[英]typedef char in a macro
I am reading the machine learning library dlib which is written in C++. 我正在阅读用C ++编写的机器学习库dlib。 I came across a code in a header file which defines a bunch of macros. 我在头文件中遇到了定义了一系列宏的代码。 I have difficulties understanding the following code 我在理解以下代码时遇到困难
#ifndef BOOST_JOIN
#define BOOST_JOIN( X, Y ) BOOST_DO_JOIN( X, Y )
#define BOOST_DO_JOIN( X, Y ) BOOST_DO_JOIN2(X,Y)
#define BOOST_DO_JOIN2( X, Y ) X##Y
#endif
// a bunch of other code
namespace dlib
{
template <bool value> struct compile_time_assert;
template <> struct compile_time_assert<true> { enum {value=1}; };
// a bunch of other definitions
}
#define COMPILE_TIME_ASSERT(expression) \
DLIB_NO_WARN_UNUSED typedef char BOOST_JOIN(DLIB_CTA, __LINE__)[::dlib::compile_time_assert<(bool)(expression)>::value]
what I do not understand is that 我不明白的是
what does the last line in the above code do? 上面代码的最后一行是做什么的?
typedef char
here is so weird, I don't understand it at all. 这里的typedef char
很奇怪,我一点也不明白。
BOOST_JOIN
, it becomes DLIB_CTA__LINE__[1]
, why it is an array? 替换为BOOST_JOIN
,它变为DLIB_CTA__LINE__[1]
,为什么是数组? Is it legal? 合法吗 This is a C++03 static assert, or at least the workaround used to simulate it. 这是C ++ 03静态断言,或者至少是用于模拟它的解决方法。
The following defines a struct only if value
is true, else the type is not defined. 以下仅在value
为true时定义结构,否则未定义类型。
template <bool value> struct compile_time_assert;
template <> struct compile_time_assert<true> { enum {value=1}; };
Which means compile_time_assert<true>::value
is perfectly correct, but compile_time_assert<false>::value
is a compilation error since compile_time_assert<false>
is not defined. 这意味着compile_time_assert<true>::value
完全正确,但是compile_time_assert<false>::value
是编译错误,因为未定义compile_time_assert<false>
。
Now, the core of the assert: 现在,断言的核心是:
DLIB_NO_WARN_UNUSED typedef char BOOST_JOIN(DLIB_CTA, __LINE__) ::dlib::compile_time_assert<(bool)(expression)>::value]
This defines a typedef on a char
array of size 1, only if expression
evaluates to true. 仅当expression
计算结果为true时,才在大小为1的char
数组上定义typedef。 If the expression evaluates to false, then there is a compile time error and the name of the typedef is a hint about the error. 如果表达式的计算结果为false,则说明存在编译时错误,并且typedef的名称是有关该错误的提示。
Note that there are some variants of this. 请注意,这有一些变体。 Some use a negative or null size instead of a compilation error, ie they define the following: 有些使用负数或null大小而不是编译错误,即它们定义了以下内容:
template <> struct compile_time_assert<false> { enum { value = -1 }; };
But actually there is a way concise formulation of this (you can see it in this question ): 但实际上有一种简洁的表述方式(您可以在此问题中看到它):
typedef char static_assert_something[expression ? 1 : -1];
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