Compiler error occurred when try to use a flexible template expression in preprocessor defines

[snnn]

When I try to compile the following codes piece under Visual C++ .net 2003, I get an error “Fatal Error C1017”

typedef ::Loki::Typelist<int,::Loki::NullType> TList;
#if ::Loki::TL::Length<TList>::value
int x;
#endif

Fatal Error C1017：
invalid integer constant expression
The expression in an #if directive did not exist or did not evaluate to a constant.

::Loki::TL::Length<TList> is a struct some like this:
struct Length{
enum { value=0}; // Indeed,the value should be the length of this Typelist(TList)
}

emmm. try to the following one instead:
enum {value=::Loki::TL::Length<TList>::value};
#if value
int x;
#endif

Ok! No error, no warning. However, as you see, that’s badly! I do not need this extra enum value.

So, Help me, please! Where is the syntax error in the former case?

Thanks!
Thanks!
Thanks!

[llmsn]

难道不会写中文,那怎么看得出我们写的中文,回答给你又有什么用呢?

[whoho]

#if ::Loki::TL::Length<TList>::value //1
int x;
#endif

1处你使用了一个编译期才能获得的常数value，预处理器是不能自己计算的
^^^^^^
#if使用的要么是字面常数，要么是#define常数

value的定义要在编译时计算之后才能得到

[whoho]

记住，模板虽然在编译期确定，但在那之前，是不确定的
预处理只是简单的替换，以及一些简单的处理

[snnn]

楼上，谢谢啦！

[jerry]

也许预处理器不认识C++的模板吧,但它认识ENUM常量,
我是猜的,仅供参考