为什么我应该避免 C++ 中的宏?

Why should I avoid macros in C++?(为什么我应该避免 C++ 中的宏?)

问题描述

我读过很多书&我应该避免在 C++ 中使用宏的教程.很好，但为什么呢?我不明白.它们非常有用，经常在 C 语言中使用.

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(非常)详细地解释一下，为什么我应该在 C++ 中避免使用它们?

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

推荐答案

宏不尊重范围规则，而是在文本级别操作，而不是在语法级别.由此产生了许多可能导致奇怪的、难以隔离的错误的陷阱.

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.

考虑以下众所周知的例子:

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，首选的替代方法是函数模板:

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; }

这是另一个导致微妙问题的案例:

Here's another case that leads to subtle problems:

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

你可能认为解决方法很简单，就是把 CHECK_ERROR 的内容包裹在 { … } 中，但是由于 ; 导致编译不了. 在 else 之前.

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.

为了避免上述问题(else 附加到 CHECK_ERROR 的 if 而不是外部 if)，应该将这样的宏包装在 do ... while (false) 中，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
", ret, msg); 
        exit(1); 
    } 
  while (false)

这对宏的含义没有影响，但确保整个块始终被视为单个语句，并且不会以令人惊讶的方式与 if 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，宏在许多层面都是危险的，因此只能作为最后的手段使用.

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

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