什么是C和C ++ preprocessors之间的区别是什么？ [英] What are the differences between the C and C++ preprocessors?

问题描述

是否有行为之间的差异在 C 和C ++ preprocessors？

Are there any differences in behaviour between the C and C++ preprocessors?

他们是由标准文本（的 C 标准和的 C ++标准）。

They are defined by different passages of standards text (section 6.10 of the C standard and section 16 of the C++ standard).

我对问这个动机是一个制作单引号一个数字分隔这一建议最近被接​​纳进入C ++ 14扩展了C ++ preprocessor语法，以适应这种变化（具体而言，它扩展了的 PP-数的定义的），我想知道这是否引入了 C 和C ++ preprocessors之间的不兼容，如果是这样，无论是第一个特征这么做。

My motivation for asking this is that a proposal for making the single quote a digit separator that was recently accepted into C++14 extends the C++ preprocessor grammar to accomodate this change (specifically, it extends the definition of a pp-number), and I'm wondering whether this introduces an incompatibility between the C and C++ preprocessors, and if so, whether it's the first feature to do so.

推荐答案

有C和C ++处理器之间的一些差异。忽略predefined宏和可用的系统头，有些差异浮现在脑海中的C和C ++的当前版本的区别是：

There are some differences between the C and C++ processors. Ignoring the differences in predefined macros and available system headers, some differences that come to mind in the current versions of C and of C++ are:

• 和和朋友是运营商，不是标识符，在C ++。这意味着 #define和＆功放;＆安培; 是在C有效，但不是在C ++和手段＃如果1和2 在C ++有效，但不是在C（除非和适当定义为宏）。


• 假和真正被允许在C ++ ＃如果前pressions，但 0 替换（像所有的标识符），这意味着＃如果真的 / C ++ / 的#else / C / #ENDIF 扩展为 C ++ 或 C ，根据不同的语言。不像和和朋友，不过，这些都不是运营商，因此可以通过的#define 中的任何一种语言重新定义。


• ：， * 和 - ＆GT; *。是C ++令牌。作为一个结果是， ## 操作符可以用来形成他们在C ++中，但不是在C。


• 原字符串在C ++的，但不是在C.结果给宏富， RX（富）x扩大在C宏，但不是在C ++中。


• 十六进制浮点常数都可以在C，但不是在C ++中。其结果是，给定宏富， 0x1p +富宏扩展在C ++中，但不是在C.

• and and friends are operators, not identifiers, in C++. This means #define and && is valid in C, but not in C++, and means #if 1 and 2 is valid in C++, but not in C (unless and is suitably defined as a macro).
• false and true are allowed in C++ #if expressions, but replaced by 0 (like all identifiers) in C. This means #if true/C++/#else/C/#endif expands to either C++, or C, depending on the language. Unlike and and friends, though, these are not operators, so may be redefined by #define in either language.
• ::, .*, and ->* are tokens in C++. As a result of that, the ## operator can be used to form them in C++, but not in C.
• Raw string literals are available in C++, but not in C. As a result, given a macro foo, R"x("foo")x" expands the macro in C, but not in C++.
• Hexadecimal floating-point constants are available in C, but not in C++. As a result, given a macro foo, 0x1p+foo expands the macro in C++, but not in C.

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