的RHS ; 只有在第一个为真时才会评估运算符。

It is safe in this case. Short-circuit evaluation means that the RHS of the && operator will only be evaluated if the first is true.

C ++允许覆盖 bool运算符&& 用于用户定义的类型。使用覆盖&&& 不会进行短路评估，因此安全性会丢失。重载此运算符并不是一个好主意。

C++ allows to override bool operator && for user defined types. Using an overriden && does not follow short-circuit evaluation, so the safety is lost. It is rarely a good idea to overload this operator.

这是一个显示超载&& 运算符：

Here's an example showing the behaviour of an overloaded && operator:

struct Foo {};

bool operator && (const Foo&, const Foo&) { return true; }

#include <iostream>

Foo make_foo()
{
  std::cout << "making foo\n";
  return Foo();
}

int main()
{
  make_foo() && make_foo(); // evaluates both expressions
}

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