为什么我应该避免在C ++中的宏？ [英] Why should I avoid macros in C++?

问题描述

我读过很多书，教程，我应该避免在c ++中的宏。好，但为什么？我不明白。它们非常有用，常用于C。

I have read in many books & tutorials that I should avoid macros in c++. Fine, but why? I don't get it. They are very useful and often used in C.

有人可以解释（非常）详细，为什么我应该在C ++中避免它们？

Could someone explain (very) detailed, why I should avoid them in C++?

推荐答案

宏不尊重范围界定规则，并且在语法级别而不是文本级别操作。

Macros don't respect scoping rules and operate at the textual level, as opposed to the syntax level. From this arise a number of pitfalls that can lead to strange, difficult to isolate bugs.

考虑下面这个着名的例子：

Consider the following well-known example:

#define max(a, b) ((a) < (b) ? (b) : (a))
⋮
int i = max(i++, j++);

在这种情况下，首选的备选方案是功能模板：

The preferred alternative in this case is a function template:

template <typename T>
T max(const T & a, const T & b) { return a < b ? b : a; }

这是另一种导致微妙问题的情况：

Here's another case that leads to subtle problems:

#define CHECK_ERROR(ret, msg) \
    if (ret != STATUS_OK) { \
        fprintf(stderr, "Error %d: %s\n", ret, msg); \
        exit(1); \
    }
⋮
if (ready)
    CHECK_ERROR(try_send(packet), "Failed to send");
else
    enqueue(packet);

您可能认为解决方案就像包装 CHECK_ERROR {...} ，但这不会编译，因为; else 。

You might think that the solution is as simple as wrapping the contents of CHECK_ERROR in { … }, but this won't compile due to the ; before the else.

为避免上述问题>附加到 CHECK_ERROR 的如果而不是外部如果），应该在 do ... while（false）中包装这样的宏，如下所示：

To avoid the above problem (the else attaching to CHECK_ERROR's if instead of the outer if), one should wrap such macros in do … while (false) as follows:

#define CHECK_ERROR(ret, msg) \
  do { \
    if (ret != STATUS_OK) { \
        fprintf(stderr, "Error %d: %s\n", ret, msg); \
        exit(1); \
    } \
  while (false)

关于宏的意义，但是确保整个块总是被视为单个语句，并且不会以令人惊讶的方式与如果语句交互。

This has no effect on the meaning of the macro, but ensures that the entire block is always treated as a single statement and doesn't interact in surprising ways with if statements.

长话短说，宏在许多层次都是危险的，因此只能作为最后手段使用。

Long story short, macros are hazardous at many levels and should thus be used only as a last resort.

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