C ++:比较C函数的返回值是否为NULL或nullptr? [英] C++: Compare return value of C function to NULL or nullptr?

问题描述

我正在用C ++进行编码，并使用一个C函数，该函数在发生故障的情况下将返回NULL.将其返回值与NULL或nullptr进行比较，将是正确的想法吗?

I'm coding in C++, and using a C function that returns NULL in case of a failure. What would be the correct think to do, compare its return value to NULL or nullptr?

if ((CreateEventEx(myEventHandleHere) == NULL)
{
    ...
}

或

if ((CreateEventEx(myEventHandleHere) == nullptr)
{
    ...
}

推荐答案

C ++标准草稿(非规范性说法):

The draft C++ standard in appendix C.4 C Standard library which is non-normative says:

在< clocale> ;、< cstddef> ;、< cstdio>中的任何一个中定义的宏NULL， < cstdlib> ;、< cstring> ;、< ctime>或< cwchar>是 在此International中由实现定义的C ++空指针常量 标准(18.2).

The macro NULL, defined in any of <clocale>, <cstddef>, <cstdio>, <cstdlib>, <cstring>, <ctime>, or <cwchar>, is an implementation-defined C++ null pointer constant in this International Standard (18.2).

各个规范部分都同意，例如18.2说:

the respective normative sections agree for example 18.2 says:

宏NULL是实现定义的C ++空指针常量 在此国际标准(4.10).194

The macro NULL is an implementation-defined C++ null pointer constant in this International Standard (4.10).194

这意味着如果您正在使用那些特定的标头NULL应该与nullptr兼容，那么我只会使用nullptr.

which would mean if you are using the those particular headers NULL should be compatible with nullptr and then I would just use nullptr.

在涵盖兼容性的附录D中，似乎没有对.h头文件做出类似的陈述，因此尽管我们希望NULL和nullptr应该兼容 null指针常量，如果从标准的角度来看它们不是最少的话，我们将感到惊讶.从实际的角度来看，这使我们陷入了困境.我们可以确定它们是兼容的，但是我们没有足够的标准信息来证明这一点.

In appendix D which covers compatibility does not seem to make a similar statement for .h header files, so although we would expect that NULL and nullptr should be compatible null pointer constants and we would be surprised if they were not from the standard point of view it seems at minimum to be underspecified. Which leaves us with a dilemma, from a practical perspective we are pretty sure they are compatible but we don't have enough information from the standard to prove it.

所以我将使用您正在使用的特定头文件所定义的NULL，或者我们可以使用!= 0，因为幸运的是C99和C ++ 11都告诉我们0是 null指针常量.

So I would use NULL as defined by the specific header file you are using or we can use != 0 since fortunately both C99 and C++11 tell us that 0 is a null pointer constant.

从C99部分6.3.2.3 指针:

值为0的整数常量表达式或此类表达式 强制类型转换为void *，称为空指针常量.55)如果为空 将指针常量转换为指针类型，结果 指针，称为空指针，保证将不等于与 指向任何对象或函数的指针.

An integer constant expression with the value 0, or such an expression cast to type void *, is called a null pointer constant.55) If a null pointer constant is converted to a pointer type, the resulting pointer, called a null pointer, is guaranteed to compare unequal to a pointer to any object or function.

和:

任何指针类型都可以转换为整数类型.除了作为 先前指定的结果是实现定义的

Any pointer type may be converted to an integer type. Except as previously specified, the result is implementation-defined

和C ++部分4.10 指针转换告诉我们:

and C++ section 4.10 Pointer conversions tells us:

空指针常量是值为零的整数文字(2.14.2) 或类型为std :: nullptr_t的prvalue.空指针常量可以是 转换为指针类型；结果是的空指针值 该类型，并且可以与对象的所有其他值区分开 指针或函数指针类型.[...]

A null pointer constant is an integer literal (2.14.2) with value zero or a prvalue of type std::nullptr_t. A null pointer constant can be converted to a pointer type; the result is the null pointer value of that type and is distinguishable from every other value of object pointer or function pointer type.[...]

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